Aperture Tutorial
This aperture tutorial introduces the fundamentals of what the camera aperture is, how it works, how you use it, and what effects its use produces.
What is the aperture?
Your eyes work like a digital camera. Light passes through the cornea, which acts like a rough lens. It then passes through a lens which can be adjusted to focus the light.
The light passes through the pupil and into your eye, where it is focused onto the fovea, a highly sensitive part of your retina which "sees" the focussed light.
The useful part of the analogy here is the pupil in your eye (the black circle in the centre). The size of the pupil is varied by the iris, which contracts and expands to control the amount of light which enters your eye.
When you look at the bright sky, your iris contracts and the pupil becomes very small, to prevent damage to your retina. In the dark, it expands as wide as it can.
Look at your eyes - the pupil is a circle and changes size such that it remains a circle. The iris is the coloured part of the eye.
(For sci-fi geeks like me, you can also use the "iris" of the Stargate as an analogy!)
So, the aperture is a circlular hole which changes in size to vary the amount of light which reaches the digital sensor.
Manipulating the aperture size
Aperture size is measured in "f-numbers". The f-number is used in optics and is the focal length of the lens divided by the diameter of the aperture.
If the focal length of the lens is fixed (let's assume) then the f-number is inversely proportional to the aperture diameter
We don't say "the aperture diamater is 20mm" because it isn't very useful. f-numbers become useful when we consider depth of field, which is directly related to aperture size.
How does it work in real terms?
As f-number is inversely proportional to aperture diameter, the f-number increases as the aperture diameter decreases, or put another way, a low f-number is a large aperture size, which will let more light in.
How do f numbers work?
You can have direct control over the aperture size using f-numbers. The f-numbers available to you will depend on your lens (for example I have a lens which says on it "70-200mm f/4 L" - the "f/4" refers to the largest available aperture size the lens can provide).
f-numbers are always written as f/number, for example f/64 or f/2.8 etc.
Just remember that a large f number means a small aperture size, and a small f number means a large aperture size.
I hope this brief aperture tutorial helps you to understand more about what the aperture is and how it works.