Dave, I've been making several attempts to reply to this since yesterday,
but for some reason my server keeps rejecting my posts. It's been doing this
occasionally lately, no idea why. Today I tried copying my yesterday's reply
into a fresh post, but it rejects that too. Yet I have no problem replying
to other posts. Curiouser and curiouser, as Alice said.
"Dave Martindale" <> wrote in message
news:cf13vr$hhd$...
> "Nostrobino" <> writes:
>
> >> Rather than go over all the issues, may I suggest you read the
> >> comprehensive entry in The Focal Encyclopedia of Photography under
> >> "Perspective". One pertinent quote on linear perspective* is:
>
> >> "Since short focal length wide angle lenses tend to be used with the
> >> camera relatively close to the subject and long focal length telephoto
> >> lenses tend to be used with the camera at relatively large distances,
> >> strong perspective is often associated with wide angle lenses and weak
> >> perspective is similarly associated with telephoto lenses, but it is
the
> >> camera position and not the focal length or type of lens that produces
> >> the abnormal linear perspective."
>
> >Obviously this is incorrect and even self-contradictory, isn't it?
>
> No, it's not. But they're using two slightly different meanings of
> perspective in the same paragraph.
>
> The wide angle photo shows much stronger perspective-related effects
> than the telephoto shot *when both are viewed with the same visual
> angle*.
>
> On the other hand, if both are viewed with the visual angle of the
> corresponding taking lens, there is no unnatural perspective at all.
>
> It's perfectly reasonable to say that the "wide angle perspective" is
> really the result of viewing the print from a greater distance,
> occupying a smaller visual angle, than is correct. Similarly, the
> "telephoto perspective" can be said to be the result of looking at the
> print from too small a distance, with too large a visual angle.
> This is just as true as saying the change in apparent perspective is due
> to the change in camera lens.
>
> So, if you assume a constant print size and viewing distance, you can
> change perspective by changing lens. If you also keep about the same
> amount of the in-focus object within the frame, you will have to change
> the camera-subject distance in proportion to the focal length. And when
> you do that, the change in apparent perspective is due to the camera
> position change - not the focal length change.
>
> On the other hand, depending on your assumptions about linear and
> angular field of view, and print size and viewing distance, all of the
> variables are tied together anyway. So it's a little bit untrue to say
> which one "causes" and which one is "caused by". The quote above is
> trying to tell you that just changing lenses *alone* will not change
> perspective, while moving the camera *alone* does change perspective.
> Both of these, however, also change the space that is included in the
> image.
>
> >Now to go on from that enlightened observation and then say that "it is
the
> >camera position and not the focal length or type of lens that produces
the
> >abnormal linear perspective" is clearly contradictory. If this were true,
> >then it would have to be possible to change THE CAMERA POSITION to one in
> >which it DOES NOT "produce the abnormal linear perspective." But you
cannot.
>
> Yes you can. If you assume a particular print size and viewing
> distance, that determines the focal length of lens *and camera position*
> that will give you a completely undistorted view.
>
> >A wide-angle lens will still produce the same "abnormal linear
perspective"
> >or "strong perspective" (as the article calls it) wherever you place the
> >camera, provided there are objects in the field of view to demonstrate
it;
>
> No, it does so as long as your viewing distance is such that the print
> occupies a smaller visual angle than the original scene. That's the
> only thing that's special about a wide-angle lens.
>
> >contrariwise, a telephoto lens will still produce its own kind of
> >perspective regardless of camera position.
>
> But whether it is absolutely correct, shows "telephoto distortion", or
> even "wideangle distortion" depends on viewing distance and print size.
>
> Dave
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