While I had the light meter and exposure test chart out for the FX6 I decided to do the same exposure level confirmation test for the FX9. No nasty surprises, the FX9’s ISO ratings certainly appear to be correct. Again using a DSC Labs exposure reference chart with 18% middle grey and 90% white plus my trusty Sekonic I tested the FX9 at both 800 ISO and 4000 ISO and my light meter and the camera were in good agreement. At 800 ISO the light meter was saying f4.01 while the camera was at f4, I suspect this tiny difference is probably down to transmission losses in the lens.
I have already done this a few times, but having seen some other tests suggesting the FX6’s ISO ratings were incorrect. So I decided to re-confirm my previous findings, which is that the ratings Sony give their cameras is correct. For the test I used a DSC labs exposure calibration chart which is an extremely accurate 18%/90% reflectivity chart and my trusty Sekonic light meter. As you can see at both 800 ISO and 12,800 ISO the light meters indicated exposure settings perfectly match the camera’s ISO ratings, shutter speed and aperture. For the 12,800 ISO test, as my light meter doesn’t go up to 12,800 ISO I set the light meter to 6400 ISO which is one stop lower than the cameras 12,800. The light meter indicated f11 which is one stop below the f16 required by the camera – confirming that the ISO rating is correct.
First of all. Unless you are actually using a lightmeter to determine your exposure, in custom mode it is far, far easier to use dB of gain. 0dB is always optimum and each time you go up 6dB the picture gets twice as bright (one stop brighter) and the noise doubles. ISO is in most cases nothing more than a rating to use in conjunction with a lightmeter to get the right picture brightness, it will not tell you how much noise you have or whether the camera is at it’s optimum setting. So don’t use ISO just because “ISO is cool and make me sound like I know what I’m doing, it makes me a cinematographer”. This isn’t a film camera, no matter how much you dress it up it is a video camera and dB tells you exactly what it is doing.
Because different gamma curves produce different brightness images the ISO rating will change depending on the gamma curve being used, this isn’t a sensitivity change, it’s an optimum brightness change. Because of this, even when you are at 0dB gain (the native setting) when you switch between different gammas the ISO rating changes. In addition because you have two different base sensitivity modes on the FX9 there are a lot of different base ISO’s (all of which are 0dB gain). I’ve prepared a table of the different base ISO’s.
In addition if you are not careful it’s possible to end up using too much gain to achieve a certain ISO as many ISO ratings can be realised at both Hi and Low Base sensitivity. You don’t want to be at 2500 ISO in Low Base for example, you would be better off using High base. The table below should help you understand when to switch up to High base from Low base. If you use dB gain, then it’s easy. More than +11dB – switch up. Don’t forget in dB mode you can also go down to -3dB.
Almost all modern day video and electronic stills cameras have the ability to change the brightness of the images they record. The most common way to achieve this is through the addition of gain or through the amplification of the signal that comes from the sensor.
On older video cameras this amplification was expressed as dB (decibels) of gain. A brightness change of 6dB is the same as one stop of exposure or a doubling of the ISO rating. But you must understand that adding gain to raise the ISO rating of a camera is very different to actually changing the sensitivity of a camera.
The problem with increasing the amplification or adding gain to the sensor output is that when you raise the gain you increase the level of the entire signal that comes from the sensor. So, as well as increasing the levels of the desirable parts of the image, making it brighter, the extra gain also increases the amplitude of the noise, making that brighter too.
Imagine you are listening to an FM radio. The signal starts to get a bit scratchy, so in order to hear the music better you turn up the volume (increasing the gain). The music will get louder, but so too will the scratchy noise, so you may still struggle to hear the music. Changing the ISO rating of an electronic camera by adding gain is little different. When you raise the gain the picture does get brighter but the increase in noise means that the darkest things that can be seen by the camera remain hidden in the noise which has also increased in amplitude.
Another issue with adding gain to make the image brighter is that you will also normally reduce the dynamic range that you can record.
This is because amplification makes the entire signal bigger. So bright highlights that may be recordable within the recording range of the camera at 0dB or the native ISO may be exceed the upper range of the recording format when even only a small amount of gain is added, limiting the high end.
At the same time the increased noise floor masks any additional shadow information so there is little if any increase in the shadow range.
