Aperture

In the following tutorial, we’ll answer the following questions:

  • What is an aperture?
  • How do I change the aperture?
  • What are those funny numbers?
  • What effect does changing aperture have?

What is an aperture?

You should be familiar with the idea of the pupil in the human eye opening or closing in dark or light situations to let more or less light in. Basically there is a circular hole which gets bigger or smaller. This is an aperture, and inside a camera lens you have exactly the same thing. An aperture is a circular hole which can be adjusted to get bigger or small to let in more or less light. In a camera you typically ‘open up’ or ‘stop down’ the aperture by one ‘stop’ at a time, which is jargon for either halving or doubling the amount of light. One stop more light is double the light and one stop less light is half the light.

How do I change the aperture?

This depends on what camera you have. On a manual exposure camera there is normally a ring on the lens with numbers such as 1.4, 2.8, 4.0, 5.6, 8, 11, 16. These are f-stop numbers and control the aperture. Just twist the ring till the aperture you want is set.

On an automatic camera you need to set your camera into the correct exposure mode (basically whichever one allows you to choose the aperture rather than getting the camera to do it for you) and then typically there is a control dial. This does vary a lot between different cameras so if in doubt, ask someone to show you.

What are those funny numbers?

When you adjust your aperture you will do so by selecting an ‘f-stop’ or ‘f-number’. This number is a ratio between the focal length of your lens, and the diameter of aperture. So for example if you have a 200mm focal length lens, and are using an f-stop of 5.6 you are using an aperture of 200/5.6 which is 35.7mm. While if you are using a 100mm focal length lens at an f-stop of 5.6 then you are using an aperture of 100/5.6 = 17.9mm.

This could get very confusing because when you use different focal length lenses it affects the amount of light being collected, and when you use different apertures it also changes the amount of light getting though the aperture. However it turns out that the ratio of the two always gives a constant amount of light. So in the two examples above, while one is 200mm with an aperture of 35.7 and the other is 100mm with 17.9, they will both give exactly the same number of photons coming though the lens. This is why f-stops are used, because they can be compared directly between different lenses and different cameras.

Its important to note that because the actual aperture used is the focal length divided by the f-stop, a large f-stop number means a small aperture and a small f-stop number means a large aperture.

Explanation of those funny numbers
The sequence of numbers: 1.0, 1.4, 1.8, 2.8, 4.0, 5,6, 8, 11, 16 at first glance looks pretty random, but there is a sensible meaning behind it. Each one is a full stop and so opens (or closes) the lens by enough to double (or halve) the amount of light. Because the aperture is a circle you do this by doubling (or halving) the area of the circle. As the area of a circle is pi r^2 (where r is the radius); if you increase r by a factor of x you increase the area by a factor of x^2. We want to increase the area by a factor of 2, so we require x^2 = 2. which means x=1.4 (roughly). If you go back to the series of f-stop numbers you will notice that each one is about 1.4 times the last one. The small differences are not big enough to cause bad effects to your photos.

What effect does changing aperture have?

There is one more thing to consider when playing with apertures. When you have a small aperture (a large f-number) you are basically approaching just having a pinhole size aperture. As anyone doing physics can (or at least should be able to) tell you, this will cause everything you look at to be in focus – we call this depth of field (DoF). On the other hand when you set a large aperture (small f-number) then you will have very little in focus, basically just what you have focused on. A large aperture (small f-number) will very short depth of field. A small aperture (large f-number) will give you a very long depth of field. Depth of Field is also affected by the focal length of the lens. When you have a longer focal length lens you will get a really shallow depth of field (which can be great for making the background in a portrait appear all out of focus) while a short lens (often called a wide-angle) gives you a really deep depth of field (which is great for crowd scenes or landscapes when you want everything to be focused). When you have a long lens you will be a much shorter depth of field then with a short lens.

However depth of field can be affected by a whole host of other things, such as whether or not your subject is parallel with your film, the size of your film and potentially a whole host of other things. For your own camera system, using just a few lenses, the best way to figure this out is to experiment and see what results you get.

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