CAMERA FACTS
SHUTTER AND APERTURE

When you take a picture in different light conditions you must vary the light allowed into the camera or your photo gets under or over exposed. You can do this by adjusting the camera's shutter speed or the aperture.

In most modern high tech cameras these are adjusted automatically. The camera usually has a photosensitive light detection sytem that will get an average reading of light from the target and adjust either aperture or shutter respectively. However in some you do have some control over this. In some compact camers there are high speed/action/sports modes which set a very fast shutter speed. SLR cameras usually allow you to either take partial or full control of these functions.

The Shutter

In film based cameras a shutter physically blocks the light from falling on the film. When you press the button to take a picture, the shutter moves out of the way usually for a very brief period of time and the film is exposed. With most modern camera shutter speed is automatically controlled by a light level sensor, but it is helpful to understand how they work

When cameras were first designed shutters were mechanical and had to be set manually. A series of common shutter speed were established which were expressed as a fraction of a second e.g. 1/60 is one sixtieth of a second. Common speeds are 1 (one second), 1/2, 1/4, 1/15, 1/30, 1/60, 1/125, 1/250 and so on. often manual control cameras have a B shutter setting. This allows you to open the shutter for a set length of time (traditionally you'd use a pneumatic Bulb to release the shutter so you wouldn't get camera shake. Hence the 'B'). This is handy if you want to use a long exposure of second to minutes (for example taking photos of the sky at night).

In most modern cameras the shutter is now controlled electronically and in some this allows even more flexibility for controlling shutter speeds (in automatic modes the shutter speed isn't stepped (i.e. 1/30, 1/60 etc.) but can be varied e.g. 1/270)

With digital cameras a light sensitive sensor called a CCD (charged coupling device - some cameras have a CMOS sensor) sits on the film plane. There is no mechanical shutter, but the sensor takes a reading for a brief period of time (just like the physical shutter).

Okay we've seen it as a method to control the length of exposure. But why else do you want some control of shutter ?

Well, if too slow and you can get camera shake. To get higher speeds you'll need to let more light in. To do this you can adjust the aperture.

If our photographing sports you might want to use higher speed to "freeze" an image. For example if you photograph a fast moving car at low speeds it can appear blurry due to movement in the time the shutter is open . Set you camera to a higher shutter speed (many automatic cameras have a "high speed" or "action" mode) and the car appears frozen in your pics.

Aperture

In addition to adjusting the shutter speed some cameras have a controllable aperture. This changes the amount of light let into the camera

An easy way to understand how apertures works is to compare them with the action of the iris and pupil in your eyes. In dark conditions your pupil dilates allowing more light in and hence lets you see better in the dark. In light conditions it constricts decreasing the amount of light let in. The size of your pupil (the opening or "aperture" allowing light into your eye) is controlled by the iris which sits behind the lenses. It acts as a physical diaphragm opening and closing to adjust the size of the pupil (aperture)

In cameras the aperture works in the same way. In many cameras it is fixed but in more advanced ones like 35mm SLR cameras it can be adjusted.

It gives users of cameras with adjustable apertures several advantages. Firstly by opening or closing the aperture you can adjust the amount of light falling into the camera and thereby change the shutter speed. Secondly by creating a small aperture you can increase the depth of field considerably

SLR cameras often express aperture in a series of measures called f- stops (e.g. f1.4). This is simply the focal length of the lenses divided by the actual diameter of the aperture e.g. the f-stop of a 50mm lens with an aperture of 25mm would be f2, if the aperture was 12.5mm it would be f4 and so on). Note how the f number rises as the aperture decreases

(1) Figures From
Michael Freeman
The 35mm Handbook
revised 1st edition, 1989,
Publisher New Burlington Books
ISBN 0 906286 21 2

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