Reducing the gain doesn’t really help either as now the brightest parts of the image from the sensor are not amplified sufficiently to reach the cameras full output. Very often the recordings from a camera with -3dB or -6dB of gain will never reach 100%.
A camera with dual base ISO’s works differently.
Instead of adding gain to increase the sensitivity of the camera a camera with a dual base ISO sensor will operate the sensor in two different sensitivity modes. This will allow you to shoot at the low sensitivity mode when you have plenty of light, avoiding the need to add lots of ND filters when you want to obtain a shallow depth of field. Then when you are short of light you can switch the camera to it’s high sensitivity mode.
When done correctly, a dual ISO camera will have the same dynamic range and colour performance in both the high and low ISO modes and only a very small difference in noise between the two.
How dual sensitivity with no loss of dynamic range is achieved is often kept very secret by the camera and sensor manufacturers. Getting good, reliable and solid information is hard. Various patents describe different methods. Based on my own research this is a simplified description of how I believe Sony achieve two completely different sensitivity ranges on both the Venice and FX9 cameras.
The image below represents a single microscopic pixel from a CMOS video sensor. There will be millions of these on a modern sensor. Light from the camera lens passes first through a micro lens and colour filter at the top of the pixel structure. From there the light hits a part of the pixel called a photodiode. The photodiode converts the photons of light into electrons of electricity.
In order to measure the pixel output we have to store the electrons for the duration of the shutter period. The part of the pixel used to store the electrons is called the “image well” (in an electrical circuit diagram the image well would be represented as a capacitor and is often simply the capacitance of the the photodiode itself).
Then as more and more light hits the pixel, the photodiode produces more electrons. These pass into the image well and the signal increases. Once we reach the end of the shutter opening period the signal in the image well is read out, empty representing black and full representing very bright.
Consider what would happen if the image well, instead of being a single charge storage area was actually two charge storage areas and there is a way to select whether we use the combined image well storage areas or just one part of the image well.
When both areas are connected to the pixel the combined capacity is large. So it will take more electrons to fill it up, so more light is needed to produce the increased amount of electrons. This is the low sensitivity mode.
If part of the charge storage area is disconnected and all of the photodiodes output is directed into the remaining, now smaller storage area then it will fill up faster, producing a bigger signal more quickly. This is the high sensitivity mode.
What about noise?
In the low sensitivity mode with the bigger storage area any unwanted noise generated by the photodiode will be more diluted by the greater volume of electrons, so noise will be low. When the size of the storage area or image well is reduced the noise from the photodiode will be less diluted so the noise will be a little bit higher. But overall the noise will be much less that that which would be seen if a large amount of extra gain was added.
Note for the more technical amongst you: Strictly speaking the image well starts full. Electrons have a negative charge so as more electrons are added the signal in the image well is reduced until maximum brightness output is achieved when the image well is empty!!
As well as what I have illustrated above there may be other things going on such as changes to the amplifiers that boost the pixels output before it is passed to the converters that convert the pixel output from an analog signal to a digital one. But hopefully this will help explain why dual base ISO is very different to the conventional gain changes used to give electronic cameras a wide range of different ISO rating.
On the Sony Venice and the PXW-FX9 there is only a very small difference between the noise levels when you switch from the low base ISO to the high one. This means that you can pick and choose between either base sensitivity level depending on the type of scene you are shooting without having to worry about the image becoming unusable due to noise.
NOTE: This article is my own work and was prepared without any input from Sony. I believe that the dual ISO process illustrated above is at the core of how Sony achieve two different base sensitivities on the Venice and FX9 cameras. However I can not categorically guarantee this to be correct.
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.
Even though I have written about these many times before the message still just doesn’t seem to be getting through to people.
Since the dawn of photography and video the only way to really change the signal to noise ratio and ultimately how noisy the pictures are is by changing how much light you put onto the sensor.
Gain, gamma, log, raw, etc etc only have a minimal effect on the signal to noise ratio. Modern cameras do admittedly employ a lot of noise reduction processes to help combat high noise levels, but these come at a price. Typically they soften the image or introduce artefacts such as banding, smear or edge tearing. So you always want to start off with the best possible image from the sensor with the least possible noise and the only way to achieve that is through good exposure – putting the optimum amount of light onto the sensor.
