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APERTURES AND SHUTTER SPEEDS:

Inside just about every camera is some form of light meter. The job of this meter is to set (or to tell you to set) the correct aperture and shutter speed combination to allow just the right amount of light through to the CCD depending on things such as weather, lighting conditions and CCD sensitivity. You now know the basic idea behind shutter speeds and apertures, but we now need to look at how we have a sequence of shutter speed settings and apertures (F stops) on a camera.

Aperture settings may start as large as F1 on a very expensive lens. F1 is a setting which lets maximum light through and is a setting only found on professional lenses. A more common maximum aperture on a non-zoom lens is F1.4, and the sequence would then run as F1.4,F 2, F2.8, F4, F5.6, F8, F11, F16 and possibly F22. Now, although this looks like a random set of numbers, they are based on a mathematical formula that is not really for this particular course. There is however a very simple relationship between each of these F stop settings that it will benefit you to know about. Obviously, in the list above, F1.4 is the maximum setting and lets in the most light. It therefor follows that F2 must be a slightly smaller aperture and let in a bit less light. Well yes, that's true, but how much less light? Fortunately the answer is pretty simple. F2 lets in HALF as much light as F1.4 and F2.8 lets in HALF as much light again as F2, and so it continues right through the range. Each time you close the aperture by one F stop (known as stopping down), you let in half as much light as the F stop before it.

Fortunately, to help keep things moderately simple, the shutter speeds work in exactly the same way. If we start at 1/4 of a second, our next shutter speed setting is likely to be 1/8 of a second, twice as fast and therefor letting in half the light. And so the shutter sequence continues, 1/15 then 1/30 then 1/60 then 1/125 then 1/250 then 1/500 then 1/1000 then 1/2000 and so on, each setting getting twice as fast and each setting letting in half as much light as the previous setting.

So let's imagine you have your camera pointed at a typical outdoor scene. Your camera's meter is telling you it has set an aperture of F8 and a shutter speed of 1/125 of a second to allow the correct amount of light to reach the CCD. We now change the aperture from F8 to F5.6. That means we have WIDENED the aperture by one F stop and the camera is now letting in TWICE as much light as is needed. How do we rectify this? We simply change the shutter speed UP one setting from 1/125 of a second to 1/250 of a second. The shutter is now open half the time it was, and is therefor letting in half the light it was, restoring the amount of light entering the camera to its correct level again. If we were to go the other way and CLOSE the aperture by one F stop, to F11, then we need the shutter to let in twice the light it was, so we would change from 1/125 of a second to 1/60 of a second and again the light levels inside the camera would be restored to the correct values. If we calculate this across the whole range of apertures, we can see that the camera's original suggestion of 1/125 of a second at F8 is identical to 1/60 at F11 and 1/30 at F16 and 1/15 at F22. Going the other way we could also use 1/250 at F5.6 or 1/500 at F4 or 1/1000 at F2.8 or 1/2000 at F2 or even 1/4000 at F1.4.

So if all of these combinations let through the correct amount of light, why would we want to change them? The answer to that lies in the photograph itself.

It should be fairly easy to understand how having your shutter open and close as fast as possible has the ability to "freeze" a moving object, and how having your shutter open too long when photographing a moving object would make it appear blurred. On this basis, the faster something is moving, the faster you need your shutter to open and close in order to freeze that motion. In a nutshell, for action photography, you may need to move away from the camera's suggested setting in order to get a faster shutter speed and freeze the action.

Taken at 1/500th of a second.

Taken at 1/30th of a second.

The aperture, you may think, being just a hole through which the light passes, has no effect on the final picture, but you'd be wrong. By altering the size of the aperture you can control a rather useful phenomenon known as DEPTH OF FIELD.

Let's take a look at depth of field and how it can be used.

 

Page 4 - Apertures and depth of field