This is a question that comes up time and time again. I’ve been using the F5 and FS7 for almost 5 years. What I’ve discovered in that time is that the one thing that people notice more than anything from these cameras is noise if you get your exposure wrong. In addition it’s much harder to grade a noisy image than a clean one.
Lets take a look at a few key things about how we expose and how the F5/FS7 works (note the same principle applies to most log based cameras, the FS5 also benefits from being exposed brighter than the suggested base settings).
What in the image is important? What will your audience notice first? Mid-range, shadows or highlights?
I would suggest that most audiences first look at the mid range – faces, skin tones, building walls, plants etc. Next they will notice noise and grain or perhaps poor, muddy or murky shadows. The last thing they will notice is a few very brightly highlights such as specular reflections that might be clipped.
The old notion of protecting the highlights comes from traditional gamma curves with a knee or highlight roll off where everything brighter than a piece of white paper (90% white) is compressed into a very small recording range. As a result when shooting with conventional gamma curves ALL of the brighter parts of the image are compromised to some degree, typically showing a lack of contrast and texture, often showing some weird monotone colors. Log is not like that, there is no highlight roll off, so those brighter than white highlights are not compromised in the same way.
In the standard gammas at 0dB the PXW-FS7, like the PMW-F5 is rated at 800 ISO. This gives a good balance between noise and sensitivity. Footage shoot at 0dB/800ISO with the standard gammas or Hypergammas generally looks nice and clean with no obvious noise problems. However when we switch to log the native ISO rating of the cameras becomes 2000 ISO, so to expose “correctly” we need to stop the aperture down by 1.3 stops. This means that compared to 709 and HG1 to HG4, the sensor is being under exposed by 1.3 stops. Less light on the sensor will mean more noise in the final image. 1.3 stops is the equivalent of 9dB. Imagine how Rec709 looks if it is under exposed by 1.3 stops or has to have +9dB of gain added in. Well – thats what log at 2000 ISO will look like.
However log has lots of spare headroom and no highlight compression. So we can choose to expose brighter than the base ISO because pushing that white piece of paper brighter in exposure does not cause it to become compressed.
If you open the aperture back up by 1.3 stops you get back to where you would be with 709 in terms of noise and grain. This would be “rating” the camera at 800 ISO or using 800 EI. Rating the camera at 800EI you still have 4.7 stops of over exposure range, so the only things that will be clipped will in most cases be specular reflections or extreme highlights. There is no TV or monitor in existence that can show these properly, so no matter what you do, they will never be true to life. So don’t worry if you have some clipped highlights, ignore them. Bringing your exposure down to protect these is going to compromise the mid range and they will never look great anyway.
You should also be extremely cautious about ever using an EI higher that 2000. The camera is not becoming more sensitive, people are often misslead by high EI’s into thinking somehow they are capturing more than they really are. If you were to shoot at 4000 EI you will end up with footage 15dB noisier than if you were shooting the same scene using 709 at 800 ISO. That’s a lot of extra noise and you won’t necessarily appreciate just how noisy the footage will be while shooting looking at a small monitor or viewfinder.
I’ve been shooting with the F5 and then the FS7 for almost 5 years and I’ve never found a situation where I going to an EI higher than 800 would have resulted in a better end result. At the same time I’ve seen a lot of 2000 EI footage where noise in the mid range has been an issue, one particular example springs to mind of a high end car shoot where 2000 EI was used but the gloss and shine of the car bodywork is spoilt because it’s noisy, especially the darker coloured cars.
Of course this is just my opinion, based on my own experience, others may differ and the best thing you can do is test for yourself.