Many people wish to bake in the cameras Exposure Index settings when shooting using CineEI in order to avoid having to make an exposure correction in post production (given that the cameras are ISO invariant when shooting Log in reality it makes vey little difference whether you add gain in the camera or in post production – gain is gain). On cameras such as the FS7, FX6 or FX9 one way to do this is by baking in the built in S-Log3 LUT. To avoid confusion – that is using the CineEI mode with the “S-Log3” LUT enabled and in the LUT settings “Internal recording” set to ON so that you are recording the “S-Log3” LUT.
While this will bake in the EI change, this technique comes with many issues. For a start, just as when you use S-Log3 in custom mode and alter the ISO, whenever you move away from the cameras base ISO you loose dynamic range. When you bake in a LUT and change the EI, you are in effect changing the ISO and there will be a corresponding loss of dynamic range. When you bake in a LUT this loss of dynamic range is exacerbated by a reduced or altered recording range.
At lower EI’s the available recording range shrinks as the LUT is made darker and at the same time upper recoding level of the LUT is reduced. At 200 EI the recording range only gets to around 78%. At the bottom end the shadows are crushed and shadow information lost by the range reduction. This then causes a post production issue because LUT’s designed for the normal S-Log3 input range of 0-94% will now be applied to recordings with a much reduced range and after application of a LUT in post the final output won’t get to 100% without further complex grading where the image will need to be stretched more than normal and this degrades quality.
At high EI’s the LUT becomes brighter but the clip point remains the same. So for each stop you go up, 1 stop of highlights just disappears beyond the LUT’s hard clip point and can’t be ever recorded. Again in post this can cause issues because when you apply a normal S-Log3 LUT the heavy clipping in the recording causes the highlights to look very heavily clipped (because they are). Again, for the best results you will need to grade your footage to allow for this.
So, in practice the idea of baking in the S-Log3 LUT to eliminate the need to do any post production corrections doesn’t work because the addition of the S-Log3 LUT introduces new limitations that will need to be corrected if you want good looking images. Plus adding the S-Log3 LUT in camera and then adding another LUT on top in post is never going to deliver the best results due to the way LUT’s divide the image into brightness zones.
And – if you are baking in the S-Log3 LUT, then really this is no longer EI as there is now no longer an offset between the exposure and the recording, you are simply recording at a higher/lower ISO.
Tag Archives: EI
ISO and EI – using the right terms makes what you are doing easier to understand.
ISO and EI are different things and have different meanings. I find that it really helps understand what you are doing if you use the terms correctly and understand exactly what each really means.
ISO is the measured sensitivity of film stock. There is no actual direct equivalent for electronic cameras as the camera manufacturer is free to determine what they believe is an acceptable noise level. So one camera with an ISO of 1000 may be a lot more or less sensitive than another camera rated at 1000 ISO, it all depends on how much noise the manufacturer things is acceptable for that particular camera.
Broadly speaking on an electronic camera ISO is the number you would enter in to a light meter to achieve the a normally exposed image. It is the nearest equivalent to a sensitivity rating, it isn’t an actual sensitivity rating, but it’s what you need to enter into a light meter if you want to set the exposure that way.
EI is the Exposure Index. For film this is the manufacturers recommended best setting for your light meter to get the best results following the standard developing process for the chosen film stock. It is often different from the films true sensitivity rating. For example Kodak 500T is a 500 ISO film stock that has an EI of 350 when shooting under tungsten light. In almost all situations you would use the EI and not the ISO.
On an electronic camera EI normally refers to an exposure rating that you have chosen to give the camera to get the optimum results for the type of scene you are shooting. ISO may give the median/average/typical exposure for the camera but often rating the camera at a different ISO can give better results depending on your preferences for noise or highlight/shadow range etc. If you find exposing a bit brighter helps your images then you are rating the camera slower (treating it as though it’s less sensitive) and you would enter your new lower sensitivity rating into your light meter and this would be the EI.
Keeping EI and ISO as two different things (because they are) helps you to understand what your camera is doing. ISO is the base or manufacturer sensitivity rating and in most (but not all) log or raw cameras you cannot change this.
EI is the equivalent sensitivity number that you may choose to use to offset the exposure away from the manufacturers rating.
If you freely interchange ISO and EI it’s very confusing for people as they don’t know whether you are referring to the base sensitivity rating or a sensitivity rating that is not the base sensitivity but actually some kind of offset.
