Tag Archives: ISO

cinematography, Shooting Tips, shoots, Technology

What does ISO mean with todays cameras?

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What is EXPOSURE?

Before diving into ISO, I think it’s first important to understand what exposure is. Exposure is the amount of light you put on to a sensor or film stock. Exposure is NOT brightness, brightness is – brightness. I can take an image in to post production and make it brighter or darker, but this doesn’t change how the image was exposed. Exposure is very specifically – how much light is allowed to hit the sensor or film stock and this is usually controlled by the shutter speed, aperture as well as perhaps ND filters.

What is ISO?

Once upon a time the meaning of ISO was quite clear. It was a standardised sensitivity rating for 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 lets not forget, we can’t swap our sensors out for more or less sensitive ones. So what does ISO mean given that we can’t actually change the sensor?

ISO is short for International Standards Organisation. And they specify many, many different standards for many different things. For example ISO 3166 is for telephone country codes, ISO 50001 is for energy management.

But in our world of film and TV there are two main 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 electronic 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, ie: it is not actually a direct sensitivity measurement.

So, from the above we can deduce that with film ISO is an actual sensitivity measurement. With an electronic camera it is a speed rating, not a measurement of the sensitivity, a rating.

Different Approaches and REI.

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 base ISO arbitrarily, based on what the manufacturer believes produces a satisfactory image. So it’s not actually a measure of the cameras sensitivity, but a rating that if entered into a standard external light meter and the shutter and aperture values from the light meter used to set the exposure will result in satisfactory looking image.
This is very different to a sensitivity measurement and variations in opinion as to what is “a satisfactory image” will vary from person to person, manufacturer to manufacturer. For example – how much noise is considered acceptable? I know a lot of people with very different opinions on this! So, there is a lot of scope for movement as to how an electronic camera might be rated and we see this in the real world where two cameras both rated at the same ISO may have very different noise levels when exposed “correctly”.

You Can’t Change the Silicon!

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 of the sensor. 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.

Higher and Lower ISO values.

Raising and lowering the ISO rating 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 is more like turning up the volume on an analog radio to make the music louder. Dual ISO cameras that claim not to add gain when switching between ISO’s typically do this by an adjustment at the sensors pixel level and this is closer to an actual sensitivity change. But generally this only gives two levels, not the multitude of ISO values offed by most cameras. While it is true that Dual ISO is different to a gain shift, it does typically alter the noise levels with the higher base ISO being slightly more noisy than the lower. With a true dual ISO sensor does do is produce the same dynamic range at both ISO’s.

Noise and Signal To Noise Ratio.

Most of the noise in the pictures we shoot comes from the sensor and sensor readout circuits and this noise level coming from the sensor is largely unchanged no matter what you do.

So, the biggest influence on the signal to noise ratio or SNR is the amount of light you put on the sensor because more light = more signal. The noise remains but with more light the signal is bigger so you get a better signal to noise ratio, up to the point where the sensor clips at which point adding more light makes no further difference.

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 can be boosted or amplified (gain is added) by increasing the cameras ISO value. This extra signal amplification makes both the desirable signal bigger but at the same time as the noise cannot be separated form the image the noise also gets bigger by the same amount. If we make the desirable picture 2 times brighter we also make the noise 2 x bigger/brighter. As a result the picture will be brighter but the noise will appear greater than an exposure where we had enough light to get the brightness we want and didn’t need to add gain or raise the ISO.

More gain = less dynamic range.

The signal to noise ratio deteriorates because the added amplification means the recording will clip more readily. Something that was right at the recordings clip point without adding gain may will end up above the clip point by adding gain. As a result the highlight range you can record reduces while at the same time the noise gets bigger. But 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 getting 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 recording brightness 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.

ISO in an electronic camera is not a sensitivity value, it is an exposure rating.

Getting rid of noise:

To combat the inevitable increase in the visibility of noise and the degraded signal to noise ratio that comes from adding 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 there will often be more noise reduction and this will start to degrade the image. It may become softer, it may become smeary. You may start to see banding, ghosting or other artefacts. Higher noise levels are also problematic for modern high compression codecs, so even if the camera doesn’t add extra noise reduction at high gain levels it is likely that the codec will do more noise reduction in an attempt to keep the recording bit rate under control.

