Monday, March 3, 2008
Quiz
Here's a little quiz for photographers: Without looking up the answer, what does the f-stop on a lens represent? I'm not asking what it does. Obviously the aperture regulates light, and higher f-stop numbers let in less light. But how is the actual number arrived at? What does 2.8 or 5.6 measure? A free print for the first person to correctly answer.
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5 comments:
It's the focal length divided by the diameter of the aperture.
As it is marked f/A, it means f divided by A.
f = focal length
A = f-stop
And the result is diameter of the aperture in millimeters.
F-Stop = (diameter of the) aperture / focal length
F-stop is a simple ratio and you compare f-stops on a relative basis rather than absolutely. A lens is circular. We can measure the amount of light entering through the lens by determining its area. The area of a circle is Pi * r^2. To double the area, r simply needs to be multiplied by the square root of 2, because the square root of 2 squared is 2! That's exactly why the values above differ by that seemingly arbitrary number.
Congratulations, Derek. You get a print. Email me your address and I'll put one in the mail. I guess I'll have to make it a bit harder next time...
My email is in the mail!
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