"Exposure" is the total quantity of light that reaches the sensor/film. A bright light for a short time or a dim light for a long time = the same "exposure".
The "aperture" (f/8, f/16, f/22, whatever) describes the hole light passes through, and "shutter speed" describes how long the light is flowing. Think of a faucet and water. You can open the faucet up wide and it takes less time to fill the glass.
Apertures are defined as "f/stops". An f/stop is the ratio between the physical size of the hole int he lens and the focal length of the lens. A 25mm hole in a 50mm lens is f/2.0. A 25mm hole in a 100mm lens is f/4.0. The reason it's doen this way is because the amount of light falling on a spot is diminished by the square of the distance the light must travel, so it takes a physically bigger hole to project the same amount of light on the sensor in a longer lens. The ratio, the f/stop, eliminates that variable from your calculations.
The smaller the number, the larger the aperture. f/2.0 is a larger aperture (lets through more light) than f/8.0....
Shutter speed describes how long light is allowed to fall on the sensor. Twice the shutter speed = twice the light (or "exposure"): 1/15 of a second is twice as long and twice the exposure compared to 1/30 of a second.
The more you "stop down" (make the aperture smaller) the longer you have to leave the shutter open to accomplish a given exposure.
Depth of Field (DOF) describes how much in front of and behind the point of focus will be in acceptable focus as far as the viewer is concerned. The smaller the aperture, the greater the DOF. Sometimes you want more (everything in focus) and sometimes you don't (selective plane of focus centered on object, BG and foreground both blurred, for example).
Will this do for starters?
R.
Message edited by author 2005-10-01 01:59:13. |
