“Color Science” is one of those currently in fashion phrases that gets thrown around all over the place today. First of all – what the heck is color science anyway? Simply put it’s how the camera sees the colors in a scene, mixes them together, records them – and then how your editing or grading software interprets what is in the recording and finally how the TV or other display device turns the digital values it receives back into a color image. It’s a combination of optical filters such as the low pass filter, color filters, sensor properties, how the sensor is read out and how the signals are electronically processed both in the camera, by your edit/grading system and by the display device. It is no one single thing, and it’s important to understand that your edit process also contributes to the overall color science.
Color Science is something we have been doing since the very first color cameras, it’s not anything new. However us end users now have a much greater ability to modify that color science thanks to better post production tools and in camera adjustments such as picture profiles or scene files.
Recently, Sony cameras have sometimes been seen by some as having less advanced or poor color science compared to cameras from some other manufacturers. Is this really the case? For Sony part of the color science issue is that historically Sony have deliberately designed their newest cameras to match previous generations of cameras so that a large organisation with multiple cameras can use new cameras without having them look radically different to their old ones. It has always been like this and all the manufacturers do this, Panasonic cameras have a certain look as do Canon etc. New and old Panasonics tend to look the same as do old and new Canon’s, but the Canon’s look different to the Panasonics which look different to the Sony’s.
Sony have a very long heritage in broadcast TV and that’s how their cameras look out of the box, like Rec-709 TV cameras with colors that are similar to the tube cameras they were producing 20 years ago. Sony’s broadcast color science is really very accurate – point one at a test chart such as a Chroma DuMonde and you’ll see highly repeatable, consistent and accurate color reproduction with all the vectors on a vector scope falling exactly where they should, including the skin tone line.
On the one hand this is great if you are that big multi-camera business wanting to add new cameras to old ones without problems, where you want your latest ENG or self-shooters cameras to have the same colors as your perhaps older studio cameras so that any video inserts into a studio show cut in and out smoothly with a consistent look.
But on the other hand it’s not so good if you are a one man band shooter that wants something that looks different. Plus accurate is not always “pretty” and you can’t get away from the fact that the pictures look like Rec-709 television pictures in a new world of digital cinematography where TV is perhaps seen as bad and the holy grail is now a very different kind of look that is more stylised and much less true to life.
So Sony have been a bit stuck. The standard look you get when you apply any of the standard off-the shelf S-Log3 or S-Log2 LUT’s will by design be based on the Sony color science of old, so you get the Sony look. Most edit and grading applications are using transforms for S-Log2/3 based on Sony’s old standard Rec-709 look to maintain this consistency of look. This isn’t a mistake. It’s by design, it’s a Sony camera so it’s supposed to look like other Sony cameras, not different.
But for many this isn’t what they want. They want a camera that looks different, perhaps the “film look” – whatever that is?
Recently we have seen two new cameras from Sony that out of the box look very different from all the others. Sony’s high end Venice camera and the lower cost FS5 MKII. The FS5 MKII in particular proves that it’s possible to have a very different look with Sony’s existing colour filters and sensors. The FS5 MK II has exactly the same sensor with exactly the same electronics as the MK I. The only difference is in the way the RGB data from the sensor is being processed and mixed together (determined by the different firmware in the Mk1 and mk2) to create the final output.
The sensors Sony manufacture and use are very good at capturing color. Sony sensors are found in cameras from many different manufacturers. The recording systems in the Sony cameras do a fine job of recording those colors as data within the files the camera records as data with different code values representing what the sensor saw. Take that data into almost any half decent grading software and you can change the way it looks by modifying the data values. In post production I can turn almost any color I want into any other color. It’s really up to us as to how we translate the code values in the files into the colors we see on the screen, especially when recording using Log or raw. A 3D LUT can change tones and hues very easily by shifting and modifying the code values. So really there is no reason why you have to have the Sony 709 look.
My Venice emulation LUT’s will make S-Log3 from an FS5 or FS7 look quite different to the old Sony Broadcast look. I also have LUT’s for Sony cameras that emulate different Fuji and Kodak film stocks, apply one of these and it really looks nothing like a Sony broadcast camera. Another alternative is to use a color managed workflow such as ACES which will attempt to make just about every camera on the market look the same applying the ACES film style look and highlight roll-off.
We have seen it time and time again where Sony footage has been graded well and it then becomes all but impossible to identify what camera shot it. If you have Netflix take a look at “The Crown” shot on Sony’s F55 (which has the same default Sony look as the FS5 MK1, FS7 etc). Most people find it hard to believe the Crown was shot with a Sony because it has not even the slightest hint of the old Sony broadcast look.
If you use default settings, standard LUT’s etc it will look like a Sony, it’s supposed to! But you have the freedom to choose from a vast range of alternative looks or better still create your own looks and styles with your own grading choices.