If you have a camera with an ISO rating of 2000 and you say “I’m shooting at 800 EI” then it’s clear that you are using a 1.3 stop exposure offset. But if you just say “I’m shooting at 800 ISO” it is less clear as to exactly what you are doing. Have you somehow changed the cameras base sensitivity or are you using an offset? While the numbers used by EI and ISO are the same, the meaning of the terms ISO and EI are importantly different.
Firmware 4.30/1.20 for the PXW-FS7 and PXW-FS7M2 Released. New ISO change in Cine-EI added.
Sony have just released a firmware update for the PXW-FS7 and PXW-FS7 II cameras. This is a minor update with only one new feature being added which is the ability to alter the recorded ISO when shooting in the Cine-EI mode.
FS7: https://pro.sony/ue_US/support/software/pxw-fs7-software-v4-30
FS7 II: https://pro.sony/ue_US/support/software/pxw-fs7m2-v120
When the camera is set to CineEI, this new function is turned on and off in the menu under System – Base Settings – Rec/Out EI Applied.
What does it do?
When shooting normally using Cine EI, assuming that no LUT is applied to SDI1/Rec the camera always records at it’s base sensitivity (2000 ISO exposure rating) with no added gain. This is done to ensure that the cameras full dynamic range is always available and that the full recording range of either S-Log2 or S-Log3 is always available.
Then the EI system is used to apply a LUT just to the viewfinder or SDI 2 for monitoring. The gain of the LUT can then be changed to provide a brighter or darker viewfinder/monitor image. For example setting the EI to 1000 EI would make the viewfinder image darker than the base setting of 2000EI by 1 stop.
Because you are viewing this darker image you would then open the cameras aperture by 1 stop to compensate. Opening the aperture up results in a brighter recording. A brighter recording, achieved by putting more light onto the sensor will have less noise than a darker exposure, so the end result is brighter recorded images with less noise.
This process is often referred to as “rating” the camera and it is in many cases preferable to “rate” the FS7 around a stop slower (Viewfinder is darker, less sensitive, so that means you end up opening up the aperture) than the base 2000 ISO rating to gain a cleaner image that typically gives much greater flexibility in post production. So many users will set the EI on an FS7 to 1000 or 800 (It’s no co-incidence that I find get the sweet spot to be 800EI which happens to match the rating that Sony give the FS7 when shooting Rec-709).
Because in the normal EI mode there is no change to the cameras actual recording gain (the recordings take place at the equivalent of 2000 ISO) there is no change to the dynamic range. The camera will always capture 14 stops no matter what you set the EI to. However if you open the aperture by an extra stop (selecting 1000 EI, which results in a 1 stop darker viewfinder image, so to compensate you open up 1 stop) you move the mid point of the exposure up 1 stop. This means you will reduce the over exposure headroom by 1 stop but at the same time you gain one stop of under exposure range. You will see 1 stop further into the shadows, plus there will be less noise, so the shadow range becomes much more useable.
Conventional EI mode and Post Production.
In post production these brightly exposed images will need some degree of adjustment. If you are doing a virgin grade from scratch then you don’t really need to do anything extra or different, you will just grade it to taste.
If you are using a LUT you will need to either use an exposure compensated LUT (I always provide these in any of my free LUT sets) or you will need to correct the exposure before applying the LUT. If you apply a standard LUT and then try to correct the exposure the results will often not be satisfactory as the LUT determines many things such as where any highlight roll-off occurs. Correcting after this can result in washed out of flat looking skin tones. So really you need to make the exposure correction to the material before it is passed to the LUT.
No loss of dynamic range with conventional EI.
It’s worth noting that even though the levels are reduced to “normal” levels when applying an exposure compensated LUT or through grading this should not reduce the dynamic range. You do not just shift the range down (which would hard clip the blacks and cause a loss of DR at the low end). What you are typically doing is reducing the gain to bring the levels down and this allows the information in the new extended shadow range to be retained, so nothing is lost and your footage will still have 14 stops of DR along with nice clean shadows and mid tones.
Is this all too difficult?
However, some people find that the need to correct the exposure prior to adding the LUT difficult or time consuming (I don’t know why, they just do. It takes no longer to add a compensated LUT than a normal LUT). Or some people find it difficult to get a good looking image from brightly exposed footage (probably because they are grading after the LUT has been applied). For these reasons Sony have added the ability to bake the EI change directly into the recording by shifting the gain of the recordings to match the selected EI.
