Camera Exposure Controls

   There are 2 controls on a camera that effect exposure. The first is aperture. This is an opening in the lens that allows light to pass through the lens in a controllable manner. This control is available because you can control the size of the opening. The aperture size is expressed in f-stops. In the water bucket scenario,  think of this as a water faucet. Where you control how far you open a faucet to control water flow, you can also control how far you open the aperture to control light flow.

   The second control is shutter speed. Shutter speed is how long the shutter in front of the film or sensor is allowed to stay open exposing the film or sensor to the light passing through the aperture. The shutter speed is expressed in a time value such as 1/60 or 1/125 of a second. Shutter speed would be comparable to how long you leave the water faucet open to fill the bucket.

  These two controls plus the sensitivity of the film (size of the bucket) or digital camera sensor work together to achieve the proper exposure or fill the bucket, if you will.

   For any given film or sensor sensitivity (ASA xxx) there is a given amount of light that is required for a proper exposure. Your camera meter will tell you what this is if your camera is set to 'manual metering' or it will set the shutter speed and aperture if it is in 'auto' mode, to give you the right amount of light to represent a middle tone properly at the given film speed or sensor sensitivity.

   Now, to try and answer the second part of the question. "How do I know what settings to use"?

Like I said, it depends. Let me explain. Let's say it is sunny outside and your using ASA 100 film. Therefore, for a middle toned subject, your cameras meter should give you settings equivalent to an f16 aperture and an exposure time of 1/125 of a second shutter speed. We know this because of the Sunny 16 rule. (click the link to read what the rule is)

Lets say you look at your depth of field preview and decide that too much is in sharp focus. Assuming your camera is in manual mode, here is what you would do. Open your aperture more to give you less depth of field (area in sharp focus) by setting the aperture to a smaller number. The next full stop aperture setting is f11. This will allow one full stop (twice as much) of light to reach the film causing it to be over exposed. To compensate for this overexposure you cut your exposure time in half by changing the shutter speed to the next full stop of 1/250 of a second.

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