2. ISO

ISO is a measure of how sensitive the sensor of your camera is to light. The term originated in film photography, where film of different sensitivities could be used depending on the shooting conditions, and it is no different in digital photography. The ISO sensitivity is represented numerically from ISO 100 (low sensitivity) up to ISO 6400 (high sensitivity) and beyond, and controls the amount of light required by the sensor to achieve a given exposure

At ‘low’ sensitivities, more light is required to achieve a given exposure compared to high sensitivities where less light is required to achieve the same exposure. To understand this, let’s look at two different situations:

Low ISO numbers
If shooting outside, on a bright sunny day there is a lot of available light that will hit the sensor during an exposure, meaning that the sensor does not need to be very sensitive in order to achieve a correct exposure. Therefore, you could use a low ISO number, such as ISO 100 or 200. This will give you images of the highest quality, with very little grain (or noise).

Taken at ISO 100, the image does not show signs of noise (even when looking at the 100% crop (right)

High ISO numbers
If shooting in low light conditions, such as inside a dark cathedral or museum for example, there is not much light available for your camera sensor. A high ISO number, such as ISO 3200, will increase the sensitivity of the sensor, effectively multiplying the small amount of available light to give you a correctly exposed image. This multiplication effect comes with a side effect of increased noise on the image, which looks like a fine grain, reducing the overall image quality. The noise will be most pronounced in the darker/shadow regions.

This image was taken as the sun was going down, meaning there was not much ambient light. Therefore, this was shot with ISO4000, however you can see very obvious noise in the 100% crop (right)

Practically Speaking: you want to keep the ISO as low as possible, as the lower the ISO, the less noise and the higher the quality of the resulting image. Outside on a sunny day, select ISO200 and see how it goes. If it clouds over, maybe select an ISO between 400-800. If you move indoors, consider an ISO of around 1600 or above (these are approximate starting points).

Most digital SLRs now have an ‘auto-ISO’ function, where the camera sets the ISO depending upon the amount of light in which you are shooting, keeping it as low as possible. Auto-ISO is a very useful tool when starting out with your camera, as it is allows you to define an upper limit i.e. where the images become too noisy such as ISO1600 or 3200, and then forget about it until situations where you specifically want to override the automatic setting, for example if taking landscape images using a tripod, you can afford to use the lowest ISO possible.