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ISO is one of three important settings on your camera that is used to take a well exposed photo. The other two are Aperture and Shutter Speed.
We regularly get questions about ISO from readers of Digital Photography School like these:
What is ISO and why is it important? What is the best setting to choose? Should I always choose the lowest one?’
In this short tutorial I want to answer each question in turn. Let’s start with a definition of ISO.
In traditional (film) photography ISO (or ASA) was the indication of how sensitive a film was to light. It was measured in numbers (you’ve probably seen them on films – 100, 200, 400, 800 etc). The lower the number the lower the sensitivity of the film and the finer the grain in the shots you’re taking.
In Digital Photography ISO measures the sensitivity of the image sensor.
The same principles apply as in film photography – the lower the number the less sensitive your camera is to light and the finer the grain.
Higher numbers mean your sensor becomes more sensitive to light which allows you to use your camera in darker situations. The cost of doing so is more grain (although cameras are improving all the time and today many are able to use high ISO settings and still get very useable images).
An example of a situation you might want to choose a higher ISO would be photographing an indoor sporting event where the light is low and your subject is moving fast. By choosing a higher ISO you can use a faster shutter speed to freeze the movement.
As mentioned – the cost of choosing higher ISO settings is that you begin to get higher grain or noise in your images the higher you go.
I’ll illustrate this below with two enlargements of shots that I just took – the one on the left is taken at 100 ISO and the one of the right at 3200 ISO.
100 ISO is generally accepted as a ‘normal’ or ‘standard’ ISO and will give you lovely crisp shots (with little to no noise/grain).
Most people tend to keep their digital cameras in ‘Auto Mode’ where the camera selects the appropriate ISO setting depending upon the conditions you’re shooting in (it will try to keep it as low as possible) but most cameras also give you the opportunity to select your own ISO also.
When you do override your camera and choose a specific ISO you’ll notice that it impacts the aperture and shutter speed needed for a well exposed shot. For example – if you bumped your ISO up from 100 to 400 you’ll notice that you can shoot at higher shutter speeds and/or smaller apertures.
When choosing the ISO setting I generally ask myself the following four questions:
If there is plenty of light, I want little grain, I’m using a tripod and my subject is stationary I will generally use a pretty low ISO rating.
If it’s dark, I purposely want grain, I don’t have a tripod and/or my subject is moving I might consider increasing the ISO as it will enable me to shoot with a faster shutter speed and still expose the shot well.
Of course the trade off of this increase in ISO will be noisier shots.
Situations where you might need to push ISO to higher settings include:
ISO is an important aspect of digital photography to have an understanding of if you want to gain more control of your digital camera. Experiment with different settings and how they impact your images today – particularly learn more about Aperture and Shutter Speed which with ISO are a part of the Exposure Triangle.
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