What is ISO in photography?

ISO definition

In photography ISO is used to specify the sensitivity of film or a digital sensor. The acronym ISO stands for “International Standards Organization”. All that means is that technically all films and digital sensors use the same standard for ISO, meaning that ISO 100 on one camera should be the same as on another camera or film.

What does ISO mean in your camera?

ISO as I mentioned simply defines the sensitivity of your film or digital camera sensor. ISO 100 is the common choice for the least sensitive ISO you can select. ISO in film goes up to around 1600, but in digital cameras ISO can literally go into the millions. On my current camera the ISO range is from 100 to 102,400. The higher the ISO the more sensitive the sensor is to light.

Why change your ISO?

Increasing ISO increases the sensitivity to light. So when you’re shooting in a low light situation you need to increase ISO to keep your shutter speed high. And you need to keep your shutter speed high to keep your images from being blurred by camera shake. This is the main reason to increase ISO.

On many cameras its simpler and just as effective to just choose Auto ISO and let the camera pick the ISO it thinks it best.

For instance you might choose shutter priority mode to pick a shutter speed of 1/500th of a second, then also set your ISO mode to Auto ISO. This will give your camera the flexibility it needs to keep your exposures consistent while maintaing the shutter speed you specified.

Why not change ISO?

Choosing a higher ISO has one significant disadvantage, it also increases the amount of noise in the image. The increase in noise is pretty dramatic at high ISOs even on modern cameras. So if you don’t need to use a high ISO, don’t use one for the simple fact that it lowers your image quality.

The side effect of the increased noise is reduced dynamic range. What this means in essence is that you will begin to lose details in the darkest shadows of the image. The problem only worsens as you increase the ISO higher. This is why most cameras will allow you to specify a maximum ISO for Auto ISO. By setting the max ISO you tell the camera not to choose an ISO that will lower the image quality too dramatically.

ISO invariance

ISO invariance basically means that choosing a different ISO is no different than underexposing the picture at a lower ISO and boosting the exposure in post production. Many modern digital cameras are considered ISO invariant. However, the fact is it’s still better to get your exposure right in camera. That way you can actually see what the pictures you took look like while you’re shooting. Additionally, the camera is going to apply its own noise reduction algorithms at high ISO that won’t be applied at lower ISOs.

Final thoughts

ISO is an integral part of photography whether you change it yourself or you let the camera change it behind the scenes. Even so it’s become less and less relevant because of the improvements in digital sensors that have increased the usable range of ISO to previously unheard of levels. Additionally ISO invariance means that getting the perfect exposure in camera isn’t 100% necessary to get a great picture in the end.

Hope this helped you understand ISO. Leave your questions and comments below!

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