In very simple words, aperture is the “size” of the hole the light goes through when it passes the lens. So large apertures will let more light go through then small apertures. Going back to the pipe allegory analogy, we can see the following: If we use smaller aperture, then to keep our exposure unchanged we have to use longer shutter speed, or higher ISO.
Measuring apertures – Aperture values are given in incremental F stops. starting fro 1/1.4 up to 1/64 (there are more F stops out there in the world but 1/1.4 to 1/64 are the most common ones). The F stop is related to the diameter of the hole though which light comes through. Actually most lenses have a diaphragm (hey! you! in the third raw. I was you laughing) that can change its diameter. Much like the Iris in our eyes. An F value of 1/1.4 will let more light in then an F value of 1/64. This is where F stops come into play
F stops – As I said before, we use F stops to note the aperture size. Since each aperture value doubles the amount of light that gets to the sensor, you will need half as fast shutter speed to get the same exposure.
Here is a list f the more common aperture values – each lets double amount of light in. I also wrote the shutter speed needed to get an equivalent exposure of f=1/2.8 with shutter speed of 1/500.
aperture
f/1.4
f/2.0
f/2.8
f/4
f/5.6
f/8
f/11
f/16
f/22
shutter
speed
1/2000
1/1000
1/500
1/250
1/125
1/60
1/30
1/15
1/8
This table is handy for knowing how to set your camera for different situations and use cases:
Say you want to do a panning shot, and your light meter shows 1/250 @ f/4. But for panning shot you need to have a shutter speed of around 1/40. You will push up three stops on shutter speed and get to 1/30. To maintain the same exposure, you will need to stop the aperture down three stops to f/11.
Say you want to freeze water in your shot, and your light meter shows 1/125 @ f/8. But for freezing water you need to have a shutter speed of around 1/500. You will push down two stops on shutter speed and get to 1/500. To maintain the same exposure, you will need to stop the aperture up two stops to f/4.
Aperture and Depth Of Field – besides controlling the amount of light that gets in your camera, aperture also control something called Depth Of Field (or DOF). Depth Of Field is the area in which things will look focused in your image. The larger your aperture is the less depth of field you have.
Aperture is one of the three main controls you can use when you are taking a picture. Along with shutter speed and ISO, aperture controls how light will hit the sensor (OK, old schoolers – hit the film).
Reference: www.diyphotography.net
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