ISO is so confusing:
But just to confuse things the use of ISO to rate an electronic cameras sensitivity has become normal. But the problem is that most people have no clue about what this really means. On an electronic camera ISO is NOT a sensitivity measurement, it is nothing more than a number that you can put into an external light meter to allow you to use that light meter to obtain settings for the shutter speed and aperture that will give you the camera manufacturers suggest optimum exposure. That’s it – and that is very different to sensitivity.
Lets take Sony’s FS7 as an example (most other cameras behave in a very similar way).
If you set the FS7 up at 0dB gain, rec-709, it will have an exposure rating of 800 ISO. Use a light meter to expose with the meters ISO dial set to 800. Lets say the light meter says set the aperture to f8. When you do this the image is correctly exposed, looks good (well as good as 709 gets at least) and for most people has a perfectly acceptable amount of noise.
Now switch the camera to S-Log2 or S-Log3. With the camera still set to 0dB the ISO rating changes to 2000 which give the impression that the camera may have become more sensitive. But did we change the sensor? No. Have we added any more gain? No, we have not, the camera is still at 0dB. But if you now expose at the recommended levels, after you have done your grading and you grade to levels similar to 709 the pictures will look quite a lot noisier than pictures shot using Rec-709.
So what’s going on?
If you now go back to the light meter to expose the very same scene, you turn the ISO dial on the light meter from 800 to 2000 ISO and the light meter will tell you to now set the aperture to f13 (approx). So starting at the f8 you had for 800 ISO, you close the aperture on the camera by 1.3 stops to f13 and you will have the “correct” exposure.
BUT: now you are putting 1.3 stops less light on to the sensor so the signal coming from the sensor is reduced by 9dB and as a result the sensor noise that is always there and never really changes is much more noticeable. As a result compared to 709 the graded S-Log looks noisy and it looks noisier by the equivalent of 9dB. This is not because you have changed the cameras sensitivity or changed because you have changed the amount of camera gain but because compared to when you shoot in 709 the sensor is being under exposed and as a result it is outputting a signal 9dB lower. So in post production when you grade or add a LUT you have to add 9dB of gain to get the same brightness as the original direct rec-709 recording and as well as making the desirable image brighter it also makes the noise 9dB higher (unless you do some very fancy noise reduction work in post).
So what do you do?
It’s common simply to open the aperture back up again, typically by 1.5 stops so that after post production grading the S-log looks no more noisy than the 709 from the FS7 – Because in reality the FS7’s sensor works best for most people when rated at the equivalent of 800 ISO rather than 2000 – probably because it’s real sensitivity is 800 ISO.
When you think about it, when you shoot with Rec-709 or some other gamma that won’t be graded it’s important that it looks good right out of the camera. So the camera manufacturer will ensure that the rec-709 noise and grain v sensitivity settings are optimum – so this is probably the optimum ISO rating for the camera in terms of noise, grain and sensitivity.
So don’t be fooled into thinking that the FS7 is more sensitive when shooting with log, because it isn’t. The only reason the ISO rating goes up as it does is so that if you were using a light meter it would make you put less light onto the sensor which then allows the sensor to handle a brighter highlight range. But of course if you put less light onto the sensor the sensor won’t be able to see so far into the shadows and the picture may be noisy which limits still further the use of any shadow information. So it’s a trade-off, more highlights but less shadows and more noise. But the sensitivity is actually the same. Its’s an exposure change not a sensitivity change.
So then we get into the S-Log2 or S-Log3 debate.
First of all lets just be absolutely clear that both have exactly the same highlight and shadow ranges. Both go to +6 stops and -8 stops, there is no difference in that regard. Period.
And lets also be very clear that both have exactly the same signal to noise ratios. S-log3 is NOT noisier than S-log2. S-log 3 records some of the mid range using higher code values than S-Log2 and before you grade it that can sometimes make it appear like it’s noisier, but the reality is, it is not noisier. Just like the differing ISO ratings for different gamma curves, this isn’t a sensitivity change, it’s just different code values being used. See this article if you want the hard proof: http://www.xdcam-user.com/2014/03/understanding-sonys-slog3-it-isnt-really-noisy/
Don’t forget when you shoot with log you will be grading the image. So you will be adjusting the brightness of the image. If you grade S-Log2 and S-Log3 to the same brightness levels the cumulative gain (the gain added in camera and the gain added in post) ends up the same. So it doesn’t matter which you use in low light the final image, assuming a like for like grade will have the same amount of noise.
