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aperature number vs f number

Started 10 months ago | Questions thread
guinness2
guinness2 Veteran Member • Posts: 4,617
Re: Hang on.....

A 10 inch f/5 telescope has an aperture of 10 inches (254mm) and a focal length of 50 inches (1270mm). A 10 inch f/10 scope would have twice the focal length and therefore twice the magnification with the same eyepiece, but it collects the same amount of light.

It is interesting numbering, is it different than in the world of a common photography?
10 inch f/10 usually means here the lens ~250 f/10 where the first is the focal length and the latter the lens speed= maximal aperture=maximal hole the lens is capable to open to.

So I feel your example is a bit misleading, despite it is physically correct.

I mean , it is worth to mention, that the lens with the speed f/5 collects more light than the one with speed f/10 of the same focal length, when both are full open.

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Canon EOS Rebel SL3
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