One of the first things that we learn when we start taking photographs seriously is that ‘aperture’, the size of the hole in the lens through which light passes, controls depth of field.
A large aperture creates shallow depth of field while a narrow one creates wide depth of field. But there’s a little more to aperture than that, let’s take a closer look at this most fundamental photographic control.
1. Doubling and halving
Apertures are often referred to as stops. Opening up the aperture by one whole stop or 1EV (exposure value) doubles the amount of light passing through the diaphragm, while closing down by one stop halves it.
However, modern cameras are usually set to adjust aperture in one third stops, something that can confuse novice photographers.
If you wish, it’s usually possible to set a camera to adjust in half or full stops via the custom menu.
The full stop aperture settings that you are most like to encounter are: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 and f/32.
Other settings such as f/3.5 and f/6.3 are fractions between these whole stops. F/3.5 could be thought of as f/2.8 and 2/3, for example, and f/6.3 as f/5.6 and 1/3.
Understanding the doubling and halving effect of aperture is helpful when setting exposure and deciding which shutter speed and/or sensitivity setting to use.
If shutter speed is kept the same, the difference in exposure between opening up the aperture from f/8 to f/5.6 is the same as pushing sensitivity up from ISO 100 to 200; the image will be one stop brighter in both cases.
Similarly, if sensitivity is kept the same, the difference in exposure between a shutter speed of 1/125 sec and 1/60 is the same as adjusting from f/8 to f/5.6; again it’s one stop brighter.