ISO is a weird part of the Exposure Triangle. There isn’t a physical component with it like there is with aperture (an opening at the back of the lens that opens and closes) or shutter speed (where a physical cover opens to reveal the sensor for a brief period of time). ISO describes how much amplification the signal receives as it’s going from the sensor to the processor. The more that signal is amplified, the grainier it gets.
So what does graininess mean for photographs?
Let’s go back in time a couple of decades.
Back in the film days, saying that a photograph was ‘grainy’ made perfect sense. Film relied upon silver halide crystals which, when exposed to light, react to it based upon their size and other characteristics. The larger the crystals, the faster the exposure can be made. This is great for sports and extremely low light situations, but it causes the pictures to look kind of like a cloud of confetti was in the air when the picture was taken.
That speckliness you see– especially in the darker areas– shows the film’s grain. Because I used an 800-speed film for this photograph, you can clearly see that grain. Had I used a slower film speed, like 100 or 200, the silver halide crystals would have been much smaller and so you wouldn’t be able to see them as well.
In the digital era, silver halide crystals have gone by the wayside, but we still worry about graininess.
Have you ever taken a picture with your smartphone in a restaurant or some other dark place without the benefit of a flash, then looked at the result and wondered why the color is weird and everything looks a little fuzzy?
I took this picture on my phone at night during a snowstorm last winter (hence the white streaks). See how the building looks a little fuzzy and the night sky is kind of a wonky purple/gray/green? That’s the result of a high ISO number. When you set the camera’s ISO number higher (800, 1600, 3200, 10,000 and up), the camera has to work harder to make the picture. High ISO numbers allow you to use faster shutter speeds in low light, so they are useful when you’re photographing indoor sports or if you can’t use a flash indoors, but you’ll want to remember that they will cause graininess, (or ‘noise’, though there’s no actual sound).
A low ISO number (like 50, 100, or 200) will give you a sharper picture that lacks graininess. The detail will be sharp and refined, and the color will be more accurate. It’s the look we’re all going for.
I photographed this rose with an ISO of 160. It’s a nice, clean image with none of the noise or grain from the previous photographs.
So why don’t we use low ISO numbers all the time? Because the lower the ISO, the more light you’ll need whether you have a sunny day, additional light from a flash, or a very long shutter speed. We don’t always have the luxury of a sunny day, we can’t always use a flash, and most situations don’t allow for long shutter speeds. So we have to push that ISO number higher to counterbalance low light. The trick is to choose an ISO number that’s as low as the situation will allow for but is high enough to give you the shutter speed/aperture combination you need.To see how ISO affects your exposure, try putting the camera into the ‘P’ mode (that’s Programmed automatic, where the camera sets the aperture and shutter speed for you, but gives you control over ISO and a few other settings).
There’s no silver bullet, but here’s a basic guide:
- Sunny day: ISO 100-400
- Indoor/Outdoor/Cloudy day: 400-800
- Nighttime, Indoor sports: 800 and up
Most current cameras have ISOs that go far higher than film ever allowed for. Mine, for example, goes to 204,800. I do not push the camera that far. I don’t need to. Usually, an ISO of 3200 is high enough for what I need to do.
Here is a series of examples of photographs taken at differing ISO settings. Notice how the graininess increases as the number increases.
Note: Cameras with small sensors (point & shoot or smartphone cameras) produce more digital noise. Why? It’s because the millions of little pixels that make up the sensor are extra tiny on those little sensors, and because they’re so tiny that 1) they don’t take in as much light as cameras with larger sensors and 2) they have to work a lot harder when the ISO is increased.
To see what happens when you change the ISO on your camera, try taking the same picture multiple times at different ISO numbers to see what happens.