#### dbrandon

##### TPF Noob!

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I see exactly what you are saying, and no i dont own a 3 foot long lens, but that shouldn't affect the maths behind aperture size.

Wikipedia ...

"In optics, the f-number (sometimes called focal ratio, f-ratio, or relative aperture[1]) of an optical system expresses the diameter of the entrance pupil in terms of the effective focal length of the lens; in simpler terms, the f-number is the focal length divided by the aperture diameter."

Goes on to give the equation

where f is the focal length, and D is the diameter of the entrance pupil

This clearly shows that f2.8 on a 50mm lens is going to be greatly different to f2.8 on a 200mm lens. Unless i am interpreting the equation completely wrongly.

Wether this means the DOF on a 200mm will be shallower due to the larger pupil, i do not know.