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 can be 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.

So what does all of this mean?

The majority of the video cameras we use today are something known as ISO invariant. This mean that the actual sensitivity of the camera doesn’t actually change, even though the camera may offer you a wide range of ISO values. Instead we are adding gain to get a brighter picture, but extra gain degrades the signal to noise ratio and limits the dynamic range.

As well as adding gain in camera we can also add gain in post production. And if the quality of the recording codec is high enough there is almost no difference between adding the gain in post production compared to adding the gain in camera. If you don’t add gain in the camera then you don’t reduce the cameras dynamic range. By moving the gain addition to post production you can retain the cameras full dynamic range and overall the end result won’t be significantly different. This is why most Log cameras use some sort of Exposure Index system that locks the camera to it’s base sensitivity as this is where the camera will exhibit the greatest useable dynamic range.

What about using Picture Profiles of different Gamma Curves?

Different gamma curves have different gain levels. So, very often you will see a camera at it’s base sensitivity (ie: no added gain) give you different ISO values depending on the gamma curve you have chosen. Again – this doesn’t meant the sensitivity of the camera is different for each gamma curve. What it actually means is that the optimum exposure (exposure = amount of light you put on the sensor) is a bit different for each gamma curve.  For example when shooting S-Log3 the sensor is exposed lower than it is with normal gammas. This darker S-Log3 exposure leaves more room for an extended highlight range. The flip side to this is that when the camera is set at the correct, no extra gain added base ISO’s “correctly” exposed S-Log3 will be noisier than correctly exposed Rec-709 or S-Cinetone, but the S-Log3 will have a greater highlight range. If you were to expose the S-Log3 and the S-Cinetone using the same aperture and shutter speed the noise would be the same.

Picture Profiles for Low Light.

A question that gets asked a lot is: What’s the best picture profile or gamma for low light?

Well,  if you have followed all of the above then you will hopefully understand that the gamma or picture profile makes no difference to the actual sensitivity of the camera.  So in reality there is very little difference between any profile or gamma curve in terms of how the camera will perform in low light. Remember: it’s always the same sensor with the same noise and same sensitivity to light no matter what other settings you have chosen.

There might be some differences in the amount of noise reduction applied in different profiles and that might make a small difference. Many of Sony cameras allow you to adjust this between off/low/mid/high. But even if you can’t change this in camera, adding a bit of extra NR in post is a common practice these days.  Really, it’s a case of choosing the profile or gamma that gives you the image you want, S-Log3 if you intend to grade, perhaps S-Cinetone if you don’t. If you bring the S-Cinetone ISO value up to match the S-Log3 ISO value, the noise in the final image from both will be more or less the same.

Using what data you have.

Perhaps the only small consideration is that under exposed S-Log3 only uses a very small part of the cameras full recording range. You won’t be making use of the full recording data range. Because of this it might be hard to grade it without the image starting to look coarse or grainy.  Because of the smaller dynamic range, similarly exposed (same aperture, same shutter speed) S-Cinetone or Rec-709 will use more of the data range and might not look quite as coarse as a result. This difference is very small, but it should be considered if you are trying to squeeze something out of an extremely under exposed situation.

cinematography, PMW-F55, PXW-FS5, PXW-FS7, Technology

Why are Sony’s ISO’s different between standard gammas and log?

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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.

 

cameras, PXW-FS5, Technology

PXW-FS5, Version 4.0 and above base ISO – BEWARE if you use ISO!!

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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.

cameras, cinematography, PXW-FS5, Technology

ISO Confusion Once Again!

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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.

cameras, cinematography, PXW-FS5

PXW-FS5 Native ISO’s

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THIS DOCUMENT IS OUT OF DATE NEW FIRMWARE HAS CHANGED THE CAMERAS ISO”S.

This is as much for my benefit as yours as I can never remember what the native ISO (0dB) is for each of the gamma curves in the FS5.