For 8 bit records S-Log2 has different benefits.
S-Log2 was designed from the outset for recording 14 stops with an electronic video camera. So it makes use of the cameras full recording range. S-Log3 is based on an old film log curve (cineon) designed to transfer 16 stops or more to a digital intermediate. So when the camera only has a 14 stop sensor you waste a large part of the available recording range. On a 10 bit camera this doesn’t make much difference. But on a 8 bit camera where you are already very limited with the number of tonal values you can record it isn’t ideal and as a result S-Log2 is often a better choice.
But if I shoot raw it’s all going to be so much better – isn’t it?
Yes, no, maybe…. For a start there are lot’s of different types of raw. There is linear raw, log raw, 10 bit log raw, 12 bit linear, 16 bit linear and they are all quite different.
But they are all limited by what the sensor can see and how noisy the sensor is. So raw won’t give you less noise (it might give different looking noise). Raw won’t give you a bigger dynamic range so it won’t allow you to capture deeper or brighter highlights.
But what raw does normally is to give you more data and normally less compression than the cameras internal recordings. In the case of Sony’s FS5 the internal UHD recordings are 8 bit and highly compressed while the raw output is 12 bit, that’s a 4 fold increase in the amount of tonal values. You can record the 12bit raw using uncompressed cDNG or Apples new ProResRaw codec which doesn’t introduce any appreciable compression artefacts and as a result the footage is much more flexible in post production. Go up to the Sony Venice, F5 or F55 cameras and you have 16 bit raw and X-OCN (which behaves exactly like raw) which has an absolutely incredible range of tonal values and is a real pleasure to work with in post production. But even with the Venice camera the raw does not have more dynamic range than the log. However because there are far more tonal values in the raw and X-OCN you can do more with it and it will hold up much better to aggressive grading.
It’s all about how you expose.
At the end of the day with all of these camera and formats how you expose is the limiting factor. A badly exposed Sony Venice probably won’t end up looking anywhere near as good as a well exposed FS7. A badly exposed FS7 won’t look as good as a well exposed FS5. No camera looks good when it isn’t exposed well.
Exposure isn’t brightness. You can add gain to make a picture brighter, you can also change the gamma curve to change how bright it is. But these are not exposure changes. Exposure is all about putting the optimum amount of light onto the sensor. Enough light to produce a signal from the sensor that will overcome the sensors noise. But also not so much light that the sensor overloads. That’s what good exposure is. Fiddling around with gamma curves and gain settings will only every make a relatively small difference to noise levels compared to good exposure. There’s just no substitute for faster lenses, reflectors or actually adding light if you want clean images.
And don’t be fooled by ISO ratings. They don’t tell you how noisy the picture is going to be, they don’t tell you what the sensitivity is or even if it’s actually changing. All it tells you is what to set a light meter to.
Once upon a time the meaning of ISO was quite clear. It was a standardised sensitivity rating of the film stock you were using. If you wanted more sensitivity, you used film with a higher ISO rating. But today the meaning of ISO is less clear and we can’t swap our sensor out for more or less sensitive ones. So what does it mean?
ISO is short for International Standards Organisation. And they specify many, many different standards for many different things. For example ISO 3166 is for country codes, ISO 50001 is for energy management.
But in our world of film and TV there are two ISO standards that we have blended into one and we just call it “ISO”.
ISO 5800:2001 is the system used to determine the sensitivity of color negative film found by plotting the density of the film against exposure to light.
ISO 12232:2006 specifies the method for assigning and reporting ISO speed ratings, ISO speed latitude ratings, standard output sensitivity values, and recommended exposure index values, for digital still cameras.
Note a key difference: ISO 5800 is the measurement of the actual sensitivity to light of film. ISO 12232 is a standardised way to report the speed rating, it is not a direct sensitivity measurement.
