I’ve taken a lot of shots at trying to explain the effect of ISO setting for raw files. Recently, I was asked to simplify my explanation. Here it is. Warning, some subtilties have been elided.
I have likened the ISO dial to the little plastic steering wheel you stick to the dashboard so your 4 year old can play at driving. It’s not quite that bad, but you get the idea.
What the ISO dial does:
- Change the brightness of the JPEG image.
- Change the brightness of the EVF preview, if that option is turned on.
- Change the brightness of the little JPEG preview image embedded in the raw file.
- Sometimes, sendsinstructions to the raw developer in the metadata.
- Sometimes, affect read noise in a way that is meaningful in some circumstances.
- Communicate information to the camera metering system.
- Affect the live histogram.
- Sometimes, affect clipping
What the ISO dial doesn’t do, except through automation:
- Affect exposure.
- Affect photon noise.
Away from base ISO, the rules for exposure are:
- Pick your shutter speed, depending on subject and camera motion
- Pick your aperture, depending on DOF
ISO plays no part in that. After you’ve picked your exposure, set ISO to any convenient ISO that avoids clipping.
Pelino says
But what about cameras that have a dual gain system? Does changing the ISO (at least as between the base ISO and the higher gain point) not affect the camera’s internal operations? I suppose this may be captured in your line about “affect read noise”, but are the effects not more extensive?
JimK says
Dual conversion gain affects read noise, not photon noise.
Paul R says
I find it frustrating that even professional cameras treat the users like children. The plastic steering wheel of the ISO dial just slightly annoying, but it blows my mind that no one gives you a true raw histogram.
How hard would it be to design a “pro” mode? Give us a raw histogram. Replace the ISO control with “low” and “high” (on a dual-gain system). Figure out something for the viewfinder … I’d be fine with it auto-adjusting so you can see what you’re looking at. I don’t need it to magically match my final result.
MikaFoxx says
Very true. Even if most cameras can benefit slightly from the first step of ISO before second conversion gain, even old Canon cameras lost any benefit after 3200 in most cases, once the read noise of the ADC was lower than the amplified photon noise.
They only do a real meaningful mode like that once you go into LOG video modes, and the ISO is at base with a shift for highlight range. Just like pro cine cameras do with software shifting of the levels aside from second base iso (or Arri, always recording both base isos with separate ADC’s)
Magic lantern did true live view raw histograms and raw clipping per channel and even back ported raw clipping to image review to the 2005 Canon 5D – so it can’t be hard.