Standard 1000 ISO
Still 800 ISO
Cinegamma 1  800 ISO
Cinegamma 2  640 ISO
Cinegamma 3  1000 ISO
Cinegamma 4  1000 ISO
ITU709 1000 ISO
ITU709(800) 3200 ISO
S-Log2 3200 ISO
S-Log3 3200 ISO

Using dB and setting it to 0dB really is so much easier with this camera!

cameras, Technology

Understanding the all important “Signal to Noise Ratio”.

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The Signal to Noise ratio is one of the key factors in determining the quality of a video or stills image. A noisy, grainy picture rarely looks as good as a low noise “clean” image. In addition it’s noise in your images will limit how far you can grade them before the picture quality becomes unacceptably poor.

Almost always what you want is the biggest possible signal with the least possible noise. In a video or film camera the signal is the desired image information or in simple terms the picture. While the noise is…. well….. the noise.

Once upon a time, when film cameras were normal for both still photography and film the noise in the pictures came primarily from the grain structure of the film stock. One of the great features of film cameras is that you can actually change that film stock to suit the type of scene that you are shooting. For low light you could use a more sensitive film stock that was actually truly more sensitive to light. However, often a very sensitive film stock will show more noise as the grain of more sensitive film is normally larger.

With video and digital stills cameras however things are quite different. You can’t normally change the sensor in a video camera and it’s the sensor that determines the sensitivity of the camera and it is the sensor that is the source of the majority of the noise.

Modern CMOS video sensors consist of two parts. The light gathering part and the readout part. The size of the pixels on a sensor is one of the key factors in determining the sensitivity and dynamic range. Small pixels are not good at capturing, converting and storing large numbers of photons of light or electrons of electricity.  Bigger pixels are much better at this, so big pixels typically mean better sensitivity and a better dynamic range. Each pixel is unique and as a result every pixel on the sensor will perform slightly differently. The signal stored in the pixels is a tiny analog signal that is easily disturbed by stray electric currents and variations in temperature. As a result of the small variations from pixel to pixel, the stray signals and heat, there is a small variation from moment to moment in the signal that comes off the pixel when it is read out and these variations are what we see as noise.

The analog signal from the pixels is passed to a circuit that converts it to a digital signal. The analog to digital conversion process normally includes some form of noise reduction circuitry to help minimise the noise. By carefully mapping the A to D circuity to the signal range the pixels provide, a sensor manufacturer can find the best combination of noise, dynamic and sensitivity. Once the signal has been converted to a digital one, the noise level, sensitivity and dynamic range is more or less locked in and can’t be changed (Some cameras have the ability to use slightly different A to D conversion ranges to help give improved noise levels at different brightness/dynamic ranges).

The bottom line of all this is that with the vast majority of video cameras the noise level is more or less fixed,  as is the sensitivity as we can’t actually swap out the sensor.

But wait! I hear you say…. My camera allows me to change the ISO or gain. Well yes it probably does and in both cases, ISO or gain, with a digital video or stills camera what you are changing is the cameras internal signal amplification. You are NOT making the camera more sensitive, you are simply turning up the volume. As anyone with any type of sound system will know, when you turn up the gain you get more hiss. This is because gain makes not only the desired signal bigger but also the noise. As a result adding gain or increasing the ISO is rarely a great thing to do.

So normally we want to use a digital camera at it’s native sensitivity wherever possible. The native sensitivity is where no gain is being added by the camera or 0dB. In ISO, well you need to find out what the native ISO is and be aware that different gamma curves will have different base ISO’s (which is why I prefer to use dB gain as 0dB = native sensitivity, least noise, best dynamic range, no matter what gamma curve).

To get the best possible image we then want to make our signal (picture information) as big as possible. As we can’t swap out the sensor, the only way to do that is to put as much light as possible onto the sensor. Obviously we don’t want to overload the sensor or exceed the limitations of the recording system, but generally the more light you get on the sensor, the better your pictures will be.

