Clive,
Those big, fast lenses like a 200mm f2.8 are mushroom shaped. The big front
ends gather lots of light (mushroom cap) and then concentrate it down into
the throat of the camera (stalk).
Hope this helps, Bob S.
[EMAIL PROTECTED] writes:
<< 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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