But for many this can prove tricky as often they will start with a standard Sony LUT or standard Sony transform. So the image they start with has the old Sony look. When you start to grade or adjust this it can sometimes look wrong because you have perhaps become used to the original Sony image and then anything else just doesn’t seem right, because it’s not what you are used to. In addition if you add a LUT and then grade, elements of the LUT’s look may be hard to remove, things like the highlight roll off will be hard baked into the material, so you need to do need to think carefully about how you use LUT’s. So try to break away from standard LUT’s. Try ACES or try some other starting point for your grade.
Going forward I think it is likely that we will see the new Venice look become standard across all of the Cinema style cameras from Sony, but it will take time for this to trickle down into all the grading and editing software that currently uses transforms for s-Log2/3 that are based on the old Sony Rec-709 broadcast look. But if you grade your footage for yourself you can create just about any look you want.
Sony are offering up to £220/€250 cash back on accessories purchased with an FS5 and up to £400/€450 cash back on accessories purchased with FS7 or FS7M2 if you purchase one before the end of March 2018. So there’s only 2 weeks left to take advantage of this offer!
So if your looking at investing in a nice camera kit with perhaps one of the excellent UWP-D radio mic kits that connect directly to the cameras MI shoe or some extra batteries this might be a great way to get some money back from Sony. There are various terms and conditions so please take a look at the promotion page for the full details. Here’s a link to the promotion page.
With the first of the production Venice cameras now starting to find their way to some very lucky owners it’s time to take a look at some features that are not always well understood, or that perhaps no one has told you about yet.
Dual Native ISO’s: What does this mean?
An electronic camera uses a piece of silicon to convert photons of light into electrons of electricity. The efficiency at doing this is determined by the material used. Then the amount of light that can be captured and thus the sensitivity is determined by the size of the pixels. So, unless you physically change the sensor for one with different sized pixels (which will in the future be possible with Venice) you can’t change the true sensitivity of the camera. All you can do is adjust the electronic parameters.
With most video cameras the ISO is changed by increasing the amount of amplification applied to the signal coming off the sensor. Adding more gain or increasing the amplification will result in a brighter picture. But if you add more amplification/gain then the noise from the sensor is also amplified by the same amount. Make the picture twice as bright and normally the noise doubles.
In addition there is normally an optimum amount of gain where the full range of the signal coming from the sensor will be matched perfectly with the full recording range of the chosen gamma curve. This optimum gain level is what we normally call the “Native ISO”. If you add too much gain the brightest signal from the sensor would be amplified too much and exceed the recording range of the gamma curve. Apply too little gain and your recordings will never reach the optimum level and darker parts of the image may be too dark to be seen.
As a result the Native ISO is where you have the best match of sensor output to gain. Not too much, not too little and hopefully low noise. This is typically also referred to as 0dB gain in an electronic camera and normally there is only 1 gain level where this perfect harmony between sensor, gain and recording range is achieved, this becoming the native ISO.
Side Note: On an electronic camera ISO is an exposure rating, not a sensitivity measurement. Enter the cameras ISO rating into a light meter and you will get the correct exposure. But it doesn’t really tell you how sensitive the camera is as ISO has no allowance for increasing noise levels which will limit the darkest thing a camera can see.
Tweaking the sensor.
However, there are some things we can tweak on the sensor that effect how big the signal coming from the sensor is. The sensors pixels are analog devices. A photon of electricity hits the silicone photo receptor (pixel) and it gets converted into an electron of electricity which is then stored within the structure of the pixel as an analog signal until the pixel is read out by a circuit that converts the analog signal to a digital one, at the same time adding a degree of noise reduction. It’s possible to shift the range that the A to D converter operates over and the amount of noise reduction applied to obtain a different readout range from the sensor. By doing this (and/or other similar techniques, Venice may use some other method) it’s possible to produce a single sensor with more than one native ISO.
A camera with dual ISO’s will have two different operating ranges. One tuned for higher light levels and one tuned for lower light levels. Venice will have two exposure ratings: 500 ISO for brighter scenes and 2500 ISO for shooting when you have less light. With a conventional camera, to go from 500 ISO to 2500 ISO you would need to add just over 12dB of gain and this would increase the noise by a factor of more than 4. However with Venice and it’s dual ISO’s, as we are not adding gain but instead altering the way the sensor is operating the noise difference between 500 ISO and 2500 ISO will be very small.
You will have the same dynamic range at both ISO’s. But you can choose whether to shoot at 500 ISO for super clean images at a sensitivity not that dissimilar to traditional film stocks. This low ISO makes it easy to run lenses at wide apertures for the greatest control over the depth of field. Or you can choose to shoot at the equivalent of 2500 ISO without incurring a big noise penalty.
One of Venice’s key features is that it’s designed to work with Anamorphic lenses. Often Anamorphic lenses are typically not as fast as their spherical counterparts. Furthermore some Anamorphic lenses (particularly vintage lenses) need to be stopped down a little to prevent excessive softness at the edges. So having a second higher ISO rating will make it easier to work with slower lenses or in lower light ranges.
COMBINING DUAL ISO WITH 1 STOP ND’s.