Rec/Out EI Applied:
So now if you enable Rec/Out EI Applied any change you make to the cameras EI settings will now also be applied as a gain change to the recordings. If you set the EI to 1000, then the recordings will take place at 1000 ISO and not 2000 ISO. This means that you do not have to make any exposure corrections in post production, just apply a standard LUT.
You will loose some of your dynamic range:
The down side to this is that you are now changing the gain of the camera. Changing the gain away from 0dB will reduce the dynamic range and affect the recording range. So, for example if you wish to shoot at 1000 EI have Rec/Out EI Applied you will be recording with -6dB gain and an effective ISO of 1000. You will have 1 stop less of shadow range as the cameras effective sensitivity is being reduced by 1 stop but the sensors clip/overload point remains the same. So when you open the aperture to compensate for the lower sensitivity you will have the same shadow range as base, but loose one stop off the top. The images will have less noise, but there will be no additional shadow information and a reduction in highlight range by 1 stop, the DR will be 13 stops.
Another side effect of this is that the peak recording level is also reduced. This is because the cameras clipping point is determined by the sensor. This sensor clip point is normally mapped to the peak recording level and the cameras noise floor is mapped to the black level (you can’t see things that are darker than the sensors noise floor no matter what level of gain you use as the noise will always be higher than the object brightness).
If you reduce the gain of the signal this level must decrease as a result. This means that S-Log3 which normally gets to around ~94% will now only reach ~85%, the change to S-Log2 is even greater (S-Log3’s peak recording level will reduce by 8.9% for every stop down you go, S-Log2 will reduce by 12% for each stop you go down ).
Raising the EI/ISO will also reduce the dynamic range as the gain is applied after the sensor. So the sensors clip point remains the same, so the brightest highlight it handles remains fixed. Adding gain after this simply means the recordings will clip earlier, but you will get a brighter mid range, brighter (but not more) shadows and a noisier picture.
What happens in post with Rec/Out EI Applied?
In post production these range and peak level changes mean that while a standard LUT will result in a correct looking mid range (because middle grey and skin tones will be at “normal” levels) there may be some problems with highlights never reaching 100% in the case of a low EI/ISO. Or being excessively clipped in the case of a high EI/ISO. Remember LUT’s are designed to work over very specific ranges. So if the input to the LUT doesn’t reach the peak level the LUT is expecting then the output from the LUT will also be reduced. So often there will still be the need to do some additional grading of highlights prior to the application of the LUT, or the need to use LUT’s designed specifically for each ISO rating (and the design of these LUT’s is more complex than a simple exposure offset).
Is it really easier?
So while this new feature will simplify the workflow for some situations where an alternate ISO/EI has been used – because exposure correction in post production won’t be needed. It may actually make things more difficult if you have bright highlights or need to be sure that your finished video meets expected standards where highlights are at 100%. You will still need to do some grading.
I don’t recommend that you use it.
Personally I do not recommend that you use this new feature. There are plenty of exposure compensated LUT’s available online (I have lots here). Tweaking the exposure of log footage in post production isn’t that difficult, especially if you use a color managed workflow. My guess is that this is aimed at FCP-X users where FCP-X applies a default LUT as standard. In this instance footage shoot with an offset exposure will look over/under exposed while footage shoot with the EI/ISO Applied will look normal (except for the highlights). So on the face of things the workflow may appear simpler. But you are loosing dynamic range and surely the primary reason for shooting with log is to maximise the dynamic range and gain the greatest possible post production flexibility. This new feature reduces dynamic range and as a result reduces post production flexibility.
Of course just because I don’t recommend it’s use, it doesn’t mean that you can’t or shouldn’t use it if it works for you, just make sure you fully understand what it is doing.
Beware the LC709 LUT double exposure offset.
The use o f the LC709 Type A LUT in Sony’s Cinealta cameras such as the PXW-FS7 or PMW-F55 is very common. This LUT is popular because it was designed to mimic the Arri cameras when in their Rec-709 mode. But before rushing out to use this LUT and any of the other LC709 series of LUT’s there are some things to consider.
