> "Chuck Skinner" <[EMAIL PROTECTED]> wrote:
> As I understand it, the 500 in 500D means that the maximum focusing
> distance (infinity set on the lens) will be 500mm. According to fcalc,
> a distance of 500mm at a 135mm focal length gives a field of view of
> approximately 65 X 97mm. Remember, this is at the maximum focusing
> distance. With a 400mm lens, it's an amazing 6 X 9mm.
Hi Chuck,
I think you might have used the formula for extension tubes by
mistake. Tubes allow the lens to focus closer, without affecting
the focal length, but at the price of a narrower effective aperture.
Putting a 500mm lens in front of a 400mm lens focussed at infinity,
produces a magnification of 500/400 = 1.25 times. If you focus at
500mm, the field of view should be 1.25 x [36mm,24mm] = [45mm,30mm]
If you had indeed used enough extension tubes or bellow to let the
400mm lens focus to 0.5m, the effective aperture is a mere f/28 !!!
See my Javascript lens calculator at:
http://members.home.net/jul.loke/
Cheers
Julian Loke
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