Next it’s worth thinking about how you might want to use the cameras ND filters. Film cameras don’t have built in ND filters. An Arri Alexa does not have built in ND’s. So most cinematographers will work on the basis of a cinema camera having a single recording sensitivity.
The ND filters in Venice provide uniform, full spectrum light attenuation. Sony are incredibly fussy over the materials they use for their ND filters and you can be sure that the filters in Venice do not degrade the image. I was present for the pre-shoot tests for the European demo film and a lot of time was spent testing them. We couldn’t find any issues. If you introduce 1 stop of ND, the camera becomes 1 stop less sensitive to light. In practice this is no different to having a camera with a sensor 1 stop less sensitive. So the built in ND filters, can if you choose, be used to modify the base sensitivity of the camera in 1 stop increments, up to 8 stops lower.
So with the dual ISO’s and the ND’s combined you have a camera that you can setup to operate at the equivalent of 2 ISO all the way up to 2500 ISO in 1 stop steps (by using 2500 ISO and 500 together you can have approximately half stops steps between 10 ISO and 650 ISO). That’s an impressive range and at no stage are you adding extra gain. There is no other camera on the market that can do this.
On top of all this we do of course still have the ability to alter the Exposure Index of the cameras LUT’s to offset the exposure to move the exposure mid point up and down within the dynamic range. Talking of LUT’s I hope to have some very interesting news about the LUT’s for Venice. I’ve seen a glimpse of the future and I have to say it looks really good!
The raw and X-OCN material from a Venice camera (and from a PMW-F55 or F5 with the R7 recorder) contains a lot of dynamic metadata. This metadata tells the decoder in your grading software exactly how to handle the linear sensor data stored in the files. It tells your software where in the recorded data range the shadows start and finish, where the mid range sits and where the highlights start and finish. It also informs the software how to decode the colors you have recorded.
I recently spent some time with Sony Europe’s color grading guru Pablo Garcia at the Digital Motion Picture Center in Pinewood. He showed me how you can manipulate this metadata to alter the way the X-OCN is decoded to change the look of the images you bring into the grading suite. Using a beta version of Black Magic’s DaVinci Resolve software, Pablo was able to go into the clips metadata in real time and simply by scrubbing over the metadata settings adjust the shadows, mids and highlights BEFORE the X-OCN was decoded. It was really incredible to see the amount of data that Venice captures in the highlights and shadows. By adjusting the metadata you are tailoring the the way the file is being decoded to suit your own needs and getting the very best video information for the grade. Need more highlight data – you got it. Want to boost the shadows, you can, at the file data level before it’s converted to a traditional video signal.
It’s impressive stuff as you are manipulating the way the 16 bit linear sensor data is decoded rather than a traditional workflow which is to decode the footage to a generic intermediate file and then adjust that. This is just one of the many features that X-OCN from the Sony Venice offers. It’s even more incredible when you consider that a 16 bit linear X-OCN LT file is similar in size to 10 bit XAVC-I(class 480) and around half the size of Apples 10 bit ProRes HQ. X-OCN LT looks fantastic and in my opinion grades better than XAVC S-Log. Of course for a high end production you will probably use the regular X-OCN ST codec rather than the LT version, but ST is still smaller than ProRes HQ. What’s more X-OCN is not particularly processor intensive, it’s certainly much easier to work with X-OCN than cDNG. It’s a truly remarkable technology from Sony.
Next week I will be shooting some more test with a Venice camera as we explore the limits of what it can do. I’ll try and get some files for you to play with.
Just a very quick note that the last UK event of the Sony Pro Tour for 2017 will be in Glasgow on Thursday the 7th of December. I’ll be there to answer any questions and to give an in depth seminar on HDR including how to shoot HDR directly with the Sony cameras that feature Hybrid Log Gamma.
The event is free, there will be a wide range of cameras for you to play with including FS5, FS7, the new Z90 and X80 as well as monitors, mixers and audio gear.
More info here: https://www.sony.co.uk/pro/page/sony-pro-tour-2017
Just to let you know that from next month Sony we will be holding a tour in selected European countries to showcase their Handheld camcorder range, and as a result starting from November they will visit 4 locations across the UK, as detailed below. I will be in attendance at the UK events offering free advice and info, so if there is anything you are struggling with why not drop in and see me:
Here’s a little insight into what some of the Sony product prefixes might mean. These may not be 100% correct but this is what I understand them to mean. There are also many exceptions to the standard naming convention, so use this as a guide only.
First letter: P = Professional, B = Broadcast, U = Utility, D = Digital, H = Studio/OB, L = LCD, O = Optical
Second letter: V = Video (means video tape if camera), M = SxS Memory, D = Optical Disc, X = XQD/SD/SxS(solid state media or digital workflow), S = System, D = Multi format, P = Projector or Printer, R = Remote Control, W = wireless
Third letter: (or 3 + 4th): W = Writer( deck or camera that can also record/camcorder), M = Monitor, C = Camera(no recording capability) CU = Camera Control Unit, CP = Compact camera(no recording), X = HD, Z = 4K, D = Digital (HDSDI?), A = Archive
Then after the 3/4 letter prefix: F = Film (digital cinema, 16 bit raw when raw included), FS = Film Style (large sensor, 12 bit raw when raw included), X = HD from factory (often but not always upgradable to 4K) , Z is 4K from factory. NX = AVCHD
The number of digits after the letters used to be significant. 2 digits was a product without an imager (PMW-50, PMW-EX30) 3 digits was a camera (PMW-200, PMW-500) and 4 digits was a deck (PMW-HD1500, F1600). However recently cameras have any number of digits.
