The f factor

While the range of shutter speeds and ISO capability is determined by your camera, the apertures are determined by the lens, and is such an important factor that the max aperture makes up part of the name – along with the focal length. As wide apertures enable more light to strike the sensor, they enable faster shutter speeds to be used, and for this reason, lenses that have a wide maximum aperture are known as ‘fast’ lenses. Aperture is measured in f-stops, in which wider apertures have smaller numbers. It makes sense when you realize these are just fractions – arrived at by dividing the focal length of the lens by aperture gives the diameter of the opening. Shooting with a focal length of 50mm at f/4, the aperture diameter will be 12.5mm (50/4). As well as the ‘full stop’ scale, which doubles or halves the light hitting the sensor, you can select third or half stops.