How are you supposed to judge ISO's effect on the image if you are looking through the viewfinder?

Question

When you look through the viewfinder, how do you prevent potentially taking bad photos by misjudging the ISO? You can see what the focus will be like and depth of field and the shutter speed isn't too difficult to judge. But the picture could look super good in the view finder but look horrible after because the ISO was set wrong. How can I make the view finder incorporate ISO before I take the photo so I don't need to take time to take multiple photos, or use live view?

twalberg · Accepted Answer

You basically can't, at least not any any current cameras that I know of. What you can do, though, is practice a lot - even if it's just throwaway shoots you do. But use your gear as much as possible under varying light conditions, so that after inspecting the results over time, you get a feel for how the ISO settings on your gear will affect the final product. And keep in mind that, if you replace your gear, the characteristics may be totally different, so you may need to repeat the exercise.

Correct answer by twalberg on April 24, 2021

Bob Macaroni McStevens · Answer

Experience and learning from mistakes.
Tools like light meters and phone apps can help.
Mirrorless cameras are another good way.
Also automatic modes like aperture priority, automatic, or program can help.
But in the end, nothing is as useful as being willing to make a bad picture. The good part is nobody else cares much if you do. So getting experience is easy if you are willing to practice.

Michael C · Answer

Many cameras allow you to turn on options to display information, such as Tv, Av, and ISO in the viewfinder. But you've still got to look at it and pay attention to the numbers within your view.
Keep in mind that "ISO" itself is not what makes many pictures noisy. It's the very minute amounts of light we are allowing  into the camera that require the high analog amplification of high ISO values that does that.
For an example of how shooting the same scene under the same light using the same exposure time and aperture results in very similar results regardless of whether the ISO was set at a high setting when shot, or whether the image was shot at ISO 100 and then pushed that many stops digitally after the fact, please see:
What is "ISO" on a digital camera?
and a follow up to it:
Should higher ISOs really be preferred (all other things being equal)?
as well as this related question:
Is it better to shoot with a higher ISO, or use lower ISO and raise the exposure in post-processing?

How are you supposed to judge ISO's effect on the image if you are looking through the viewfinder?

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