For example PXW-X200:
P = Professional X = Solid state media W = Writer – X = HD camcorder.
Example BVW-400 (Betacam SP camcorder – remember those!)
B = Broadcast V = Video Tape W = Writer
P = Professional X = Solid State Media W = Writer – FS = Film Style.
Example PMW-F55 (Slight odd-ball this one as it was the very first XAVC camera, perhaps should really have been a PXW-F55 although as it’s SxS and has the XDCAM codec PMW works too).
P = Professional M = SxS Memory W = Writer – F = Film (Digital Cinema).
Example PDW-700 (Optical disc camcorder)
P = Professional D = Disk W = Writer.
P = Professional M = SxS Memory W = Writer.
F65 = F65 Ultimate digital cinema camera, no prefix and a 35mm sensor, not 65mm as the name suggests. Other “F” only cameras used the sensor size for the name. The F35 had a super 35mm sensor and F23 which had a 2/3″ sensor.
CBKZ = Software upgrade option.
CBK = Camera build kit
HDW = HDCAM Writer.
SRW = HDCAM SR Writer.
DSR = Digital DVCAM camera/camcorder/deck.
I don’t know what the HXR prefix stands for, this line of normally AVCHD solid state camcorders used to come from a different group within Sony to the broadcast group. These two groups now work together so the product numbering is now more consistent, but there are still many product names that don’t follow the convention.
It’s interesting to note that there are very few “Broadcast” B** products these days except for BVM (Broadcast Video Monitor) monitors. Most camcorders are now P** even if they are most definitely broadcast cameras, for example the PXW-X500.
If anyone would like to add to this list or correct any errors please let me know by adding a comment. Any input/additions are most welcome!
Damn, I just purchased a Sony A6000 to take to Norway next week and this pops up. It’s the latest camera in the compact camera range from Sony that started with the NEX-5 and NEX-3, great little cameras that take great photos and have been timelapse work-horses for me.
The A6300 specs are beyond impressive. It has a new 25MP sensor with an improved type of construction that improves sensitivity. It’s only APS-C so I’m not expecting A7 MK2 performance, but it should do very well when the light levels are low.
One of the headline features for me though is it’s ability to shoot 4K XAVC-S that is originated from a 6K image coming off the sensor. On top of that this truly pocket sized camera has what appear to be the full compliment of cinegammas as well as S-log2 and S-log3. Now before everyone gets too excited, do remember that XAVC-S is 8 bit whether in HD or SD, but even so this is an amazing feature set for this kind of camera.
If that isn’t enough it can even shoot in HD at upto 120fps!
The price? Well the body only is $999 USD. It’s E-Mount so as usual you can put all kinds of lenses on it from Sony power zooms to PL mount primes and everything in between. For more information on what on paper at least appears to be a remarkable little camera click here.
If you find this useful please consider buying me a coffee or a beer. I’m not paid to write these articles.
One of the really nice features of the Sony A7s and Sony’s other Alpha cameras, including the A6300, A6500 etc is the ability to use different gamma curves and in particular the Sony S-Log2 gamma curve.
What are gamma curves?
All conventional cameras use gamma curves. The gamma curve is there to make the images captured easier to manage by making the file size smaller than it would be without a gamma curve. When TV was first developed the gamma curve in the camera made the signal small enough to be broadcast by a transmitter and then the gamma curve in the TV set (which is the inverse of the one in the camera) expanded the signal back to a normal viewing range. The current standard for broadcast TV is called “Recommendation BT-709”, often shortened to Rec-709. This gamma curve is based on standards developed over 60 years ago and camera technology has advanced a lot since then! Even so, almost every TV and monitor made today is made to the Rec-709 standard or something very similar. Many modern cameras can capture a brightness range, also known as dynamic range, that far exceed the Rec-709 standard.
The limitations of standard gammas.
As gamma effects the dark to light range of the image, it also effects the contrast of the image. Normal television gamma has a limited dynamic range (about 6 to 7 stops) and as a result also has a limited contrast range.
Normally the gamma curve used in the camera is designed to match the gamma curve used by the TV or monitor. This way the contrast range of the camera and the contrast range of the display will be matched. So the contrast on the TV screen will match the contrast of the scene being filmed and the picture will look “normal”. However the limited dynamic range may mean that very bright or very dark objects cannot be accurately reproduced as these may exceed the gammas dynamic range.
The over exposure typical of a restricted range gamma such as Rec-709 is commonly seen as bright clouds in the sky becoming white over exposed blobs or bright areas on faces becoming areas of flat white. Objects in shade or shadow areas of the scene are simply too dark to be seen. But between the overexposed areas and any under exposure the contrast looks natural and true to life.
