How to choose ISO
ISO – This is the sensitivity of the sensor to light. Much the same as it was in the days of film the lower the ISO number the less sensitive the sensor is to light and the higher the ISO number the more sensitive the sensor is to light.
As mentioned in a previous lesson, ISO numbers are arithmetic; that is, an ISO number that is twice as high as another ISO number is twice as sensitive to light (+1 EV*). Each time an ISO film speed is doubled, the exposure should be halved. When the ISO is halved (-1 EV*), the exposure should be doubled; for example, if the correct camera setting is 1/250 second at f/16 with ISO 100 film, the same subject photographed with ISO 200 film would require only half the exposure or 1/500 second at f/16 or 1/250 second at f/22, and so on.
The factory default setting for ISO in many cameras is “Auto”. This can be convenient at times to allow you to concentrate on a subject without having to think about setting ISO. It is especially helpful when photographing some types of action, such as a child or animal, where the subject may move from an area of bright light to shadows and back again. However, there are times when you will want to take control of the ISO. Whether your goal is to maintain image quality or obtain consistent exposures here are some guidelines for choosing the ISO yourself;
How to Chose ISO:
When choosing the ISO I generally ask myself the following four questions:
1.Light – Is the subject well lit? If yes, use a low ISO setting for best quality.
2.Tripod – Will I be using a tripod? Use a low ISO with a tripod for best image quality in low light.
3.Moving – Is my subject moving or stationary? Using a higher ISO allows for a faster shutter speed to freeze the action.
4.Grain – How much will noise or lack of detail affect the final image? A sharp image with noise will be more acceptable than a blurred image with no noise.
What is Image Noise?
“Image noise” is the digital equivalent of film grain for analog cameras. Alternatively, one can think of it as similar to the subtle background hiss you may hear from your audio system at full volume. For digital images, this noise appears as random speckles on an otherwise smooth surface and can significantly degrade image quality. Although noise often detracts from an image, it is sometimes desirable since it can add an old-fashioned, grainy look which is reminiscent of early film. Some noise can also increase the apparent sharpness of an image. Noise increases with the sensitivity setting in the camera, length of the exposure, temperature, and even varies amongst different camera models.