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How Does ISO Work?

The last two articles concentrated on aperture and shutter speed, and their relation to each other. This time we’re going to look at ISO and how this connects with the other two.

ISO actually stands for International Standards Organization, which doesn’t give you much of a clue as to what it actually does!

What is ISO?

Basically, ISO is a measurement of how sensitive your camera is to the available light. The lower the ISO number on your camera, the less sensitive it is, while the higher you go with the ISO numbers, the more sensitive it is.

When you increase your ISO above a certain level, you can get more light in low light conditions, but that increase in light comes with a trade-off in terms of picture quality. The higher the ISO, the more digital noise or grain becomes apparent in your images. Noise literally looks like small grains all over your image.

You can see an example of this below. This image was taken at high ISO, and if you look closely, you can see tiny specks all over the image, but it’s especially noticeable in the sky areas.

Base ISO is the lowest ISO number your camera can go to. Some start at ISO 60, others at ISO 100. This is the lowest ISO number of your sensor that can produce a photo of highest quality (no noise.)

When possible, try to stick to your camera’s base ISO number for the highest quality images, unless you are trying to get a noisy, grainy effect or there is no other way to get the shot.

How ISO is Measured

It’s important to know that ISO increases by the power of 2. The sequence goes: 100, 200, 400, 800, 1600, 3200, 6400 etc. This means that each time you adjust your ISO upwards, you are doubling the sensitivity of your sensor. ISO 200 makes your camera twice as sensitive to light as ISO 100, while ISO 400 makes it four times more sensitive to light as ISO 100.

Because increasing ISO makes more light available to your sensor, it means you can shoot at faster shutter speeds in low-light situations without the need for a tripod. In my opinion though, it’s always better to use a tripod and shoot at slower shutter speeds than shoot handheld at a higher ISO and have a seriously grainy image.

Yes, you can get noise reduction software, or use the noise reduction in Lightroom or Photoshop, but you have to use it sparingly, or you can end up with unwanted effects. I’ll show you a quick way to reduce noise in Lightroom later in the article.

The image above was taken with a low ISO and a low shutter speed. The image is clear and there is no obvious noise in the photo.

How Does ISO Affect Shutter Speed and Aperture?

Because ISO affects the amount of light coming in to your camera – like shutter speed and aperture – changing it has an effect on them, and hence your camera’s exposure.

For example, say you have your camera at ISO 100, shutter speed of 1/125ths of a second at f/5.6 and the exposure is fine. If you increase your ISO to 200, your photo will now be overexposed, as your sensor becomes twice as sensitive to the light coming in. You would have to increase your shutter speed, decrease your aperture (larger f-number) or both to regain correct exposure at ISO 200.

The same is true for the opposite scenario. Say you have your camera at ISO 800, shutter speed 1/125ths of a second, aperture at f/5.6, and it’s correctly exposed. If you then put your ISO down to 400, your image will be underexposed, because you’ve reduced your camera sensor’s sensitivity to light by half through dropping the ISO. You would have to reduce your shutter speed, open up your aperture (smaller f-number) or do both to gain correct exposure again.

So, if you adjust one of the three – aperture, shutter speed or ISO – you will have a direct effect on your exposure.

As you become more used to using the three together, dialing in the correct settings for any given lighting situation will become second nature to you, but it means getting your camera off auto or program mode and in to fully manual to experiment.

It can seem like you’ll never get the hang of working all three together to produce a correctly exposed image, but you will! It’s like driving a manual-shift car or truck – you think you’ll never be able to work the clutch, accelerator, gearstick and everything else, but now you just jump in your car and drive without thinking about it. You’ll get to this stage with your camera settings too. It just takes practise.

Noise – The Unwanted Effect of High ISO

Sometimes you have no choice but to shoot at a high ISO – and that means your images will have tiny, grainy little specks and splodges of color. Different cameras deal with noise differently, and if you are going to make a habit of low-light shooting, you should perhaps consider buying a camera that has a reputation for low noise levels at high ISO’s.