Log gamma, such as Sony’s S-Log2, allows the camera to capture a much greater brightness range or dynamic range than is possible when shooting with conventional television gamma. Dynamic range is the range from light to dark that the camera can capture or the range that the monitor or TV can display within one image. It is the range from the deepest blacks to the brightest whites that can be captured or shown at the same time.
There are some things that need to be considered before you get too excited about the possibility of capturing this much greater dynamic range. The primary one being that if the camera is set to S-log2 and the TV or monitor is a normal Rec-709 TV (as most are) then there is no way the TV can correctly display the image being captured, the TV just doesn’t have the range to show everything that the camera with it’s high range log gamma can capture accurately.
Fixed Recording Range For Both Standard and Log Gamma.
The signal range and signal levels used to record a video signal are normally described in percent. Where black is 0% and the brightest thing that can be recorded is normally recorded at 100% to 109%. Most modern video cameras actually record the brightest objects at 109%. The important thing to remember though is that the recording range is fixed. Even when you change gamma curve the camera is still constrained by the zero to 109% recording range. The recording range does not change whether you are recording Rec-709 or S-log2. So log gamma’s like S-Log2 must squeeze a much bigger signal range into the same recording range as used by conventional Rec-709 recordings.
In order to record using S-log2 with the A7s you need to use a picture profile. The picture profiles give you several recording gamma options. For S-log2 you should use Picture Profile 7 which is already set up for S-log2 and S-Gamut by default (for information on gamuts see this article). In addition you should ALWAYS use the cameras native ISO which is 3200 ISO and it is normally preferable to use a preset white balance. Using any other ISO with S-log2 will not allow you to get the full benefit of the full 14 stops of dynamic range that S-log2 can deliver. In most of the Alpha cameras you now also have the ability to use a different version of S-log, – S-Log3 and this is found in picture profiles 8 and 9. You can use S-Log3 if you wish, but S-Log2 was designed from the outset by Sony to work with digital camera sensors. S-Log3 is based on an older curve designed for film transfers to a 10 bit recording. As a result when using a camera that only has 8 bit recording with a limited number of code values, S-Log2 tends to be more efficient and yield a better end result. This is what it was designed for.
Grey Cards and White Cards.
Before I go further let me introduce you to grey and white cards in case you have not come across them before. Don’t panic you don’t have to own one, although I would recommend getting a grey card such as the Lastolite EzyBalance if you don’t have one. But it is useful to understand what they are.
The 90% White Card.
The 90% white card is a card or chart that reflects 90% of the light falling on it. This will be a card that looks very similar in brightness to a piece of ordinary white paper, it should be pure white, some printer papers are bleached or coloured very slightly blue to make them appear “brilliant white” (as you will see later in many cases it is possible to use an ordinary piece of white paper in place of a 90% white card for exposure).
The Grey Card.
The 18% grey card, also often called “middle grey” card, is a card that reflects 18% of the light falling on it. Obviously it will appear much darker than the white card. Visually to us humans an 18% grey card appears to be half way between white and black, hence it’s other name, “middle grey”.
Middle grey is important because the average brightness level of most typical scenes tends to be around the middle grey brightness value. Another key thing about middle grey is that because it falls in the middle of our exposure range it makes it a very handy reference level when measuring exposure as it is less likely to be effected by highlight compression than a 90% white card.
Exposing White and Middle Grey.
Coming back to Rec-709 and conventional TV’s and monitors. If we want a piece of white paper to look bright and white on a TV we would record it and then show it at somewhere around 85% to 95% of the screens full brightness range. This doesn’t leave much room for things brighter than a white piece of paper! Things like clouds in the sky, a shiny car, a bright window or a direct light source such as a lamp or other light. In order to make it possible for S-log2 to record a much greater dynamic range the recording level for white and mid tones is shifted down. Instead of recording white at 85%-95%, when using S-log2 or S-Log3 it is recommended by Sony that white is recorded at around 60%. For S-Log2 Middle grey moves down too, instead of being recorded at 42%-43% (the normal level for Rec-709) it’s recorded at just 32% with S-Log2 (S-log3 uses 41%).
By recording everything white (ie a white piece of paper) and darker in a lower range, we free up lot of extra space above the white recording level, within the full recording range, to record all those bright highlights in any scene that would be impossible to record with conventional gammas where there is only 10% to 20% from white at 90% to the peak of the recording range at 100 to 109%.
As S-Log2 and S-Log3 normally shift a lot of the recording levels downwards, if we show a scene shot with S-Log2 or S-log3 that has been exposed correctly on a conventional TV or monitor it will look dark due to the lower recording levels. In addition it will look flat with very low contrast as we are now squeezing a much bigger dynamic range into the limited conventional Rec-709 display range of a normal TV or computer monitor.