As the sensors noise output remains more or less constant, the best signal to noise ratio will be gained when you put a lot of light on the sensor. This generates a very large signal, so the signal becomes big compared to the noise and the noise becomes only a small percentage of the overall image.

If we are unable to get enough light onto the sensor to expose it fully then it is often tempting to add some gain to make the picture brighter. 6dB of gain is the equivalent to 1 stop of exposure. Just like f-stops, each time we go up a stop we are doubling. So adding 6db of gain doubles everything. It makes the picture the equivalent of one stop brighter, but it also doubles the noise. Adding 12dB gain multiplies the noise 4 times, adding 18dB multiplies the noise 8 times.

What if instead of adding gain to make the picture brighter we let 4x more light fall on the sensor (2 stops)? Well the image gets brighter by the equivalent of 2 stops but as we are not adding gain this means the desirable signal, the picture is now going to be the equivalent of 12dB bigger than the noise than it was before we added the 2 stops of light. That’s going to give you a much cleaner looking image.

How do you get more light onto the sensor? There are many ways such as using a faster lens with a larger aperture that will let more light through. Or you could try using a slower shutter speed (I often find it beneficial in low light to use a 1/24th or 1/25th shutter if there is not too much motion to cause the image to become excessively blurred). Then of course you can also add light to your scene by lighting it. It’s very rare to find noisy and grainy night scenes in feature films and that’s because the night scenes normally have well lit foregrounds but keep dark backgrounds to maintain the sensation of night time or darkness. High contrast is the key to good looking night scenes, well lit foregrounds or actors with deep, dark shadows and backgrounds.

The desire to have a good signal to noise ratio is one of the reasons why when shooting in log or raw you want to expose as brightly as you can (while still maintaining consistent exposure from shot to shot, scene to scene). It’s a little bit harder with standard gammas as we have things like the knee or highlight roll off to deal with. Plus the need to have a shot that looks correct straight out of the camera. But at the end of the day the best results are almost always gained when the gain is kept to a minimum (but don’t use negative gain as this can effect the dynamic range) and the amount of light falling on the sensor as high as possible.

In the next article I’ll give you an interesting experiment to try on a PMW-F5, F55 or PMW-FS7 that is very revealing  about the way ISO, gain, exposure and noise behaves that will show why exposing log or raw at +1 to +2 stops is so important.

cinematography, Shooting Tips

Why gain is bad for your dynamic range.

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One way to reduce the noise in a video camera image is to reduce the cameras gain. One way to increase the brightness of the image is to add gain.

We all know that increasing the gain to lets say +6db will increase noise and generally the reverse holds true when you reduce the gain, the noise typically reduces and this may be helpful if you are going to do a lot of effects work, or just want a clean image.

However in most cases adding or removing gain reduces the cameras dynamic range as it will artificially clip or limit your low key or high key parts of the image. The maximum illumination level that a camera can capture is limited by the sensor or the gamma curves that the camera has. The black level or darkest part of the image is the point where the actual image signal compared to the sensor noise level is high enough to allow you to see some actual picture information (also known as noise floor). So the dynamic range of the camera is normally the range between the sensors noise floor and recording or sensor clipping point.

To maximise the cameras dynamic range the designers will have carefully set the nominal zero db gain point (native ISO) so that the noise floor is at or very close to black and the peak recording level is reached at the point where the sensor itself starts to clip.

The gain of the camera controls the video output and recording level, relative to the sensors signal level. If you use -3db gain you attenuate (reduce) the relative output signal. The highlight handling doesn’t change (governed by the sensor clipping or gamma curve mapping) but your entire image output level gets shifted down in brightness and as a result you will clip off or loose some of your shadow and dark information, so your overall dynamic range is also reduced as you can’t “see” so far into the shadows. Dynamic range is not just highlight handling, it is the entire range from dark to light. 3db is half a stop (6db = 1 stop) so -3db gain reduces the dynamic range by half a stop, reducing the cameras underexposure range without (in most cases) any change to the over exposure range, so overall the total dynamic range is reduced.

