> In a message dated 5/23/01 6:50:27 AM Pacific Daylight Time,
> [EMAIL PROTECTED] writes:
>
> << Looking at the EOS Magazine article again, the length of the 400mm f4
is
> 317mm.
> The DO version is 233mm. So, *technically*, Canon has a 300mm lens that
> gives
> you 400mm magnification? Will this change the "compression" of the image
> (i.e., where the background appears closer than it really is) or am I
over
> thinking this? The DO just bends the light steeply allowing the same
> compression, etc. in a shorter barrel?
>
> 317mm seems quite a bit short of 400mm. >>
>
The lens has no effect on the "compression" of the image. That is determined
by the distance between the film plane and the subject of the photograph.
There are various ways to design lenses. Some (retrofocus) result in lenses
that are physically longer than their focal length (often used for Wide
angle lenses so that the rear element doesn't crash into the mirror or the
shutter. Some result in lenses that are physically shorter than their focal
length. But those have nothing to do with perspective.
John
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