This on screen reduction in contrast and the darker levels are actually perfectly normal when shooting using log gamma, this is how it is supposed to look on a normal monitor or TV. So don’t be alarmed if when shooting using S-Log your images look a little darker and flatter than perhaps you are used to when shooting with a standard gamma. You will adjust the S-Log footage in post production to restore the brightness and contrast later.
The post production adjustment of S-Log2 and S-log3 is very important and one of the keys to getting the very best finished images. The S-Log recording acts as a digital negative and by “processing” this digital negative in post production (normally referred to as “grading”) we manipulate the large 14 stop dynamic range of the captured image to fit within the limited display range of a Rec-709 TV in a pleasing manner. This may mean pulling up the mid range a bit, pulling down the highlights and bit and generally shifting the brightness and colour levels of different parts of the image around (see PART 2 for more post production information).
SLog-2 and 10 bit or 8 bit data.
Originally Slog-2 was designed for use on high end digital cinema cameras such as Sony’s F65 camera. These cameras have the ability to record using 10 bit data. A 10 bit recording can have up to around 1000 shades of grey from black to white. The A7s however uses 8 bit recording which only has a maximum of 235 shades from black to white. Normally 8 bit recording is perfectly OK as most transmission and display standards are also 8 bit. Shoot with an 8 bit camera and then display that image directly via an 8 bit system and nothing is lost. However when you start to grade and manipulate the image the difference between 8 bit and 10 bit becomes more significant. If you start to shift levels around, perhaps stretching out some parts of the image then the increased tonal resolution of a 10 bit recording helps maintain the very highest image quality. Photographers that have shot using both jpeg and raw will know how much more flexibility the 12 bit (or more) raw files have compared to the 8 bit jpeg’s. However they will also know that 8 bit jpeg’s can be also adjusted, provided you don’t need to make very large adjustments.
Contrary to popular belief heavy grading of 8 bit footage does not necessarily lead to banding in footage across smooth surfaces except in extreme cases. Banding is more commonly a result of compression artefacts such as macro blocking. This is especially common with very highly compressed codecs such as AVCHD. The 50Mbps XAVC-S codec used in the Sony Alpha cameras is a very good codec, far superior to AVCHD and as a result compression artefacts are significantly reduced, so banding will be less of an issue than with other lower quality codecs. If you’re going to shoot using S-Log2, some grading will be necessary and as we only have 8 bit recordings we must take care to expose our material in such a way as to minimise how far we will need to push and pull the material.
Getting Your Exposure Right.
When S-Log2 was developed the engineers at Sony produced tables that specified the correct exposure levels for s-Log2 which are:
As you can see the nominal “correct” exposure for S-Log2 is a lot lower than the levels used for display on a typical Rec-709 TV or monitor. This is why correctly exposed s-log2 looks dark on a conventional TV. The implication of this is that when you grade your footage in post production you will have to shift the S-log2 levels up quite a long way. This may not be ideal with an 8 bit codec, so I decided to carefully test this to determine the optimum exposure level for the A7s.
The panel of images below is from the A7s recording S-log2 and exposed at the Sony recommended “correct” 32% middle grey level. The correct exposure was determined using a grey card and an external waveform monitor connected to the cameras HDMI output. Then the S-log2 was corrected in post production to normal Rec-709 levels using a Look Up Table (LUT – more on LUT’s in part 2). You can also see the viewfinder display from the camera. If you click on the image below you can expand it to full size. Sorry about the shadow from the laundry line, I didn’t see this when I was shooting the test shots!
From this you can see just how dark and low contrast looking the original correctly exposed S-log2 is and how much more vibrant the corrected Rec-709 image is. I have also indicated where on the cameras histogram middle grey and white are. Note how much space there is to the right of white on the histogram. This is where the extra highlight or over exposure range of S-log2 can be recorded. When correctly exposed S-log2 has an exposure range of 6 stops above middle grey and 8 stops under.
Over Exposing or “Pushing” S-log2.
If we deliberately raise the exposure level above the Sony recommended levels (known as pushing the exposure), assuming you grade the image to the same final levels some interesting things happen.
For each stop we raise the exposure level you will have 1 stop (which is the same as 6db) less noise. So the final images will have half as much noise for each stop up you go. This is a result of exposing the image brighter and as a result not needing to raise the levels in post as far as you would if exposed at the normal level.
You will loose one stop of over exposure headroom, but gain one stop of under exposure headroom.
Bright highlights will be moved upwards into the most compressed part of the log gamma curve. This can result in a loss of texture in highlights.
Skin tones and mid tones move closer to normal Rec-709 levels, so less manipulation is need for this part of the image in post production.
This last point is important for the A7s with it’s 8 bit codec, so this is the area I looked at most closely. What happens to skin tones and textures when we raise the exposure?
Exposing at +1, +2 and +3 Stops.
Below are another 3 panels from the A7s, shot at +1 stop, +2 stops and +3 stops. Again you can click on the images if you wish to view them full size.
Looking at these results closely you can see that when you increase the exposure by 1 stop over the Sony specified correct level for S-log2 there is a very useful reduction in noise, not that the A7s is particularly noisy to start with, but you do get a noticeably cleaner image.