When you add gain the reverse happens. Generally how far the sensor can see into the shadows is limited by the sensors noise floor. Add 6db of gain and you will make the darkest parts of the image brighter by 6db, but you will also raise the noise level by the same amount. So while you do end up with brighter shadow details you can’t actually see any more picture information because the noise level has increased by the same amount. At the top end as the brightest sensor output is mapped to the maximum recording level at 0db, when you add gain this pushes the recording level beyond what can be recorded, so you loose 6db off the top end of your recordings because the recordings and output clips 6db earlier. So positive gain maintains the same shadow range but reduces the highlight recording range by 6db.

However you use it gain tends to reduce your dynamic range. Adding gain to cope with poor lighting tends to be the lesser of the two evils as generally if your struggling for light then overexposure and blown out highlights is often the last of your worries.

Negative gain is sometimes used in camera to try to reduce noise, but the reality is that you are loosing dynamic range. Really a better solution would be to expose just a tiny bit brighter and then bring your levels down a bit in post production.

Uncategorized

What is ISO and how does it compare to gain?

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With more and more people using 35mm size sensors, more of the old traditional filming styles and techniques are trickling down from the high end to lower and lower production levels. This is a good thing as it often involves slowing down the pace of the shoot and more time being taken over each shot. One of the key things with film is that you can’t see the actual exposure on a monitor as you can with a video camera. A good video assist system will help, but at the end of the day exposure for film is set by using a light meter to measure the light levels within the scene and then you calculate the optimum exposure using the films ISO rating.
So what exactly is an ISO rating?

Well it is a measure of sensitivity. It tells you how sensitive the film is to light, or in the case of a digital stills or video camera how sensitive the sensor is to light. Every time you double the ISO number you are looking at doubling the sensitivity. So ISO 200 is twice as sensitive as ISO 100. ISO 1600 is twice as sensitive as ISO 800 etc.
Now one very important thing to remember is that ISO is a measure of sensitivity ONLY. It does not tell you how noisy the pictures are or how much grain there is.  So you could have two cameras rated at 800 ISO but one may have a lot more noise than the other. It’s important to remember this because if you are trying, for example, to shoot in low light you may have a choice of two cameras. Both rated with a native sensitivity of 800 ISO but one has twice as much noise as the other. This would mean that you could use gain (or an increased ISO) on the less noisy camera and get greater sensitivity, but with a final picture that is no more noisy than the noisier camera.
How does this relate to video cameras?

Well most video camera don’t have an ISO rating, although if you search online you can often find someone that has worked out an equivalent ISO rating. The EX1 is rated around 360 ISO. The sensitivity of a video camera is adjusted by adding or reducing electronic gain, for example +3db, +9db etc. Every 6db of gain you add, doubles the sensitivity of the camera. So taking an EX1 (360 ISO) if you add 6db of gain you double the sensitivity and you double the ISO to 720 ISO, but you also double the amount of noise.
Now lets compare two cameras. The already mentioned EX1 rated at approx 360 ISO and the PMW-350 rated at approx 600 ISO. As you can see from the numbers the 350 is already almost twice as sensitive as the EX1 at 0db gain. But when you also look at the noise figures for the cameras, EX1 at 54db and 350 at 59db we can see that the 350 has almost half as much noise as the EX1. In practice what this means is that if we add +6db gain to the 350 we add +6db of noise so that brings the noise level 53db, very close to the EX1. So for the same amount of noise the 350 is between 3 and 4 times as sensitive as the EX1.
Does your head hurt yet?
There is also a good correlation between sensitivity and iris setting or f-stop. Each f stop represents a doubling or halving of the amount of light going through the lens. So 1 f-stop is equal to 6db of gain, which is equal to a doubling (or halving) of the ISO. You may also hear another term in film circles and that is the T-stop. A T stop is a measured f-stop, it includes not only the light restriction created by the iris but also any losses in the lens. Each element in a lens will lead to a reduction in light and T stops take this into account.

So there you go. The key thing to take away is that ISO (and even the 0db gain setting on a video camera) tells you nothing about the amount of noise in the image. Ultimately it is the noise in the image that determines how much light you need in order to get a decent picture, not the ISO number.