Within the digital camera ISO rating system there are 5 different standards that a camera manufacturer can use when obtaining the ISO rating of a camera. The most commonly used method is the Recommended Exposure Index (REI) method, which allows the manufacturer to specify a camera model’s EI or base ISO arbitrarily based on what the manufacturer believes produces a satisfactory image. So it’s not really a measure of the cameras sensitivity, but a rating that if used with a standard external calibrated light meter to set the exposure will give a satisfactory looking image. This is very different to a sensitivity measurement and variations in the opinion as to what is a satisfactory image will vary from person to person. So there is a lot of scope for movement as to how an electronic camera might be rated.
As you cannot change the sensor in a digital camera, you cannot change the cameras efficiency at converting light into electrons (which is largely determined by the materials used and the physical construction). So you cannot change the actual sensitivity of the camera to light. But we have all seen how the ISO number of most digital cameras can normally be increased (and sometimes lowered) from the base ISO number.
Raising and lowering the ISO in an electronic camera is normally done by adjusting the amplification of the signal coming from the sensor, typically referred to as “gain” in the camera. It’s not actually a physical change in the cameras sensitivity to light, it like turning up the volume on a radio to make the music louder. Dual ISO cameras that claim not to add gain when switching between ISO’s typically do this by adjusting the way the signal from the sensor is converted from an analog signal to a digital one. While it is true that this is different to a gain shift it does typically alter the noise levels as to make the picture brighter you need to sample the sensors output lower down and closer to the noise floor. Once again though it is not an actual sensitivity change, it does not alter the sensors sensitivity to light, you are just picking a different part of it’s output range.
Noise and Signal To Noise Ratio.
Most of the noise in the pictures we shoot comes from the sensor and the level of this noise coming from the sensor is largely unchanged no matter what you do (some dual ISO cameras use variations in the way the sensor signal is sampled to shift the noise floor up and down a bit). So the biggest influence on the signal to noise ratio is the amount of light you put on the sensor. More light = More signal. The noise remains the same but the signal is bigger so you get a better signal to noise ratio, up to the point where the sensor overloads.
But what about low light?
To obtain a brighter image when there the light levels are low and the picture coming from the sensor looks dark the signal coming from the sensor is boosted or amplified (gain is added). This amplification makes both the desirable signal bigger but also the noise bigger. If we make the desirable picture 2 times brighter we also make the noise 2 x bigger. As a result the picture will be more noisy and grainy than one where we had enough light to get the brightness we want.
The signal to noise ratio deteriorates because the added amplification means the recording will clip more readily. Something that is close to the recordings clip point may be sent above the clip point by adding gain, so the range you can record reduces while the noise gets bigger. However the optimum exposure is now achieved with less light so the equivalent ISO number is increased. If you were using a light meter you would increase the ISO setting on the light meter to get the correct exposure. But the camera isn’t more sensitive, it’s just that the optimum amount of light for the “best” or “correct” exposure is reduced due to the added amplification.
So with an electronic camera, ISO is a rating that will give you the correct brightness of recording for the amount of light and the amount of gain that you have. This is different to sensitivity. Obviously the two are related, but they are not quite the same thing.
Getting rid of noise:
To combat the inevitable noise increase as you add gain/amplification most modern cameras use electronic noise reduction which is applied more and more aggressively as you increase the gain. At low levels this goes largely un-noticed. But as you start to add more gain and thus and more noise reduction you will start to degrade the image. It may become softer, it may become smeary. You may start to see banding ghosting or other artefacts.
Often as you increase the gain you may only see a very small increase in noise as the noise reduction does a very good job of hiding the noise. But for every bit of noise thats reduced there will be another artefact replacing it.
Technically the signal to noise ratio is improved by the use of noise reduction, but this typically comes at a price and NR can be very problematic if you later want to grade or adjust the footage as often you won’t see the artefacts until after the corrections or adjustments have been made. So be very careful when adding gain. It’s never good to have extra gain.
With Sony’s log capable cameras (and most other manufacturers) when you switch between the standard gamma curves and log gamma there is a change in the cameras ISO rating. For example the FS7 is rated at 800 ISO in rec709 but rated at 2000 ISO in log. Why does this change occur and how does it effect the pictures you shoot?
As 709 etc has a limited DR (between around 6 and 10 stops depending on the knee settings) while the sensor itself has a 14 stop range, you only need to take a small part of the sensors full range to produce that smaller range 709 or hypergamma image. That gives the camera manufacturer some freedom to pick the sweetest part of the sensors range. his also gives some leeway as to where you place the base ISO.