Below are 4 crops from the same images, after grading. I really recommend you view these images full size on a good quality monitor. Click on the image to view larger or full size.
The noise reduction at higher exposures compared to the base exposure is very clear to see if you look at the black edge of the colour checker chart (the coloured squares), although the difference between +2 and +3 stops is very small. You can also see further into the shadows in the +3 stop image compared to the base exposure. A more subtle but important effect is that as the exposure goes up the visible texture of the wooden clothes peg decreases. The grain can be clearly seen at the base level but by +3 stops it has vanished. This is caused by the highlights creeping into the more compressed part of the log gamma curve. The same thing is happening to the skin tones in the +3 stop image, there is some reduction of the most subtle textures.
From this we can see that for mid tones and skin tones you can afford to expose between 1 and 2 stops above the Sony recommended base level. More than 2 stops over and brighter skin tones and any other brighter textures start to be lost. The noise reduction gain by shooting between one and 2 stops over is certainly beneficial. The down side to this though is that we are reducing the over amount of exposure headroom.
Given everything I have seen with this 8 bit and almost every other 8 bit camera my recommendation is to shoot between the Sony recommended base S-log2 level and up to two stops over this level. I would try to avoid shooting more than 2 stops over as this is where you will start to see some loss of texture in brighter skin tones and brighter textures. Exactly where you set your exposure will depend on the highlights in the scene. If you are shooting a very bright scene you will possibly need to shoot at the Sony recommended level to get the very best over exposure headroom. If you are able to expose higher without significantly compromising any highlights then you should aim to be up to 2 stops over base. But whatever you do never expose darker than the Sony base level, this will normally look really nasty.
Determining The Correct Exposure.
The challenge of course is determining where your exposure actually is. Fortunately as we have seen, provided you in the right ball park, S-log2 is quite forgiving, so if you are a little bit over exposed it’s probably not going to hurt your images much. If you have a waveform monitor then you can use that to set your exposure according to the table below. If you don’t have proper white or grey cards you can use a piece of normal white paper. Although slightly less accurate this will get you very close to where you want to be. Do note that white paper tends to be a little brighter than a dedicated 90% reflectivity white card. If you don’t have any white paper then you can use skin tones, again a bit less accurate but you should end up in the right zone.
If you don’t have an external waveform monitor then you do still have some good options. Sadly although the camera does have zebras, these are not terribly useful for S-log2 as the lowest the zebras can go is 70%.
Light Meter: You could use a conventional photography light meter. If you do choose to use a light meter I would recommend checking the calibration of the light meter against the camera first.
Mark 1 Eyeball: You could simply eyeball the exposure looking at the viewfinder or rear screen but this is tricky when the image is very flat.
In Camera Metering: The cameras built in metering system, like the majority of DSLR’s is calibrated for middle grey. By default the camera uses multi-point metering to measure the average brightness of several points across the scene to determine the scenes average brightness and from there set the correct base S-log2 exposure.
When you are using S-Log2, auto exposure in most cases will be very close to the correct base exposure if you use the default Multi-Zone exposure metering. The camera will take an average exposure reading for the scene and automatically adjust the exposure to the Sony recommended 32% middle grey exposure level based on this average. In the P, A and S modes you can then use the exposure compensation dial to offset the exposure should you wish. My recommendation would be to add +1 or +2 stops via the dial. Then observe the histogram to ensure that you don’t have any significant over exposure. If you do then reduce the exposure compensation. Lots of peaks to the far right of the histogram is an indication of over exposure.
Manual Exposure And Internal Metering.
If you are exposing manually you will see a small M.M. indication at the bottom of the LCD display with a +/- number. In the eyepiece viewfinder this appears as a scale that runs from -5 to +5, in S-log2 only the -2 to +2 part of the scale is used. In both cases this is how far the camera thinks you are away from the optimum exposure. + meaning the camera is over exposed, – meaning under.
In the image above we can see the M.M. indication is +0.3, in the eyepiece you would see a small arrow one bar to the right of “0” , indicating the cameras multi zone metering thinks the shot is just a little over exposed, even though the shot has been carefully exposed using a grey card and external waveform monitor. This error is probably due to the large amount of white in the shot, white shirt, white card, test charts with a lot of brighter than grey shades. In practice an error of 0.3 of a stop is not going to cause any real issues, so even if this was exposed by setting the exposure so that you have “M.M. 0.0” the exposure would be accurate enough. But it shows that multi point exposure averaging is easily confused.
The scene above is a fairly normal scene, not excessively bright, not particularly dark. If shooting a snow scene for example the cameras multi point averaging would almost certainly result in an under exposed shot as the camera attempts to bring the bright snow in the scene down to the average middle grey level. If shooting a well lit face against a very dark background then the averaging might try to bring the background up and the shot may end up overexposed.
If you want really accurate exposure then you should put the cameras metering system into the spot metering mode where instead of taking an average of various points across the scene the camera will just measure the exposure at the very center of the image.
