If you remember playing with a magnifying glass you will recall
that the image goes down to a point before it widens out again.
So the narrow point can go through the lens mount without being
cut off but the amount of light is not diminished.
--Tom
Clive Williams wrote:
>
> Any elementary book on photography explains that aperture values are
> expressed in terms of the diameter of the opening as a fraction of the
> focal length, so for a 100mm lens, f/4 indicates a diameter of 25mm. OK
> so far, but how does this work at wide apertures on long lenses? I
> can't measure a line longer than about 50mm across the throat of a K
> mount, and yet I could fit to it a lens of 200mm with a maximum aperture
> of f/2.8, which would require a diameter of nearly 71mm.
>
> What I don't understand, Inspector, is how a 71mm-wide disc of light can
> pass through a 50mm-wide hole. There are those on the List who
> evidently understand optics beyond the elementary level, so can anyone
> please explain?
>
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