The Arri cameras are rarely used in Rec-709 mode for anything other than quick turn around TV. You certainly wouldn’t normally record this for any feature or drama productions. It isn’t the “Arri Look” The Arri look normally comes as a result of shooting using Arri’s LogC and then grading that to get the look you want. The reason it exists is to provide a viewable image on set. It has more contrast than LogC and uses Rec 709 color primaries so the colors look right, but it isn’t Rec-709. It squeezes almost all of the cameras capture range into a something that can be viewed on a 709 monitor so it looks quite flat.
Because a very large dynamic range is being squeezed into a range suitable to be viewed on a regular, standard dynamic range monitor the white level is much reduced compared to regular Rec-709. In fact, white (such as a white piece of paper) should be exposed at around 70%. Skin tones should be exposed at around 55-60%.
If you are shooting S-Log on a Sony camera and using this LUT to monitor, if you were to expose using conventional levels, white at 85-90% skin tones at 65-70%, then you will be offsetting your exposure by around +1.5 stops. On it’s own this isn’t typically going to be a problem. In fact I often come across people that tell me that they always shoot at the cameras native EI using this LUT and get great, low noise pictures. When I dig a little deeper I often find that they are exposing white at 85% via the LC709 LUT. So in reality they are actually shooting with an exposure the equivalent of +1 to +1.5 stops over the base level.
Where you can really run into problems is when you have already added an exposure offset. Perhaps you are shooting on an FS7 where the native ISO is 2000 ISO and using an EI of 800. This is a little over a +1 stop exposure offset. Then if you use one of the LC709 LUT’s and expose the LUT so white is at 90% and skin tones at 70% you are adding another +1.5 stops to the exposure, so your total exposure offset is approaching 3 stops. This large an offset is rarely necessary and can be tricky to deal with in post. It’s also going to impact your highlight range.
So just be aware that different LUT’s require different white and grey levels and make sure you are exposing the LUT at it’s correct level so that you are not adding an additional offset to your desired exposure.
Using LUT’s for exposure – choosing the right LUT.
If using a LUT to judge the exposure of a camera shooting log or raw it’s really important that you fully understand how that LUT works.
When a LUT is created it will expect a specific input range and convert that input range to a very specific output range. If you change the input range then the output will range will be different and it may not be correct. As an example a LUT designed and created for use with S-Log2 should not be used with S-Log3 material as the the higher middle grey level used by S-Log3 would mean that the mid range of the LUT’s output would be much brighter than it should be.
Another consideration comes when you start offsetting your exposure levels, perhaps to achieve a brighter log exposure so that after grading the footage will have less noise.
Lets look at a version of Sony’s 709(800) LUT designed to be used with S-Log3 for a moment. This LUT expects middle grey to come in at 41% and it will output middle grey at 43%. It will expect a white card to be at 61% and it will output that same shade of white at a little over 85%. Anything on the S-Log3 side brighter than 61% (white) is considered a highlight and the LUT will compress the highlight range (almost 4 stops) into the output range between 85% and 109% resulting in flat looking highlights. This is all perfectly fine if you expose at the levels suggested by Sony. But what happens if you do expose brighter and try to use the same LUT either in camera or in post production?
Well if you expose 1.5 stops brighter on the log side middle grey becomes around 54% and white becomes around 74%. Skin tones which sit half way between middle grey and white will be around 64% on the LUT’s input. That’s going to cause a problem! The LUT considers anything brighter than 61% on it’s input to be a highlight and it will compresses anything brighter than 61%. As a result on the output of your LUT your skin tones will not only be bright, but they will be compressed and flat looking. This makes them hard to grade. This is why if you are shooting a bit brighter it is much, much easier to grade your footage if your LUT’s have offsets to allow for this over exposure.
If the camera has an EI mode (like the FS7, F5, F55 etc) the EI mode offsets the LUT’s input so you don’t see this problem in camera but there are other problems you can encounter if you are not careful like unintentional over exposure when using the Sony LC709 series of LUTs.
Sony’s 709(800) LUT closely matches the gamma of most normal monitors and viewfinders, so 709(800) will deliver the correct contrast ie. contrast that matches the scene you are shooting plus it will give conventional TV brightness levels when viewed on standard monitors or viewfinders.
If you use any of the LC709 LUT’s you will have a miss-match between the LUT’s gamma and the monitors gamma so the images will show lower contrast and the levels will be lower than conventional TV levels when exposed correctly. LC709 stands for low contrast gamma with 709 color primaries, it is not 709 gamma!
