What is ISO in photography?

What is ISO?

ISO refers to the sensitivity to light of the recording medium, or the sensor in digital cameras or film. The higher the ISO, the more sensitive the sensor/film is to light.

How does ISO work?

The right combination of aperture, shutter speed, and ISO will give you the correct exposure. In the film days, once you had a roll in your camera you were stuck with whatever ISO that roll was. But with digital, you can change the ISO for every shot if you wanted to, which gives you incredible flexibility.

As mentioned, the higher the ISO, the more sensitivity to light. So an ISO of 100 is not very sensitive to light and will require a longer shutter speed or larger aperture to allow more light to hit the sensor. An ISO of 800 is more sensitive and will require less light for a correct exposure all other things being equal.

ISO numbers usually double up, and each step gives one more stop of light. Common numbers are:

100 - 200 - 400 - 800 - 1600 - 3200

Newer cameras even go to 6400 and higher.

Why is ISO important?

Because as with aperture and shutter speed, it has a significant influence in the final photograph. However, as with everything in photography, there is a trade off.

A high ISO is more sensitive to light. It allows you to shoot in low light situations because it'll let you shoot using a faster shutter speed that with low ISO. For example, imagine that at ISO 100 the meter in the camera gives me the following combination of aperture and shutter speed:

• ISO 100, f/2.8, 1/15

At 1/15th of a second, the shutter speed is too slow to effectively shoot hand held without causing blur just by your own movement. But if we increased the ISO to 400, we'd gain 2 stops of light, bringing the shutter speed up to 1/60th of a second, or:

• ISO 400, f/2.8, 1/60

Unfortunately, the trade of is digital noise in digital cameras or grain in film. The higher the ISO the noisier/grainier the image, and this is not always desirable. Film grain can look pretty cool sometimes, but digital noise is practically always terrible and it's usually difficult to remove effectively.

As a general rule, shoot with the lowest ISO you can get away with to get the sharpest image with the least amount of noise. However, if you need to, don't be afraid to bump the ISO as far as you need to. Sometimes it's better to get a noisy shot than no shot at all!

Fortunately, camera manufacturers are getting really good at managing noise in digital cameras and new models tend to perform much better at high ISO. Also, keep in mind that generally bigger sensors are better at handling noise in higher ISO, so if you're into shooting at night or in low light situations, it may be worth investing in a full frame digital SLR.