Aperture - What Is It?
Aperture is simply the term used to describe the size of the "hole" in a lens where light passes through the instant the shutter is pressed and the picture taken. It is open only for that (usually) split second and the light passing through is what forms our image - either projected onto light sensitive film, or today much more likely to be a digital sensor.
Aperture is measured in "f stops" and many cameras typically have aperture settings like these marked on the lens where it joins the camera and is adjustable by us.
f 2.8, f 4, f 5.6, f 8, f 11, f 16 and f 22
More expensive cameras may have more options.
The term "stop" came about as aperture used to be no more than a series holes of fixed sizes cut in a strip of metal, which was then slid one way or the other to adjust the amount of light admitted when the shutter was opened.
Each stop either doubles or halves the size of hole and amount of light allowed to pass through the lens, so for instance f 11 allows twice as much light as f 16 but only half as much as f 8.
"Wait!" I hear you say "surely you mean the other way around?" In fact - and strangely - aperture sizes are measured in reverse order - the smaller the number the wider the aperture, so f 22 is much smaller than f 2.8 for example.
There is a crucial relationship and balance between the size of the hole - our aperture - and the time the shutter is open - "shutter speed" - typically measured in tiny fractions of a second. Given that light must fall on the film or sensor for a minimum period of time in order to form the image, and this can be varied by more or less light over a shorter or longer period, we must either have a larger opening and shorter time, or smaller opening and longer time. We can infinitely vary this relationship to achieve certain results.
For example, say we want to photograph a bird in flight. This would typically mean we have to set a very fast shutter speed to capture the detail of rapid movement of wings. Set the shutter speed too low and the wings appear to be no more than a blur. However, a very fast shutter speed in low light would result in not enough light falling on the film or sensor to create a proper, usable image. We therefore have to create a balance between shutter speed and aperture.
This generally means that to achieve a fast enough shutter speed to capture motion, we need to increase the size of our aperture - the amount of light we allow onto the sensor - in order to have a correctly exposed image the faster the shutter speed the less light is captured so we need to increase the size of the hole to compensate.
Unfortunately, we tend to get what we pay for and cheaper lenses do not allow us to increase aperture size as much as more expensive models known as "fast" lenses. A balance must be struck according to our needs and our fortune.