Sony’s LC709 Type A LUT is very popular as it mimics the way an Arri Alexa might look. That’s fine but you also need to be aware that the correct exposure levels for this non-standard LC gamma are middle grey at around 41% and white at 70%.
An easy trap to fall into is to set the camera to a low EI to gain a brighter log exposure and then to use one of the LC709 LUT’s and try to eyeball the exposure. Because the LC709 LUT’s are darker and flatter it’s harder to eyeball the exposure and often people will expose them as you would regular 709. This then results in a double over exposure. Bright because of the intentional use of the lower EI but even brighter because the LUT has been exposed at or close to conventional 709 brightness. If you were to mistakenly expose the LC709TypeA LUT with skin tones at 70%, white at 90% etc then that will add almost 2 stops to the log exposure on top of any EI offset.
Above middle grey with 709(800) a 1 stop exposure change results in an a 20% change in brightness, with LC709TypeA the same exposure change only gives a just over 10% change, as a result over or under exposure is much less obvious and harder to measure or judge by eye with LC709. The cameras default zebra settings for example have a 10% window. So with LC709 you could easily be a whole stop out, while with 709(800) only half a stop.
Personally when shooting I don’t really care too much about how the image looks in terms of brightness and contrast. I’m more interested in using the built in LUT’s to ensure my exposure is where I want it to be. So for exposure assessment I prefer to use the LUT that is going to show the biggest change when my exposure is not where it should be. For the “look” I will feed a separate monitor and apply any stylised looks there. To understand how my highlights and shadows, above and below the LUT’s range are being captured I use the Hi/Low Key function.
If you are someone that creates your own LUT’s an important consideration is to ensure that if you are shooting test shots, then grading these test shots to produce a LUT it’s really, really important that the test shots are very accurately exposed.
You have 2 choices here. You can either expose at the levels recommended by Sony and then use EI to add any offsets or you can offset the exposure in camera and not use EI but instead rely on the offset that will end up in the LUT. What is never a good idea is to add an EI offset to a LUT that was also offset.
ISO Confusion Once Again!
I’m going to keep bringing this up until people start to take note and understand that with an electronic camera ISO is NOT sensitivity.
With an electronic camera ISO is a guide to the required shutter speed and aperture needed to get the correct exposure. This is different to sensitivity. The ISO rating of a video camera and it’s sensitivity are closely related, but they are not quite the same thing. Because different gamma curves require different exposures the ISO rating for each gamma curve will be different even though the gain and actual sensitivity of the camera may be exactly the same.
Lets take the Sony PXW-FS5 as an example.
If you shoot using the standard camera settings you should expose white at 90%, middle grey will be around 42% and skin tones typically around 70%. At 0dB gain the camera the camera will display an ISO equivalent rating of 1000 ISO. So let’s say you are using a light meter. You set it to 1000 ISO and it tells you you need an aperture of f5.6 to get the right exposure.
Now you change to S-Log2. If you do nothing else your white card will now be at around 75% and middle grey will be around 40%. At 0dB gain the camera will show an equivalent ISO of 3200 ISO.
But hang on – The camera is still at 0dB gain, so there is no change in sensitivity. .But the camera is over exposed, S-Log2 is supposed to be exposed with white at 59% and middle grey at 32%.
So we go to our light meter and change the ISO on the light meter from 1000 ISO to 3200 ISO. Because the light meter now “thinks” the camera is more sensitive by almost 2 stops it will tell us to close the aperture by nearly 2 stops. So we go to the camera and stop down to f10 and bingo, the image is exposed correctly.
But here’s the important thing – The camera hasn’t become any more sensitive. We haven’t replaced the sensor with a different, more sensitive one (as you would do with a film camera where you actually change the film stock). We are still at 0dB gain (even though the camera tells us this is the equivalent to a higher ISO).
The only reason that ISO number changes is so that if we were using an external light meter we would get the recommended exposure levels for the gamma curve we are using. In this example closing the aperture increase the highlight range that the camera would be able to cope with and this helps us get that full 14 stop range from the camera, although closing the aperture means less light on the sensor so the pictures end up a little noisier as a result – That is unless you choose to rate the camera at a different ISO by over exposing the log a bit.
ISO is useful, but you need to understand that it isn’t really sensitivity. After all we can’t change the sensors on our video cameras and that would be the only way to truly change the sensitivity. Any “sensitivity” change is really nothing more than a gain or amplification change. Useful but not the same as changing the actual sensitivity. Gain will make a dark picture brighter but it won’t allow you to see something that the sensor can’t detect.
