What is ISO?
Were you a photographer in the film days? If you took photos back then, you had to buy film. It came in a box, probably yellow, but possibly green, and it had a big number on it; 100, 200, 400, etc.
Those numbers were the ASA film speed. ASA stood for the American Standards Association. The higher the number, the ‘faster’ the film, or the more sensitivity to light. If you accidentally bought some ASA 1600 film (not likely as it was expensive), your images turned out grainy. That was the analog equivalence of digital noise. If you were in the middle of a shoot and needed faster film, you took the film out of your camera and put in a fresh roll at the correct speed.
Today, in digital cameras, the sensitivity to light is adjustable in-camera. You do this with a setting called ISO. ISO stands for International Standards Organization. ISO is numbered the same way as film. On the low end, most cameras operate at ISO 100, the same as the old ASA 100 film. Some newer cameras go lower. On the other end, it gets higher every year. It is probably higher than you are likely to need unless you are shooting in almost total darkness.
Why is it important?
Like that of the film, the speed of a modern digital sensor is part of the exposure triangle. The exposure triangle consists of aperture (the size of the hole that allows light in), shutter speed (how long that hole stays open), and ISO (the sensitivity of the sensor to the light).
As with the other factors, ISO is measured in stops, and also, like the other two, each stop doubles or halves the light. This bears repeating; each ISO setting doubles or halves the amount of light, or in this case, sensitivity to light that hits the sensor. So, ISO 200 is twice as sensitive to light as ISO 100, so you could shoot at a faster shutter speed or a smaller aperture.
So why not just raise it to 64,000 or higher and leave it?
Remember the grain I mentioned in film? It’s still there, only now it’s called noise. With each new update in camera technology, the noise gets less noticeable, and post-processing software is getting better. But it’s still the case that the higher the ISO, the noisier the image. It is up to you and your use what is acceptable.
How do you use ISO in photography?
Many beginners and hobbyists leave their cameras in some variation of automatic mode. This means the camera will decide all three factors of the exposure triangle and change them as needed. I think ISO is the best reason not to do that. Many cameras and most cellphones will crank up the ISO before adjusting other factors when the light isn’t good. That’s why most cellphone images look plastic; too high an ISO coupled with too much noise reduction.
Manual or auto ISO settings?
Some professional photographers always shoot in manual mode. They will manually set all three factors and then adjust one or more when the light or situation changes. Personally, I only do this when shooting with strobes or flashes, or in very unusual lighting conditions.
Many photographers, including myself, shoot in either aperture priority or shutter priority. This means you select one of those factors, and the camera selects the other. But what about ISO?
Well, unless you select Auto ISO, you choose that as well. So in one of these modes, you are manually selecting two sides of the exposure triangle, and the camera selects the third. If your camera supports auto ISO, I highly recommend you learn how to use it and incorporate it into your daily shooting.
Choosing an ISO setting
So what setting is best? As with almost every question in photography, the answer is, it depends. But usually, you should avoid digital noise, so a lower ISO is best. Rather than fiddling with this setting all day, I tend to set it once and leave it unless there is a significant shift in the light.
For years, I would set it as ISO 100 if I was outdoors on a sunny day, 400 if it was cloudy or the light was variable, and 800 if I went inside or into deep shade. Now, I use auto ISO and set all the parameters in the camera, including my maximum allowable ISO and my minimal acceptable shutter speed. There is no point in getting a noise-free image if it suffers from camera shake.
No matter which mode you shoot in, you always need to double-check your settings as the light changes. I usually shoot in aperture priority with my ISO setting as described above. This means the camera will select the shutter speed. But I check that shutter speed with every shot to make sure it isn’t too slow. If I move into the shadows and the camera drops the shutter speed to less than 1/30th second or so, that’s a problem. In that case, I have to adjust the aperture or ISO. Which one? You guessed it; it depends.
That is why I like auto ISO so much. Once I set those limits, the camera will do its best. If I try to shoot outside those boundaries because the light is too low, it will warn me. I can then decide what to do about it. If I’m shooting on a tripod, it doesn’t matter; I will use the lowest ISO and let the shutter speed drop to where it needs to. Most photographers with any experience are familiar with and know how to use aperture and shutter speed. But ISO is the third part of that triangle, and we must take it into account. Learn how to select the best ISO for each situation and raise your photography to a new level.