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ISO and noise

The concept of ISO sensitivity was introduced in Chapter 1 (ssss1). Many cameras can regulate it automatically but, as suggested already, this is one of the most important settings of all, so it really pays to understand what it means and at least consider taking direct control.

Ridge above Matho Valley, Ladakh (Chiz) Manual mode is a good choice for controlling tricky exposures

The ISO setting doesn’t just affect the interplay between aperture and shutter speed. It also has a direct impact on the quality of the final image. Increasing the ISO tends to reduce the dynamic range and the intensity of colours, but what’s usually most noticeable is an increase in image noise. Noise is random variation in brightness and colour which produces a kind of speckly interference in images. Most cameras have various noise reduction features, and you can also address it when processing images on the computer, but strong noise reduction can make the image softer overall. The range of ISO settings which is genuinely usable depends on your tolerance of noise and how you view the images (large screens or large prints show up faults more readily). Small-sensor cameras suffer much more from noise, so the usable ISO range runs out more quickly. The top limit for decent-size prints from a compact is probably 800 or even 400 ISO, whereas a full-frame SLR will still be delivering acceptable results at 3200 or higher.