It is much easier to understand dB gain with an electronic camera as it actually tells you exactly what the camera is doing and it is actually my recommendation that people use gain rather than ISO for all of the above reasons. The use of ISO on electronic cameras is very badly understood, in part because it’s a largely meaningless term because it doesn’t tell us how sensitive the sensor is, how much gain we are using or how much noise we are adding. Give any experienced camera operator a camera and ask them how noisy will it be a 18dB gain and they will have a pretty good idea of what the pictures will look like. Give them the same camera and ask them how noisy will it be at 8000 ISO and they won’t have a clue.
The problem is ISO is trendy and fashionable as that’s what “cinematographers” use. But lets be honest with ourselves – we are using electronic video cameras, whether that’s a Red, Alexa or FS5 so really we should be using the correct terminology for an electronic camera which is gain. It would eliminate an aweful lot of confusion and tell us how much noise and grain our pictures will have. It’s noise and grain will levels will determine how good a clip looks and how much we can grade it, so we need to clearly understand how much gain is being added in camera and dB gian tells us this. ISO does not.
Side Note: Modern film stocks will often have 2 ratings, the ISO or actual measured sensitivity of the film stock plus the EI or Exposure Index which is the recommended setting for the light meter to get the best exposure. In some respects the ISO rating of a video camera is closer to the EI rating of a film stock. Perhaps we should stop calling it ISO and use the term EI instead, this would be me appropriate and signify that it is a reference for best exposure rather than true sensitivity.
UPDATE: A comment on facebook was why not display both ISO and Gain side by side. This is an obvious solution really. Why do camera manufacturers force us to choose either ISO or gain? Why can’t we use a hybrid of the 2? I see no technical reason why cameras can’t show both the gain and ISO at the same time – Problem solved.
Ultimate Guide for Cine EI on the Sony PXW-FS7
Ultimate Guide to CineEI on the PXW-FS7 (Updated May 2016).
INTRODUCTION:
This guide to Cine-EI is based on my own experience with the Sony PXW-FS7. There are other methods of using LUT’s and CineEI. The method I describe below, to the best of my knowledge, follows standard industry practice for working with a camera that uses EI gain and LUT’s.
If you find the guide useful, please consider buying me a beer or a coffee. It took quite a while to prepare this guide and writing can be thirsty work.
S-Log, Latitude, Dynamic Range and EI S-log. Or how to modify your exposure range with EI S-Log
The big issue most people have when working with log and exposing mid grey at 38 is that when you look at it on a standard monitor without any lookup tables it looks underexposed. The assumption therefore is that it is underexposed or in some way too dark to ever look right, because that’s what people used to working with conventional gammas have become programmed to believe over many years from their experience with conventional gammas.
So, for confidence you add a lookup table which converts the log to a Rec-709 type gamma and now the image looks brighter, but as it now has to fit within Rec-709 space we have lost either some of our high end or low end so we are no longer seeing the full range of the captured image so highlights may be blown out or blacks may be crushed.
It’s important for people to understand the concept of gamma and colour space and how the only way to truly see what a camera (any camera) is capturing is to use a monitor that has the same gamma and colour space. Generally speaking lookup tables don’t help as they will be taking a signal with a large range and manipulating it to fit in a small range and when you do that, something has to be discarded. If you were to take an F3 set to S-log and expose mid grey at 38 and show that on one of the nice new Sony E170 series monitors that have S-log gamma and place that next to another F3 with Rec-709 shooting mig grey at 45% and a similar but conventional 709 monitor the lower and mid range exposures would be near identical and the S-log images would not look under exposed or flat. The S-log images however would show an extra 2 stops of dynamic range.
Furthermore it has to be remembered that log is log, it is not linear. Because of its non linear nature, less and less brightness information is getting recorded as you go up the brightness range. As our own visual system is tuned to be most accute in the mid ranges this is normally fine provide you expose correctly putting mid tones in the more linear, lower parts of the S-log curve. Start putting faces to high up the S-log curve and it gets progressively harder to get a natural look after grading. This is where I think a lot of people new to log stumble. They don’t have the confidence to expose faces at what looks like a couple of stops under where they would with a standard gamma, so they start bringing up the exposure closer to where they would with standard gamma and then have a really hard time getting faces to look natural in the grade. Remember that the nominal S-Log value for white is 68 IRE. Part of the reason for this is that above about 70 IRE the amount of compression being applied by log is getting pretty extreme. While there is some wriggle room to push your exposure above or below the nominal mid grey at 38 it’s not as big as you might expect, especially dealing with natural tones and overexposure.
