ISO is frequently the last setting that people look at and only when they’re losing light. I typically set ISO first and it’s probably because for quite awhile now I’ve been using auto ISO. This is something that generally works quite well on mirrorless cameras and to a degree on some DSLRs.
I find that my cameras make very good choices in auto ISO especially when there’s bright light and deep shadows as well as on really overcast days. I set the parameters for the base ISO and the maximum ISO as well as the minimum shutter speed.
When it comes to shooting with film cameras you are pretty much stuck with the ISO of the film that you choose until you finish a roll. You can push film to a higher ISO for low light conditions but you have to develop the entire roll for that setting.
The great thing about digital cameras is that you can change your ISO for every shot you take and it doesn’t have an effect on the others.
ISO is the light sensitivity of the imaging sensor. Digital cameras have what’s called nominal sensitivity or their natural sensitivity which has an effect on their dynamic range. It is very much similar to the ISO for particular films. The higher the ISO number is, the less light is required for a properly exposed image.
On my Fuji cameras the nominal sensitivity is 200 ISO. If I select to shoot at 400 ISO or I let the camera choose that ISO in the auto mode, “the exposures captured by the image sensor will be underexposed by one f-stop. Selecting ISO 800 corresponds to an underexposure of 2 f-stops” etc.
Each full jump to the next higher ISO setting equals one f-stop. Without going into a lengthy explanation that I don’t fully understand, digital cameras use a few different technologies to compensate for this while still providing acceptable images within a certain range.
Fortunately digital cameras have gotten quite good and eliminating noise and grain. I often shoot at 1600 ISO and occasionally even at 3200 ISO with very good results. It’s a good thing to sit down and take a number of photos with different ISO settings to see the actual difference to see how your camera responds in the same conditions.
As well, RAW photos often are a better choice if you are shooting at the limits of ISO in your camera. At least you can process the photo accordingly. If you aren’t doing so already. you may want to start your day of shooting by choosing ISO first.