Now that we have aperture and shutter speed covered, the final part of your exposure triangle is your ISO setting. If you have any experience with film photography, your ISO is equivalent to your film speed. Essentially, your camera can increase your shutter speed artificially by increasing your ISO setting (typically ranging from 100 to 1600) for those moments when you need an extra boost. The more you increase your ISO, the faster your shutter speed will be.
The downfall to increasing your ISO digitally is that you’ll also be increasing the “noise” in your image – this would be known as “grain” for analog. It’s the camera’s way of reaching a faster shutter speed by having random pixels vary in brightness and color – the more you increase your ISO, the more noise will be present. In film photography (especially black and white), grain is usually pleasing and is used for artistic purposes by adding texture to an image. However, digital noise is of lesser quality and appears more pixelated than film grain, and is usually an unwanted side-effect.
This doesn’t mean that ISO will neccesary ruin your image – in most cases, it won’t be very noticeable or detract from your image. And if it comes down to capturing the photo you want with some ISO, and not capturing it at all, I think most will choose to handle the increase in noise. This is why I refer to ISO as my “emergency switch” – the last resort when all else fails.
ISO noise is not always going to look the same at a given setting (let’s say 800) depending on what is in your composition. Noise is more apparent on darker tones, and when there is little variation in the tones themselves. For example, a photo of the night sky with a high ISO will produce some noticeable noise across the sky – both because of the darker tones, and because the sky has little variation. Combine this with the fact that ISO becomes more apparent the longer your shutter is open, and you’ve got the perfect storm for a high-noise image. However, if you were photographing a “busy” scene – such as the snowfall image below – the digital noise will become lost and be hard to notice.
With the images above, I wanted to keep a deep depth of field, but also capture the falling snow. Since I was working with limited light, I couldn’t reach a fast enough shutter speed to capture the flakes without widening my aperture – which I didn’t want to do as that would make my depth of field more shallow.
My only alternative was to increase my ISO from 100 to 1600, which let me reach a fast enough shutter to freeze the flakes as they fell. Since this was a busy scene with many lighter tones, the pixelation from the increased ISO is barely noticeable – even to the trained eye.
Also know that your sensor size determines how obvious your noise will appear – the larger the sensor, the less likely noise will become a distraction. For example, an ISO setting of 800 will generally look horrible on a compact camera (such as those on cell phones and iPads), decent on your average DSLR, and almost unnoticeable on a full-frame sensor. So for landscape photographers who use an SLR, digital noise is not much of an issue – especially when it comes down to getting the image you want.
If you’re using a tripod, and you don’t need a fast shutter speed to capture movement, then your ISO should remain at 100. The tripod will eliminate any issues you would encounter with camera shake, so for the highest-quality image, your ISO should be on the low side.
However, if you’re working hand-held or your shutter speed needs to be fast, then things get a bit more complicated. If you find yourself in a low-light situation without a tripod (i.e. indoors or during the twilight) where you’ve opened your aperture as far as it will go and still can’t get a sharp enough image, it’s worth increasing your ISO in order to capture the moment.
You can also increase your ISO when you have to keep your shutter speed at a certain setting – such as when you’re photographing a moving subject. You can run into this situation more often than you think – for example, when you’re trying to capture falling rain or snow. If your lens is open all the way and your shutter speed is still too slow to capture the action, increasing your ISO will give you a bump in shutter speed.
Another scenario where your ISO can help is when you are photographing handheld and need to keep a deep depth of field – in other words, your aperture is small, and not much light is being let in through your lens. If your shutter speed is too slow, you’ll run into issues with camera shake, ultimately affecting your image sharpness and clarity. By increasing your ISO (and thus increasing your shutter speed), you can capture a sharp landscape without having to mount your camera.
So with this new knowledge, you can see how your ISO, shutter speed, and aperture work together to create what is called the exposure triangle. If you change your ISO, you affect your shutter speed. If you change your shutter speed, you increase or decrease the amount of time that the light is shown on your sensor – which means that you need to either widen or close your aperture to compensate. Alternatively, if you adjust your aperture for a different depth of field, you need to change your shutter speed to compensate for the increased or decreased amount of light let in through your lens.
In short, all three settings work in harmony to create the exposure you want – you can’t adjust one setting without changing another if you want to maintain the same exposure.
It’s best to focus more on aperture and shutter speed and how those two work together by experimenting with your camera and taking as many manual photos as you can. Sooner or later, you’ll come across a situation where you can’t increase your shutter speed by adjusting your aperture, which is when you should pull the “emergency switch” and bump up your ISO.
You can remove the noise later in post process if necessary, but most noise removal workflows will only soften your image, which will detract from your sharpness. I would use noise removal only if it’s incredibly distracting, or if it’s on areas where sharpness isn’t important – such as the blank night sky. Regardless, it’s always better to have a photo with noise than no photo at all, which makes ISO a great asset to have for those special situations.