If you do want to shift your middle grey point this is where the EI S-log function and a light meter comes into it’s own, it’s what it’s designed for.
First something to understand about conventional camera gain, dynamic range and latitude. The latitude and sensitivity of the F3 is governed by the latitude and sensitivity of the sensor, which is a little under 13 stops. Different amounts of gain or different ISO’s don’t alter the sensors latitude, nor do they alter the actual sensitivity, only the amount of signal amplification. Increasing the camera gain will reduce the cameras output dynamic range as something that is 100 IRE at 800 ISO would go into clipping if the actual camera gain was increased by 6db (taking the ISO to 1600) but the darkest object the camera can actually detect remains the same. Dark objects may appear brighter, but there is still a finite limit to how dark an object the camera can actually see and this is governed by the sensor and the sensors noise floor.
EI (Exposure Index) shooting works differently, whether it’s with the F3, F65, Red or Alexa. Let’s consider how it works with the PMW-F3. In EI S-Log mode the camera always actually outputs at 800 ISO from the A/B outputs. It is assumed that if your working with S-Log you will be recording using an external 10 bit recorder connected to the A/B outputs. 422 is OK, but you really, really need 10 bit for EI S-Log. At 800 ISO you have 6.5 stops of over exposure and 6.5 under when you shoot mid grey at 38 or expose conventionally with a light meter.
Now what happens when you set the camera to EI 1600? Understand that the camera will still output at 800 ISO over the A/B outputs to your external recorder, but also note that 6db gain (1 stop) is added to the monitor output and what you see on the LCD screen, so the monitor out and LCD image get brighter. As the cameras metering systems (zebras, spot meter, histogram) measure the signal on the monitor side these are also now offset by +6db or + 1 stop.
As the camera is set to EI 1600 we set our light meter to 1600 ISO. If we make no change to our lighting the light meter would tell us to stop down by one stop, compared to our original 800 ISO exposure.
Alternately, looking at the camera, when you switch on EI 1600 the picture gets brighter, your mid grey card would also become brighter by one stop, so If we use the cameras spot meter to expose our grey card at 38 again we would need to stop down the iris by one stop to return the grey card to 38 IRE (for the same light levels as we used for 800). So either way, whether exposing with a light meter or exposing using the cameras built in metering, when you go from EI 800 to EI 1600 for the correct exposure (under the same lighting) you would stop down the iris by one stop.
Hope those new to this are still with me at this point!
Because the cameras A/B output is still operating at 800 ISO and you have stopped down by one stop as that what the light meter or camera metering told you to do because they are operating at EI 1600, the A/B output gets darker by one stop. Because you have shifted the actual recorded output down by one stop you have altered you exposure range from the original +/- 6.5 stops to + 7.5 stops, -5.5 stops. So you can see that when working at EI 1600 the dynamic range now becomes + 7.5 stops and -5.5 stops. Go to EI 3200 and the dynamic range becomes +8.5 stops and -4.5 stops.
So EI S-log gives you a great way of shifting your dynamic range centre while giving you consistent looking exposure and a reasonable approximation of how your noise levels are changing as you shift your exposure up and down within the cameras dynamic range.
EI S-Log doesn’t go below 800 because shifting the dynamic range up the exposure range is less beneficial. Lets pretend you have an EI 400 setting. If you did use it, you would be opening up the iris by one stop, so your range becomes +5.5 and -7.5 stops compared to your mid grey or light metered exposure. So you are working with reduced headroom and you are pushing your mid range up into the more highly compressed part of the curve which is less desirable. I believe this is why the option is not given on the F3.
Camera Gain: It doesn’t make the camera more sensitive! (also relevant EI S-Log).
This is something that’s not well understood by many people. It helps explain why the PMW-F3 (and other cameras) EI S-Log function is so useful.
You see, camera gain does not normally actually change the cameras ability to capture photons of light. A CCD or CMOS sensor has a number of photo sites that capture photons of light and convert those photons into electrons or electrical charge. The efficiency of that capture and conversion process is fixed, it’s known as the QE or quantum efficiency. There are a lot of factors that effect this efficiency, such as the use of micro lenses, whether the sensor is back or front illuminated etc. But all of these factors are physical design factors that do not change when you add extra camera gain. The sensitivity of the sensor itself remains constant, no matter what the camera gain is set to.
