What is "aperture"?
Aperture refers to the size (or width or diameter) of the opening of a lens diaphragm inside a photographic lens.
The size of the diaphragm opening in a camera lens regulates the amount of light that passes through onto the film inside the camera while the shutter curtain of the camera is open.
The size of the aperture inside a lens is usually adjustable, and its usually also adjusted in fixed steps (or stops).
f stops
Aperture size is usually measured in f-numbers or f-stops. These are the numbers engraved on the lens barrel and are pretty well standard on all cameras. For example, most convential lenses have at least the following f-stops: f16; f11; f8; f5.6; f4; f2.8; f2.
Each of these values represents a doubling of the amount of light entering the camera, reading from left to right, or halving if you read them from right to left.
That is, f11 will let in twice as much light as f16; f8 twice as much as f11 etc. Therefore, f16 lets in half as much light as f11.
Confusing... as the numbers get lower, the amount of light increases. That said, other than the numbering, everything else is consistent.
(© Jared C. Benedict)
A LARGE aperture is a BIG hole that lets in LOTS of light (but has a SMALLER f-stop number).
A very common way of expressing aperture is, unforturnately for the beginner, simply in terms such as "small" or "large" and these are referring to the size of the hole - not the number on the barrel of the lens.
You will get used to it - trust me.
Impact of aperture on bulb photgraphy
In bulb photography the aperture is very significant and is an extremely powerful tool.
For example, if you are trying to photograph a person who is attempting to remain still, and (let's say) the length of the shot at f16 is 20 seconds (an awfully long time for a person to remain still) then if you were to change the f-stop to f4 (increase the aperture) you could (on the face of it anyway) take the shot in about 1 second or less.
Conversely, if you are uncertain as to how much light is going to get onto the film when you take a shot, the longer the exposure, the more room for error you have (the greater your flexibility).
Simplistically, a one second error on a 2 second shot can be either half as much light as you need (a disaster) or about 1 and a half times as much as you need, whereas a 1 second error on a 20 second shot is pretty well insignificant.
Despite all this, the most visually obvious impact is the same as other types of photography: depth of field.
Historical context
As with many photographic terms, the term "f-stop" has been around many years.
Early cameras did not have an adjustable diaphram (like that made famous in the opening credits of James Bond films). Instead they used a set of panels to control exposure. Each panel had a different sized hole drilled or cut into the middle.
A panel (if one were used at all) would literally stop the amount of light that entered the camera.
