> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Julian Loke
> Sent: Sunday, March 11, 2001 8:26 PM
> To: [EMAIL PROTECTED]
> Subject: Re: EOS Magnification with 500D Close-Up Adapter.
>
>
> > "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 !!!
My mistake was in saying that the maximum _focusing_ distance was 500mm.
Actually, the maximum _working_ distance is 500mm; that is, 500mm in front
of the lens.
Thus, with my 80-200 2.8 set at infinity, the maximum focusing distance
becomes about 700mm, giving a field of view (with the lens set at 200mm) of
about 60 X 90mm.
Assuming that the 100-400 zoom is about 400mm long, that gives a maximum
focusing distance of 900mm with a field of view of 30 X 45mm. So our methods
are in agreement!
Chuck
Chuck
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