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How do you adjust photographic exposure?
September 12, 2011 11:05PM
With the mechanical shutter cameras the exposure [the amount of light getting to the film emulsion] is determined by two things.
1. The length of time [1/60th of a second for example] the shutter remains open to admit light from the subject, to the film emulsion surface. This is true for both focal plane shutters, and shutters built into a lens.
2. The diameter of the opening in the iris diaphragm [the device you set, utilizing the "f-stop" setting on a movable ring on the lens] in the lens.
The larger the diameter of the opening, the greater the area of the opening, and therefore, the more light allowed to enter at any given time. My memory is not what it used to be, but I think I recall that for every unit of increase of the radius of the iris diaphragm opening, the amount of light allowed to pass is increased by a factor of 4 times!
Also, if I recall correctly, each of the "clicks" [detent positions] on the f-stop ring represents a change of the amount of light passing through the lens by a factor of two times [refered to as one "stop"].
With the new digital cameras, the "shutter speed" is NOT mechanical, but controlled electronically. I do know that some of the digital cameras with interchangable lenses also still have the iris diaphrams, allowing one to control depth of "field," or focus. I am not certain about the presense of a mechanical diaphragm in the less expensive digitals which do not have interchangable lenses.
I hope this helps you to begin understanding the basics of exposure, until an expert can improve this answer.j3h.