ISO in digital photography

By: Mradula Shet

ISO or ASA is used to indicate how sensitive traditional photography film is sensitive to light and was measured using the numbers 100, 200, 400 etc seen on films. Smaller numbers indicate the low film sensitivity and the finer grain of the shots taken.

In case of digital photography, ISO measures the image sensor sensitivity based on the same film photography principles- lower numbers indicate lower sensitivity to light and a finer grain.

It is better to use higher ISO settings in dark situations where you need faster shutter speeds like an indoor sports event where you need to freeze action in lower light. Higher ISO settings mean you get noisier shots. This is illustrated in these shots where the ones to the left were taken using lower ISO settings and the ones to the right were taken using higher ISO settings.

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100 ISO is the normal setting which gives you lovely, crisp shots with little noise and grain.

People generally set their digital cameras in Auto Mode wherein the camera selects the right ISO setting (the lowest possible) based on the shooting conditions. However most cameras let you select your own ISO too.

ISOIn case you override your camera and use a specific ISO, you find that it affects the aperture and shutter speed you need for a well exposed shot. For example, bumping your ISO from 100 to 400 helps you shoot at higher shutter speeds using smaller apertures.

Questions to help you choose ISO

There are four basic questions to ask and answer to find the best ISO setting.

  1. The first is related to light, where you need to find out if the subject is well lit.
  2.  The second relates to your deciding to use a tripod or not.
  3. The third question is based on whether you want a grainy shot or a silent shot.
  4. The fourth question is whether the subject is a moving or stationary one.

In case there is lots of light, you want little grain, are using a tripod and have a stationary subject, it is better to use a pretty low ISO rating. However if it is dark, you need grain, are not using a tripod and you have a moving subject, you should increase the ISO as it helps you shoot with a faster shutter speed and still lets you expose the shot. Here you will see that where there is not enough light, setting the ISO too low produces a poor shot:

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You do however need to be ready for noisier shots with the increased ISO. Situations where a higher ISO setting is preferable are:

  • Concerts with low light and no-flash zones
  • Indoor sports events with fast moving subjects and limited light.
  • Places like churches and art galleries that have rules against using a flash, and as it’s indoors are not well lit.
  • Birthday parties where candles are blown out in a dark room can be ruined by a bright flash. An increased ISO is better for capturing the scene.

All this proves that ISO is important in digital photography and you need to understand it if you want to gain control on your digital camera. Try out different settings, and how they affect your images today. Concentrate on Aperture and Shutter Speed as well as ISO is important for mastering the Exposure Triangle.

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