Think of it as a way to increase the base brightness of the total image. Only, instead of individual photosensitive crystals in your film, you now have a set number of pixels, regardless of ISO. As a result, it can be a little misleading to say that ISO directly relates to your sensor’s sensitivity, and instead more accurately ISO directly relates to the brightness of each individual pixel. When a pixel is blown out (becoming too bright), the result is the graininess that appears in high ISO photos. The more pixels that get blown out, the more “grain” appears and the worse your photo looks. So why wouldn’t you just always shoot at the lowest possible ISO? The answer, like with most things in photography, is “it depends.”
When should I change my ISO?
In my opinion, the beautiful thing about getting to guide photography tours, (and nature photography in general) is that no two scenes are ever the same. Sure, there may be similar light, similar subjects, or even similar clouds (or lack thereof), but at the heart, there’s never going to be the same scene shot twice from the same spot in the same conditions with the same results. As such, ISO can be a dynamic part of your use of the exposure triangle and a way to ensure great results each time, no matter what the light is doing. Let’s cover a few specific instances when ISO plays a big role.
Landscape Photography on a Tripod
When shooting landscape scenes without a moving component (ie. no animals or people in the frame), know that your clearest result will come from an ISO of 100. If you’re shooting a mirrorless system, know that the Sony, Olympus and Panasonic mirrorless systems have all been designed to have their best ISO actually at 200, so shoot at 200 for best results. You Canon and Nikon folks? 100 is your sweet spot.
Why can you count on 100 (or 200) to be your hard line go to for landscapes on a tripod? Because you’re better off using shutter speed to increase total light in your scene than ISO. The use of a sturdy tripod will result in the ability to really slow your shutter way down and get the clearest results with minimal noise caused by high ISO.