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Possible Duplicate:
What does the f-stop printed on the lens refer to?

I understand that for a DSLR you can modify the aperture(the F number) to allow more or less light to enter the camera.

But than what does F2.8 in "Sigma 10mm/F2.8 EX DC Fisheye HSM compatible Canon " mean? How does the F number of lens compare with the F number of DSLR?

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You are mistaken on the first point, changing F-number on the DSLR only instructs the lens to open/close the iris (aperture) to let more or less light in. Nothing changes on the DSLR, except that the metering calculations take the new f number into account.

F/2.8 on the lens means the maximum size of the opening is the focal length divided by 2.8, although how much light the lens lets in is a function of the f-number alone (i.e. two f/2.8 lenses with different focal lengths will let in the same amount of light).

See also

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