One thing that’s becoming very clear with the on-going FS5 discussions is that the use of ISO with video camera is confusing the hell out of people. ISO is an almost meaningless sensitivity measure for a video camera. Especially with a video camera that uses different types of gamma curves. The ISO rating is there so that when you use an external light meter you will get the correct exposure, but it doesn’t necessarily tell you a great deal about the cameras actual sensitivity.

Take a Sony S-log camera. If you set the camera to 0dB gain and expose white card or piece of paper using standard gamma correctly (white at 90%), then still at 0dB gain switch to S-log does the piece of paper get brighter? The answer is NO it does not. White will drop to about 70% depending on the log curve chosen. That should be telling you that the actual sensitivity of the camera is NOT changing. If you set the camera to ISO, the ISO indication will change from 1000 ISO to 3200 ISO when you switch between standard gamma and log on an FS5 for example, but these are both 0dB gain and the picture brightness does not significantly change when you switch between standard gamma and log (the picture does become flatter, don’t confuse this for “brighter”. Use a waveform or histogram and measure the levels of a fully exposed scene).

If in both cases, S-Log and Standard Gamma, the gain is 0dB, even thought the ISO rating is changing, the actual sensitivity of the camera is not changing, the only thing that’s changing is the shape of the gamma curve.

The ISO rating changes because for normal gammas you would expose white at 90% while for log it is exposed darker at 60%. So if using an external light meter you want the light meter to tell you to close the aperture relative to standard gammas so that white ends up at 60% (the lower white level makes space for more dynamic range above white).

When you change your external light meter from 1000 ISO to 3200 ISO the light meter will tell you to close the aperture by a stop and a half and that will give you that correct and now darker log exposure.

This is why I hate the use of ISO in video cameras because it is miss-leading people. The FS5 (and the same applies to the FS7, F5, F55 etc) isn’t actually any more sensitive to light in log than in standard gammas (video camera sensitivity is governed by the sensors efficiency at converting light into electrons and we cannot change the sensor). Neither does it have any more gain at 3200 ISO in log compared to 1000 ISO in standard gammas. Both are 0dB. That’s why when you shoot at 3200 ISO in Log you don’t get any more noise and you don’t get any additional NR artefacts…. because the camera isn’t actually becoming any more sensitive and it’s not adding any extra gain, it’s just a rating change to make sure an external light meter would give you the correct log exposure. If people actually went back and used the correct and appropriate terms for video cameras: ie: dB we wouldn’t have all this stupid confusion, it would be obvious that 0dB = 0dB.  The gain and thus sensitivity in standard gammas with 0dB is actually the same as in Log at 0dB. If people actually took the time to look at their monitors it would also be pretty obvious that the sensitivity does not change.

Sadly all this is resulting in some pretty ridiculous statements like: I use log in low light because it’s the only way I can get 3200 ISO without adding extra noise. The reality is that the only thing that’s really changing is the little number on the viewfinder overlay, in standard gamma it says 1000, in log it says 3200. But the sensitivity and noise levels are not changing. S-Log3 does give raised shadows, but those raised shadows will show more noise due to the raised shadow levels, but for a like-for-like brightness they are no more noisy than any other gamma. Using the wrong terminology or miss-understanding how a video camera works is resulting in very silly mistakes. Once upon a time video camera operators used to rely on monitors and waveform displays to get exposure and gain levels right. But this isn’t cool anymore because we are all now “Directors of Photography” and DP’s don’t use gain or shutter speeds they use ISO and angles. This is nonsense and the desire to sound hip and cool is resulting in poor or miss-guided camera operation.

Following on from the ongoing discussions about the way the noise reduction on the PXW-FS5 works I’d like to try to answer a few points that have been raised.

Noise reduction is incorporated in to almost every single video camera on the market today. Without it cameras would not be achieving the sensitivity levels that we are becoming accustomed to. Big sensors have helped increase sensitivity, but noise reduction also plays a major part. 4 years ago the typical sensitivity of a video camera was around the equivalent of 300 ISO. Today 1000 ISO is common. The laws of physics have not changed but image processing has.

However image processing has limitations. Noise reduction is a trade off as it typically introduces other artefacts including blurring or softening of the image, smearing of the image when there is motion, posterisation, blockiness or banding.

The more noise you introduce into the image the harder the NR has to work and as a result the worse, or more noticeable the artefacts will become.

Every time you add 6dB of gain (or double the ISO) of a sensor you will also double the underlying noise level, that is a very significant noise increase.

Different sensors will exhibit different noise characteristics. A sensor with a very high pixel count may exhibit finer grain, but may be less sensitive, have aliasing issues, skew issues or heat problems. A sensor with fewer pixels may have coarser grain but be more sensitive have fewer aliasing issues, fewer skew issues and be less prone to overheating. A sensor with an excess of pixels compared to the final image resolution may be easier to noise reduce as the excess sensor resolution can be used during the NR process without degrading the end image, but the high pixel count may introduce some of the other issues listed above.

