ISO: Adjusting Your Camera’s Sensitivity To Light
The ISO setting affects how sensitive the camera’s sensor is to the light that falls on it. It is the third element in the triangle that determines the overall exposure of the image, alongside aperture and shutter speed.
ISO stands for the International Standards Organisation, and harks back to the days when photographs were captured only on film, rather than a digital sensor. The ISO ratings created an industry standard for the sensitivity of different film types. ISO 100 film was less sensitive, so ideal for use in brighter situations, such as outdoors, whereas ISO 400 film was more sensitive and used for indoor shots, for example. The trade-off for using film that was more sensitive was ‘grain’ – visible speckles or texture on the final shot.
The digital version of grain is ‘noise’. Ramping up a digital camera’s sensitivity to light causes the pixels on the sensor to record detail that doesn’t actually exist, resulting in ‘noisy’ images that have coloured pixels in areas that should just be one even tone. It’s the visual equivalent of playing an old recording at a high volume – a background hiss eventually becomes audible. This is a phenomenon present in all electronic devices that transmit signals, but one that is becoming less and less of a concern in digital photography due to the pace of progress by the manufacturers and software developers.
Only a few years ago, any digital photographs taken at ISO 800 or above would be negatively affected by the presence of noise. On new equipment, it is now possible to photograph at ISO 1600 and above without any problems.
Setting the ISO
You will need to select the ISO setting whenever using Aperture Priority, Shutter Priority or Manual Mode. On most DSLRs, there is an ISO button which you hold down while rotating the main control dial. The current setting will be visible on the main control panel while the ISO button is pressed.
Typical ISO settings start from 100 and then double each time – 200, 400, 800, 1600, 3200 – although many cameras now offer settings in between, too. The lower the number, the less sensitive the sensor is to light. Each time you double the number, you double the sensitivity and therefore the brightness of the exposure.
To fully understand how the ISO affects exposure, take a series of shots in different light levels at each setting. Study your images afterwards to find out at which setting noise becomes visible on your DSLR, particularly checking the darker areas where it tends to be most evident. Compare these shots with photos taken on a compact camera or smart phone in low light. As smaller devices have less space, they have much smaller image sensors. Although a manufacturer might boast about the high number of mexa-pixels that a compact camera or smart phone can capture, these pixels are smaller than the ones on a DSLR sensor, and are therefore much more likely to misfire, resulting in digital noise.
If you have a more recent DSLR and/or one with a large sensor you are likely to be pleasantly surprised by how high the ISO can go before you start to see a detrimental effect on image quality. Constant improvements in photographic technology mean that digital noise will continue to becoe less and less of an issue, and image processing software will become better at mitigating it when it does occur.