Switching base ISO mid recording in Cine-EI is causing some metadata issues in Resolve and perhaps other applications, so I strongly recommend you do not switch the base ISO mid shot.
DaVinci Resolve now reads the metadata from footage shot by the FX6 and FX9 in the Cine-EI mode to automatically add the correct exposure offset. So, shoot at 800 ISO base with the EI set to 200 and Resolve will add a -2 stop offset to the footage so that it looks the same as it did when you shot. Shoot at 800 ISO base and 3200 EI and again the correct +2 stop offset is applied.
However if you shoot at 800 base ISO, perhaps with 800 EI and then half way through the shot change the base ISO to high and 12,800 ISO, perhaps with 12,800 EI Resolve gets a bit confused. It will use the new base ISO but the original EI and as a result from the point where you switch base ISO the footage will look extremely under exposed.
So, if you must change the base ISO, it is better to stop recording, switch base and start recording again.
Tag Archives: ISO
Chart of Sony Dual ISO Base Levels
Here’s a handy chart of the base ISO levels for Sony’s cinema line cameras including Venice, the FX9, FX6, FX3 and FX30 as well as the A7SIII and A7IV. The new Sony FR7 is the same as the FX6. I’ve include the base ISO’s for both S-Log3 and S-Cinetone. If you use other gammas the base levels may be different to the S-Cinetone base level, so these values should only be used for S-Cinetone and S-Log3. You can click on the image for a bigger version or left click on it to download it.
As explained above there is a difference in the way the dual ISO functions work between the FX6/FX3/A7SIII and the other cameras. Venice, the FX9 and FX30 have sensors with two distinctly different sensitivities. These cameras offer near identical performance at either the low or high base ISO. Sony call these cameras “Dual Base ISO” as in most cases the two base ISO’s can be used in exactly the same way depending on which best suits the light level you are working at and a near identical image produced.
The other cameras (FX6, FX3, A7SIII) probably have a dual gain sensor plus additional processing to deliver their 2 distinctly different sensitivity ranges. The result is that there is a more visible increase in noise at the high range (compared to the Dual Base ISO cameras) plus a very slight reduction in dynamic range. However, the noise level in the high base setting is significantly lower than you would have by adding gain to get to the same level and the upper base sensitivities are very usable and allow for shooting at very low light levels.
For more information on Dual Base ISO sensors take a look here: https://www.xdcam-user.com/2019/11/what-is-dual-base-iso-and-why-is-it-important/
Handy Tips For Using The Sony Variable ND Filter Values.
Sony rate the ND filters in most of there cameras using a fractional value such as 1/4, 1/16, 1/64 etc.
These values represent the amount of light that can pass through the filter, so a 1/4 ND lets 1/4 of the light through. 1/4 is the equivalent to 2 stops ( 1 stop = half, 2 stops = 1/4, 3 stops = 1/8, 4 stops = 1/16, 5 stops = 1/32, 6 stops = 1/64, 7 stops = 1/128).
These fractional values are actually quite easy to work with in conjunction with the cameras ISO rating.
If you want to quickly figure out what ISO value to put into a light meter to discover the aperture/shutter needed when using the camera with the built in ND filters, simply take the cameras ISO rating and multiply it by the ND value. So 800 ISO with 1/4 ND becomes 800 x 1/4 = 200 (or you can do the maths as 800 ÷ 4). Put 200 in the light meter and it will tell what aperture to use for your chosen shutter speed.
If you want to figure out how much ND to use to get an equivalent overall ISO rating (camera ISO and ND combined) you take the ISO of the camera and divide by the ISO you want and this gives you a value “x” which is the fraction in 1/x. So if you want 3200 ISO then take the base of 12800 and divide by 3200 which gives 4, so you want 1/4 ND at 12800.
Which Sony ISO RatingS Are Correct?
In the course of my tests with the FX3 and comparing it with the FX6 and FX9 I discovered a strange anomaly with the FX3 and A7SIII ISO ratings when compared to the FX6 and FX9.
The FX3’s default picture profile is PP11 and S-Cinetone. If you have an FX6 or FX9 these cameras also default to S-Cinetone in SDR mode. In the FX6 and FX9 the base ISO for S-Cinetone is 320 ISO. Therefore you would assume that if you also set the A7SIII or the FX3 to 320 ISO and expose all the cameras the same, same aperture, shutter etc that the exposures would match.
BUT THE EXPOSURES DON’T MATCH!!
The FX3 and the A7SIIII are just over 1 stop brighter than the FX6 and FX9 when all the exposure settings are matched. I tested all the cameras with the same lens to ensure this wasn’t a lens issue, but it isn’t the lens.
I then went on to test other gamma/picture profile settings and I found a just over 1 stop difference between the FX3 and my FX6/FX9 in any similar combination EXCEPT S-LOG3!
When using Picture Profile 2 on the FX3 which is uses Sony’s “Still” gamma and then using the “Still” Profile on the FX6 there is a difference of around 1 stop. If I set the FX3 to PP3 (ITU-709) and the FX6 to ITU-709 then the difference is again around 1 stop, in every case the FX3 is brighter except when you select S-Log3 where the FX3 and the FX6/FX9 match almost perfectly!
I find this very strange. They should not be different. My light meter suggests to me that the FX6/FX9 are correct.
Comparing to my light meter I believe the FX6/FX9 ratings to be correct and the FX3 to be between 1 and 1.3 stops brighter than it should be when using gammas that are not S-Log3. What I really don’t understand is why the FX3/A7SIII match the FX6/FX9 when using S-Log3 but do not match when using the other profiles, normally I would expect to see a consistent offset. This further makes leads me to be sure this is not a problem with my light meter, but something else.
I would love to hear from anyone else that’s able to take a look at the ISO ratings of the A7SIII and compare it with an FX6 or FX9.
The bottom line is – DON’T EXPECT TO PUT THE SAME EXPOSURE SETTINGS INTO BOTH AN FX3 AND AN FX6/FX9 AND GET THE SAME RESULTS, because you won’t, unless you are using S-Log3, then they match.
Also in the clip metadata I found that 0dB for S-Cinetone is 100 ISO, and whether this is a coincidence or not, if I set the FX3 to 100 ISO and the FX6 to 320 ISO and then match shutter speed and aperture then the exposures are very close.
This one has left me confused!!!!
FX9 ISO Rating Confirmation Test
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.
FX6 ISO rating Confirmation Test
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.
Base ISO Levels for the FX9
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.
What is Dual Base ISO and why is it important?
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 – 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.
Noise, ISO, Gain, S-Log2 v S-Log3 and exposure.
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: https://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.