So to get an image with an acceptable noise level, low skew, a desirable level of sensitivity without overheating is a very fine balancing act. Different cameras will perform differently and each will have strength’s and weaknesses. A Sony A7S has big pixels, so it’s very sensitive and only needs low levels of NR at the base sensitivity compared to say an FS7 with it’s smaller pixels. But the A7S suffers from a lot of image skew that is unacceptable for many types of video production plus there are heat issues when shooting for long periods. Meanwhile the FS7, while less sensitive has very, very low skew levels and no overheating.

But what about the new PXW-FS5? There is much discussion about the noise reduction in this camera. First of all it is a different camera to any other. So it will not perform the same as any other. It uses a different combination of sensor and processing to any other camera on the market.

Does it perform as well as an A7R in low light? Actually it’s not all that different in terms of base sensitivity. What about if you add gain? Well the image quality of both cameras deteriorates when you add gain. Both become noisier and both have more NR artefacts. The A7RII is possibly a better performer in terms of noise and artefacts, maybe due to the higher pixel count allowing the use of a lot of pixel binning. Does this mean that the FS5 is somehow broken or defective? No, it just means that the A7RII has the edge in low light. Do remember that the A7RII suffers from a lot of image skew, really appalling flash band issues and overheats on long shots. Does that mean the A7RII is broken, because after all the FS5 doesn’t have these issues problems. Of course not, these are just limitations of these two very different cameras.

Anyway coming back to the noise reduction. As we can’t actually turn off the NR on the FS5 it’s hard to understand exactly how beneficial it is, even though it does introduce some inevitable artefacts (NR will always have undesirable side effects), especially when you add more than 6dB of gain. But we can get some clues. The edge tearing artefact that can be see on vertical motion at high gain levels appears to simply be noise leaking through the temporal NR when there is a lot of vertical motion, and it’s pretty obvious that there’s actually a lot of noise being hidden.

Why does this only occur in 4K? Well that’s probably because in HD the camera has a surplus of sensor resolution, so different noise reduction processes that soften the image can be used instead of temporal NR as the softening won’t be noticed in HD.

Another clue as to how well the NR works is that if you quickly switch the gain switch from one gain level to another it takes a couple of frames for the temporal NR to catch up so for one or two frames the temporal NR level is reduced (not eliminated just reduced a bit). If you look at these frames it gives some insight into how amazingly effective the temporal NR is and as well as temporal NR the FS5 is also employing spatial NR, so this is only part of the story.

I have a couple of frame grabs. One is a frame where the temporal NR is stabilised and doing it’s thing reducing the noise in the picture. This is with +12 dB of gain applied and to be honest it’s a pretty clean looking image for a 4000 ISO shot, imagine adding +24dB gain to an EX1 or PMW-500 to get up to 4000 ISO and what that would look like! The other frame is a frame grabbed as the camera is switching between +6dB to +12dB, so what you are seeing is about 6dB’s worth of temporal noise reduction. Just look at all that very nasty looking noise. Notice the very blocky areas in the shadows, in motion these can be seen to be fluttering from frame to frame as the NR kicks in, it’s really nasty. The noise is having a serious impact on the image resolution. On the far left table leg in the close up you can no longer see the wood grain. Now imagine what at least double if not 3 times that would look like because if you were to shoot at 3200 ISO with the standard gammas without NR, that’s what it’s going to look like. It would almost certainly overload the internal coded resulting in compression artefacts that you won’t see while shooting, only when you play the footage back when quite possibly it’s too late to do anything about it.

So next time you look at a noise reduction artefact do think about just how dreadful the image would look without any NR. We are not talking about seeing a little bit more noise and grain with reduced NR but a blizzard of noise and grain. It’s also worth remembering that all the camera manufacturers are doing this. Also consider what a typical 1/2″ or 2/3″ broadcast camera would look like with +24dB of gain added, it really would be unusable, yet the FS5 can deliver an image at 4000ISO that is really not all that bad. It’s not perfect, it does have some artefacts but really it’s quite remarkable what a camera like this can do thanks to modern noise reduction processes and large sensors.

Temporal NR: Temporal noise reduction works by taking a single pixel and measuring it’s average output over several frames. Using this average value a correction can be applied to the current frame that helps reduce the instantaneous brightness fluctuations that are seen as noise. However if there is motion in that area this can create nasty local blurring or smearing effects. On option is to include a process that detects motion and locally reduces or eliminates the noise reduction during motion to counter the smear or blurring. Either way motion in the image introduces undesirable artefacts.