Monday, October 24, 2011

WHAT IS AN F STOP?... FOR THOSE WHO DON'T WANT TO KNOW IT ALL.

If you compare making a proper exposure to filling a bucket it may be easier to understand what an f stop is.  To properly expose an image at a given film speed or ASA or ISO (why so many names?) two things have to happen at the same time.  The shutter has to open for a given amount of time (shutter speed) and with a given diameter (f-stop).  The f-stop is actually a ratio of the diameter of the shutter opening compared to the focal length of the lense.  The focal length of the lense can be from around 18mm, very wide angle, to 1000mm, very long telephoto.  The larger the diameter of the shutter hole, the smaller the f-stop number expression.  f2= very wide opening, f22= very small opening.
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This doesn't explain why you use one or the other, just what it is.  Thanks, and keep shootin.