Camera sensors, DR, ISO and Exposure

[alexander uribe]

I’ve recently started to incorporate using iso to better take advantage of exposing my scenes to maximize dynamic range in either the highlights, shadows or a mix depending on what I’m filming. For example, if I’m filming daytime exteriors I’ll typically kick up my iso to around 1000 and ND down to reign in my exposure and retain better dynamic range in the highlights or if I’m filming a scene with shadows I’ll lower my iso and light the scene bring my exposure to here I want it and retain better (and lower noise) shadow detail. I think that I mostly understand why I should be doing this but I’ve recently started thinking about my first example—daytime exteriors—and need some help understanding how it works. My issue is that while I bring up my iso (also increasing the noise floor) I’m still having to ND to expose properly and am thus cutting off the amount of light hitting the sensor. Shouldn’t this cause issues with sensor noise, more than say if I kept my iso lower and used less ND, obviously this would not bias my DR towards the highlights like it would if I used a higher iso. Thoughts appreciated, thanks!

[Tyler Purcell]

Well, unlike film, single ISO range imagers will have a fixed ISO. Many are 800, which would give the optimal amount of dynamic range in the highlights and blacks. Most cameras are pretty clean at 800, even older ones. 

When you increase the ISO, you're basically adding gain through the preamps and this will cause noise. The noise comes from many things, most of which is temperature. When things start to heat up a lot, they get noisy. This is why companies like Arri have developed very clever imager cooling systems, to help compensate. It's why smaller cameras like DSLR's which can't afford that luxury, resort to de-noising through clever image manipulation on the fly. When you record externally to raw through the HDMI, the signal is always WAY nosier than the internal compressed codec. 

On a compressed codec (non-raw) camera, you probably will want to use the ISO trick you explained to help retain DR in the highlights. If you're using a raw camera, the ISO can be changed in post. So if you shoot 1000 for instance, you're just under exposing the imager. You can do the same thing by shooting 800, just put the zebra's on and set them to 90% and if you see them, you're over exposed. That's a trick I've used for 30 years, it works really well to make sure you're not over exposing.