Post by daviddewar on May 26, 2011 19:16:37 GMT -5
Understanding Exposure
by PS-RagE on Wed Oct 22, 2008 11:49 pm
In order to take control of your camera and get away from "green camera" mode you have to understand the theory of exposure. Controlling exposure is the most critical aspect of the photographic process.
The first and most basic photographic concept is that of "stops". One "stop" is the halving or doubling of any setting.
There are three settings that determine the actual exposure of any image. They are: shutter speed, aperture and ISO (aka. film speed).
Shutter speed is the length of time the camera's shutter blades are opened allowing light to reach the sensor. It is, of course, measured in seconds or fractions of a second. As you change shutter speeds, you change the way motion is captured. The faster the shutter speed, the more an image is frozen. The longer the shutter speed, the more an image is blurred. Changes in shutter speed are measured in stops. From 1/125" to 1/500" is a full stop faster. From 1/125" to 1/60" is a full stop slower.
Aperture is a measurement of light intensity and is expressed in f-stops. The f-stop is a ratio of the opening in the lens versus its focal length. The smaller the f-stop number, the larger the opening and thus the more light that can reach the camera's sensor. The standard f-stop series is f1, f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22 and f32. Each progression represents the doubling or halving of the amount of light to reach the sensor. f5.6 is half the light of f4 and f2.8 is double the amount of light. Varying your aperture affects the depth of feild of an image. The depth of feild refers to how much of the photograph is in focus. For example, when taking someone's portrait, you might use a larger aperture (smaller f-stop number) so as to keep the background out of focus. When photographing a landscape, you want the entire image to be sharp so you would use a small aperture (larger f-stop number).
In a properly exposed image, shutter speed and aperture are exact opposites of each other. If you "stop down" (make smaller) the aperture, you must increase the shutter speed. For example, on a sunny afternoon, your meter indicates a properly exposed image with a shutter speed of 1/2000" and an aperture of f5.6. However, for this photo we desire a large depth of feild so that the foreground and background are both in focus. We decide to "stop down" to f16 which is three full stops: f5.6 to f8 to f11 to f16. Since we have decreased the amount of light reaching the sensor, we now have to increase the length of time the shutter is open. Therefore, we decrease the shutter speed by three full stops also: 1/2000" to 1/1000" to 1/500" to 1/250"
The third setting that affects the correct exposure of an image is the ISO setting or film speed. Changing this setting alters the sensor's (or film's) sensitivity to light allowing you to further increase your shutter speed or stop down your aperture. ISO is, you guessed it, also changed in stops: 100, 200, 400, 800, etc. Note that as you increase the ISO you also introduce noise into the image as the amplifier gain is increased. For example: we are shooting a baseball game with 200mm lens "wide open" at its a maximum aperture of f2.8. However, in the dim evening light the highest shutter speed we can get is only 1/30" of a second. This is far too slow to freeze the action of the game so we decide to increase the ISO setting from its base of 200 up to 3200 (four stops). This now allows us to increase our shutter speed by four full stops to 1/500" of a second.
A work in progress - to be continued......
A. Dale McLean
aka PS-RagE
www.bugography.com
PS-RagE
Site Admin
Posts: 586
Joined: Wed Oct 22, 2008 5:21 pm
Location: Porcupine, Ontario
Top