NB: This series of posts will convey my understanding of photographic exposures. While I hope it serves a useful purpose, I by no means describe this series as a be-all, end-all resource on exposure. As ever, if you feel I have made a factual error, please correct me.
Exposure is a combination of how long and how much light an imager (digital sensor or film) is exposed to by the lens and shutter. ISO/ASA (and less commonly nowadays DIN) is the standardised measure of an imager’s light sensitivity (i.e. sensitivity dictates how much light and how long for). In Part One of this series, I shall focus on “how much” (aperture).
The aperture of a lens controls both the amount of light admitted and the depth of field of the photograph. Apertures are given as “f/ stops” or simply “stops”, which represent the lens focal length, divided by the lens diaphragm diameter. Large apertures have low numbers, e.g. f/1.4, whilst small apertures have large numbers, e.g. f/16. The minimum and maximum apertures vary between lenses, and may also vary across the range of a zoom lens.
At a small aperture, less light is admitted. You can see this visually in Fig. 1 and 2, where there is more light able to travel through the lens in Fig. 1 than Fig. 2. This results in longer shutter speeds (or higher ISO) than a larger aperture (more on shutter speeds and ISO in later posts). In low light conditions, you can open the aperture to admit more light. In bright light, you can close it down to admit less light.
Depth of field describes the area in focus fore and aft of the precise focus point. Low depth of field (low f/ number) (Fig. 3) gives little in focus either side of the subject, effectively isolating the subject from its surroundings. This is quite effective in portraiture. Conversely, a large depth of field (high f/ number) (Fig. 4) keeps more in focus either side of the subject and is often used for landscapes. Two examples of ancient rules are late landscape photographer Ansel Adams’ “f/64 Club” and late photojournalist Henri Cartier-Bresson’s “f/8 and be there” mantra.
Focus distance also plays a role: the closer you focus at a given aperture, the more you reduce the depth of field. As Fig. 3 demonstrates, even a smallish aperture (for smaller format cameras) of f/6 limits depth of field. On a personal note, this is why I tend to take close-up photographs at a maximum aperture of f/11, but most frequently f/16 or f/22. It also provides the perfect excuse to have a bigger flashgun. However, be wary of diffraction, which manifests itself at smaller apertures, and causes a loss of sharpness. The apertures at, and extent to which this occurs, vary by camera and lens combination.
So, in summary, aperture controls the amount of light coming into a camera, and contributes to controlling the depth of field of a photograph.
The (somewhat generalised) way I remember it is as follows:
Low f/ number = low light, low depth of field
High f/ number = high light, high depth of field