You can then use a grey card to very accurately set the exposure. Simply place the circular shaped symbol at the center of the viewfinder display over a grey card and set the exposure so that M.M is 0.0 for the correct S-Log2 base exposure. To expose 1 stop over with a grey card, set M.M. +1.0 and two stops over M.M. +2.0 (not flashing, flashing indicates more than +2 stops).
One small issue with this is that the camera will only display a M.M. range of -2.0 to +2.0 stops. Provided you don’t want to go more than 2 stops over base then you will be fine with a grey card.
Using White Instead of Grey:
If you don’t have a grey card then you can use a 90% reflectivity white target. As white is 2 stops brighter than middle grey when S-Log2 is correctly exposed the 90% white should indicate M.M +2.0.
Once you have established the correct exposure you can then open the iris by 1 or two stops to increase the exposure. Or halve the shutter speed to gain a one stop brighter exposure. Each time you halve the shutter speed your exposure becomes one stop brighter, so divide the shutter speed by 4 to gain a 2 stop increase in exposure. As always you should observe the histogram to check for any over exposure. White peaks at the far right of the histogram or disappearing completely off the right of the histogram is an indication of over-exposure. In this case reduce your exposure back down towards the base exposure level (M.M 0.0 with a a grey card).
I recommend using an exposure between the “correct” base S-Log2 exposure level of middle grey at 32% and two stops over this. I would not recommend going more than 2 stops over over base.
In the P, A and S auto exposure modes, when using the default multi-zone metering the camera will set the base S-log2 exposure based on the average scene brightness. For most typical scenes this average should be very close to middle grey. This exposure can then be increased (brightened) by up to 2 stops using the exposure compensation dial.
In manual exposure the “M.M.” number displayed at the bottom of the viewfinder display is how far you are from the correct base S-log2 exposure. M.M. +2.0 indicates +2 stops over base. If using multi zone metering (the cameras default) this exposure will be based on the scenes average brightness.
If you set the metering to “Spot” you can use a grey card centred in the image to determine the correct base exposure and up to 2 stops of over exposure via the M.M. indication when shooting manually.
In Part 2:
In part two I will take a look at grading the S-log2 from the A7s and how to get the very best from the S-log2 images by using Look Up Tables (LUT’s).
Don’t forget I run storm chasing and Northern Lights expeditions every year. These are amazing expeditions by snowmobile up on to the Finnmarksvidda. We go ice fishing, dog sledding, exploring, cook a meal in a tent and enjoy traditional Norwegian saunas.
I’ve been doing a lot of work on shooting SLog-2 with the A7s. I realised almost straight away that a set of LUT’s for this camera would really help speed up my grading and testing. In addition as the camera is only 8 bit I have found that I am actually getting the best results from the Slog-2 if I over expose it just a little bit, depending of course on the scene. So I created a set of LUT’s that includes compensation for shooting at the nominal correct exposure as well as either 1 or 2 stops over exposed. In all there are 20 LUT’s in two sets. One is Rec-709 based LUT’s and the other Filmic LUT’s to act as starting points for further grading.
I am in the process of creating the complete workflow and SLog-2 guide for the A7s which I should be publishing later next week which will have much more information on how to use these LUT’s. But in the mean time here are the LUT’s if anyone want’s to play (and I would like feedback on what you think of them).
The LUT naming goes something like this:
AC A7S 709(800) ZERO
AC (That’s me!). A7S (The Camera).
709(800) = Output gamma or style. ZERO = Exposure off set.
The exposure offset refers to the number of stops the footage is over exposed by relative to the normal SLog2 exposure level of Middle Grey at 32% and 90% white at 59%. ZERO menas no exposure offset. 1STOP would be used when the SLog2 was exposed 1STOP over and in this case the LUT then shifts the exposure back down 1 stop to compensate.
709 = Vanilla Rec-709, very contrasty, but limited highlight response and hard clip of over exposure.
709(800) = Rec-709 gamma with 800% (high) dynamic range. Will be slightly low contrast but deals much better with over exposure or bright highlights than vanilla 709.
Film-Like1 = An extended range gamma with highlight roll off (+400% range), slightly de-saturated, slightly more film like color (small red/yellow removed).
Filmic2 = Extended range low contrast gamma with very good over exposure handling. Slightly de-saturated. Good grading start point.
Filmic3 = Similar to Filmic2 but a little more contrast at the expense of a little less highlight roll off.
Click on the links below to download the LUT sets. PLEASE DO NOT HOST THESE ELSEWHERE OR DISTRIBUTE THESE ELSEWHERE OR VIA ANY MEANS OTHER THAN A LINK TO THIS PAGE.
I have just published a major update to my guide to Cine-EI on the PMW-F55 and F5. The guide now goes in to a lot more depth. I have tried to make it easy to understand but it is also quite technical, I have deliberately included the technical background stuff so that hopefully you will understand why Cine-EI and LUT’s work the way they do. I’ve added a whole new section on exposure methods for some of the different LUT’s as well as how to create your own LUT’s.
Please take a look if you use these cameras. Soon I will add a section on post production.