I suspect Sony chose 800 ISO for the FS7 and F5 etc as that’s the sensors sweet spot, I certainly don’t think it was an accidental choice.
What is ISO on an electronic camera? ISO is the equivalent sensitivity rating. It isn’t a measure of the cameras actual sensitivity, it is the ISO rating you need to enter into a light meter if you were using an external light meter to get the correct exposure settings. It is the equivalent sensitivity. Remember we can’t change the sensor in these cameras so we can’t actually change the cameras real sensitivity, all we can do is use different amounts of gain or signal amplification to make the pictures brighter or darker.
When you go switch the camera to log you have no choice other than to take everything the sensor offers. It’s a 14 stop sensor and if you want to record 14 stops, then you have to take 100% of the sensors output. The camera manufacturer then chooses what they believe is the best exposure mid point point where they feel there is an acceptable compromise between noise, highlight and lowlight response. From that the manufacture will get an ISO equivalent exposure rating.
If you have an F5, FS7 or other Sony log camera, look at what happens when you switch from rec709 to S-Log2 but you keep your exposure constant.
Middle grey stays more or less where it is, the highlights come down. White will drop from 90% to around 73%. But the ISO rating given by the camera increases from 800ISO to 2000ISO. This increased ISO number implies that the sensor became more sensitive – This is not the case and a little missleading. If you set the camera up to display gain in dB and switch between rec709 (std gamma) and S-Log the camera stays at 0dB, this should be telling you that there is no change to the cameras gain, no change to it’s sensitivity. Yet the ISO rating changes – why?
The only reason the ISO number increases is to force us to underexpose the sensor by 1.3 stops (relative to standard gammas such as rec709 and almost every other gamma) so we can squeeze a bit more out of the highlights. If you were using an external light meter to set your exposure if you change the ISO setting on the light meter from 800 ISO to 2000 ISO the light meter will tell you to close the aperture by 1.3 stops. So that’s what we do on the camera, we close the aperture down a bit to gain some extra highlight range.
But all this comes at the expense of the shadows and mid range. Because you are putting less light on the sensor if you use 2000 ISO as your base setting the shadows and mids are now not as good as they would be in 709 or with the other standard gammas.
This is part of the reason why I recommend that you shoot with log between 1 and 2 stops brighter than the base levels given by Sony. If you shoot 1 stop brighter that is the equivalent to shooting at 1000 ISO and this is closer to the 800 ISO that Sony rate the camera at in standard gamma. Shooting that bit brighter gives you a much better mid range that grades much better.
The new version 4.0 firmware for the PXW-FS5 brings a new lower base ISO range to the camera. This very slightly reduces noise levels in the pictures. If you use “gain” in dB to indicate your gain level, then you shouldn’t have any problems, +6dB is still +6dB and will be twice as noisy as 0dB. However if you use ISO to indicate your gain level then be aware that as the base sensitivity is now lower, if you use the same ISO with version 4 as you did with version 3 you will be adding more gain than before.
Version 3 ISO in black, version 4 ISO in Blue
Standard 1000 ISO – 800 ISO
Still 800 ISO- 640 ISO
Cinegamma 1 800 ISO – 640 ISO
Cinegamma 2 640 ISO – 500 ISO
Cinegamma 3 1000 ISO – 800 ISO
Cinegamma 4 1000 ISO – 800 ISO
ITU709 1000 ISO – 800 ISO
ITU709(800) 3200 ISO – 2000 ISO
S-Log2 3200 ISO – 3200/2000 ISO
S-Log3 3200 ISO- 3200/2000 ISO
At 0dB or the base ISO these small changes (a little under 3dB) won’t make much difference because the noise levels are pretty low in either case. But at higher gain levels the difference is more noticeable.
For example if you often used Cinegamma 1 at 3200 ISO with Version 3 you would be adding 12dB gain and the pictures would be approx 4x noisier than the base ISO.
With Version 4, 3200 ISO with Cinegamma 1 is an extra 15dB gain and you will have pictures approx 6 time noisier than the base ISO.
Having said that, because 0dB in version 4 is now a little less noisy than in version 3, 3200 ISO in V3 looks quite similar to 3200 ISO in version 4 even though you are adding a bit more gain.
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.