Camera gain is applied to the signal coming out of the sensor. It’s a bit like turning up the volume on a stereo amplifier. If you have a quite piece of music, turning up the volume makes it louder, but the original piece of music is still a quiet piece of music. Turning up the volume on your stereo, as well as making the music louder will also make any hiss or background noise in the music louder and it’s exactly the same with a video camera. As you increase the gain, as well as the wanted video signal getting bigger (brighter) all the unwanted noise also get bigger. So adding gain on your video camera doesn’t actually make the camera more sensitive, but it does make what light the camera has captured brighter in the recordings and output, giving the impression that the camera has become more sensitive, however this is at the penalty of increased background noise.
As well as adding gain to the image in the camera, we can also add gain in post production. Traditionally gain has been added in camera because the gain is added before the recording process. In the uncompressed analog days the recording process itself added a lot of noise. In the digital age the process of compressing the image adds noise. 8 bit recordings have quite small number of grey shades. So any gain added in post production amplifies not only the camera signal but also the added recording or compression noise so generally gives an inferior result to adding gain in camera. With an 8 bit signal the stretching of the relatively few grey shades results in banding.
Now, however the use of lower noise sensors and much improved 10 bit or higher recording codecs or even uncompressed recording means that adding gain in post as opposed to in camera is not such a bad thing. In some cases you can use post production noise reduction prior to adding post gain and by leveraging the processing and rendering power of a computer, which will normally be of greater quality than the in camera processing, you can get a cleaner, lower noise output than you would using in camera gain. So before you flick on the gain switch of your camera, if your using only very light 10 bit or higher compression (HDCAM SR, Cineform, ProRes HQ) or uncompressed do consider that you may actually be better waiting until you get into post before you add gain.
Some modern cameras, like Red or the Sony F3 can use something called EI gain. EI gain does not actually add any gain to the recorded signal (or signal output in the case of the F3). Instead it adds gain to the monitor output only and adds metadata to the recording to tell the post facility or conversion software to add gain. This way you see on the monitor what the image should look like when the gain has been added, but the recording itself has no gain added giving the post production team the ability to fine tune exactly how much gain is applied.
You may have seen that Sony are releasing a free firmware update in the next week or so for the PMW-F3 that incorporates a new version of S-Log. You will still need to have purchased the S-Log upgrade in order to use S-Log, but now there are two variations of S-Log, normal S-Log and EI S-Log.
EI S-log differs from the original S-Log in that you can select either S-Log or EI S-Log mode in the menu. When EI S-Log is selected you have the ability to then add EI gain to the MLUT’s (Monitor Look Up Tables or LUT’s). When you switch the EI ISO to 1200 for example, the additional gain is added to the LUT to give the equivalent S-Log + 3db gain output on the monitor out and to the SxS card recordings as using S-Log with gain. But the actual S-Log output on the A-B dual link outputs remains fixed at 800 ISO. The benefit of this is that what you see on the monitor out represents what you will end up with after post production with added gain or lift, it’s a way of pre-visualising what you will finish up with, without compromising your recordings.
Theoretically, if the cameras native 0db gain point is represented by 800 ISO. Which is what you have when using 50/60i standard gamma at 0db. Reducing the cameras S-Log gain like this by 6db compared to the previous or standard S-log base ISO of 1600. Should yield a 6db (1 stop) dynamic range improvement. Given that S-Log already improves the dynamic range by about 1.5 stops, then on paper at least, EI S-Log should yield a 2.5 stop improvement over the 11 stops the standard and cinegammas give. That would make the F3 a camera capable of 13.5 stops which is quite remarkable. I hope to be able to measure the actual DR very soon and see if this is really the case. Anyway, whatever the outcome of the DR measurements, the EI option is a nice one to have as it will allow you to underexpose a little (to gain extra headroom) when you shoot and then use the added EI LUT gain to check that even after gain gets added in post the images will still be noise free enough for use in your production.
A further feature of the update is the ability to change the R and B gain when shooting in S-Log. This will allow you to tweak your white balance. Currently when shooting s-Log you can only use a preset white balance dialled in in 100k steps, there is no option to white balance the camera using a grey/white card.