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P3D Re: formats & focal lengths
- From: George Themelis <gthemelis@xxxxxxxxx>
- Subject: P3D Re: formats & focal lengths
- Date: Tue, 5 Oct 1999 03:42:02 -0600
--- Rod Sage <rsage@xxxxxxxxxxxxxxxxxxxx> wrote:
> The Realist shows the same perspective vertically as the full
> frame image, but the full frame image will still show a wider
> perspective. In effect, the Realist will project a normal
> image and the full frame will project a wide angle image.
Great! You did not understand my point and you are propagating
the same confusion regarding perspective and size of the image.
Let me try again: Perspective is tied to viewing. You can go
ahead and crop an image as much as you like. If you do not
change your viewing method (i.e. magnification) then you have
not affected the perspective or "effective focal length" of the
taking lens.
For example, let's compare a full frame image taken with a 35mm
lens and a half frame image taken with a 35mm lens. Assume that
they are both projected in a screen with the same projector.
They both show the same perspecive which is essentially the
relationship between taking and viewing angle of view. This has
not changed. Yes, you do see more in the full frame image but
that does not make it "wider perspective".
Now, assume that you are projecting these images with different
projectors and you adjust them so the widths are matching. In
effect you are magnifying the smaller image twice as much as the
larger image. This affects the perspective. If you crop the
half frame image to 2:3 ratio like the full frame image then the
two images will have the same size on the screen. To match the
perspectives in this case you will have to use a 70mm lens for
the full frame image.
Reality now: We are using the same viewers to view slides in
41x101 format, even though some slides are mounted in 4p mounts
and others are mounted in 8p mounts. We are using the same
projector to project them. So we can compare the focal lengths
of the taking lenses directly. To match the perspective of a
Realist lens you have to use 35mm lenses in your SLRs.
=====
George Themelis (DrT-3d@xxxxxxxxxxxxxxxx)
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