On Sat, 11 Aug 2001 07:15:47 -0400 Morgan Sparks <msparks@xxxxxxxxxxxx>
writes
> Can someone please explain this?
>
> Morgan Sparks
Morgan,
The reason a 50/2.8 will capture more stars than a 28/2 has to do
primarily with the fact that a star is a true point light source. No
matter how long your lens is (within practical limits) it still has zero
size. So all of the light collected by the front element will get focused
at the same point on the film. With an extended object, like a galaxy or
a barn door, the longer the lens is the more the light gets "spread out"
at the film plane.
So my 200/4 (50mm aperture) will image stars of the same brightness as my
100/2 (also 50mm aperture). The 100/2 will fit more of them on the frame,
of course. Once I aim my camera at an extended object, like my house for
instance, now the two lenses are still collecting the same number of
photons, but since the 200/4 has a higher magnification it is spreading
those photons over 4 times the area (inverse square law), and needs 4
times the exposure (2 stops). But since a star is the same size when
imaged by a 100mm lens as a 200mm lens (or by an 8mm lens or a 1000mm
lens, for that matter) the focal length doesn't make any difference.
Not sure if this helps or confuses,
Paul Schings
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