Shutter Speeds & Apertures - Page 2
An Explanation of Apertures
As well as letting more or less light into the camera the size of the aperture you choose governs the 'Depth of Field'. Depth of field means the amount of the picture, from foreground to background, that is in sharp focus. A smaller aperture will give you a greater depth of field and a larger aperture will give you a more restricted depth of field. This characteristic can be used to good effect in many ways.
If you are taking vast landscapes on a sunny day, the chances are that everything will be in focus and you will not notice this phenomenon at all. Depth of field, or the lack of it, is much more noticeable when taking close-ups. As I mentioned in the section on moving subjects, it is often desirable to render the background of your picture out of focus. This is easy to achieve by selecting a larger aperture to restrict the depth of field.
Conversely, when photographing very small objects (as in the picture opposite) getting everything in focus can be quite a challenge and may require a very slow shutter speed in order to be able to use the smallest aperture available.
The Longer the Lens the Shallower the Depth of Field
The focal length of the lens makes a difference to the depth of field available, the longer the lens the more restricted the depth of field. A wide angle lens will give you almost limitless depth of field. This is because the Depth of Field is governed by the absolute size of the aperture rather than the relative size. F-stop numbers such as f5.6 and f11 are fractions of the focal length of the lens so, as the lens gets longer, the hole gets bigger and vice versa. (see 'the technical stuff' below.)
Tip - If depth of field is important to either make sure everything is in focus or to throw some things out of focus, select the 'Aperture Priority', Av, mode on your camera. In this mode you select the aperture and the camera selects the shutter speed according to the available light to give the correct exposure.
Tip - If you are shooting in bright light and want to restrict the depth of field, use a neutral density filter in front of the lens to reduce the light entering the lens. These are available in different densities, 2x, 4x, 8x etc. each one cutting the light in half, quarter, eighth etc. In extreme circumstances you can screw a couple of them together. Although they are 'neutral density' filters and should not effect the color balance, if you use two or more together you might need a little color correction in the computer.
Technical Stuff - Shutters Speeds and Apertures
What do the numbers mean?
If you look at the exposure display in your viewfinder you will see two numbers. On a normal sunny day you might see something like '125 16' or '500 5.6'. The first number is the 'shutter speed' and is simply the time that the shutter will be open for, expressed as a fraction of a second. So 125 means that the shutter will be open for 1/125th of a second, and 500 means that it will be open for 1/500th of a second. 1/500th of a second is referred to as a 'faster' shutter speed than 1/125th, which is a 'slower' shutter speed. In reality all it means is that the shutter is open for a set amount of time.
The second number, sometimes referred to as the f-stop, tells you the size of the hole (aperture) in the lens. This number is also a fraction. The number represents the focal length of the lens divided by the diameter of the aperture. So an aperture that is 10mm in diameter in an 80mm lens will have an f-number of f/8 and the setting f/16 on the same lens will be 5mm across.
From this you can see that if you change the lens to one of, say, 160mm focal length then the size of the f8 aperture will be 20mm. However, because the diaphragm is now twice the distance from the film the same amount of light will reach the film. This is a bit complex but if you have a mathematical bent and you draw it all on paper you will see why (see inverse square law). If not, just take my word for it. Now you can see that a larger 'f' number, say f/16, is actually a smaller hole and lets in less light than f/8.
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Large aperture = small f number |
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Small aperture = larger f number |
All may not be as it seems with focal lengths
To make matters even more complicated, modern lenses, sophisticated beasts that they are, are not always physically the same as their focal length. So the good old f-stop acts as a nominal indicator of how much light will reach the sensor, rather than an accurate measurement of aperture size. This amount of light is independent of the focal length of the lens.
Also see my tutorial - ISO rating for Film Speed
Getting away from the auto settings.
An explanation of the mechanics of exposure and the side effects of choosing different aperture/shutter speed combinations.
Overriding the automatic metering system.
What they are trying to tell you.
Setting up your camera to take a series of pictures at different exposures.
Another piece of the exposure puzzle.
An explanation, strictly for the jargon heads.
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