Re: Earthquakes and epicentres

"Peter Duncanson (BrE)" <mail@xxxxxxxxxxxxxxxxxx> wrote
On Mon, 13 Apr 2009 20:23:37 +0100, "Peter Duncanson (BrE)"
On Mon, 13 Apr 2009 20:12:11 +0200, Steve Hayes <hayesmstw@xxxxxxxxxxx>
On Mon, 13 Apr 2009 08:16:17 -0700, Evan Kirshenbaum <kirshenbaum@xxxxxxxxxx>
Steve Hayes <hayesmstw@xxxxxxxxxxx> writes:

As far as I am aware, foreshortening is a quality of images produced by
telephoto lenses -- is that wrong, or is there some other meaning I have
missed?

I don't know it in that sense. I learned it as a property of
perspective. The OED describes it as

Of the effect of visual perspective: To cause (an object) to be
apparently shortened in the directions not lying in a plane
perpendicular to the line of sight. Of a draughtsman: To delineate
(an object) so as to represent this apparent shortening.

What does it mean with lenses?

Exactly what you descrbe, or at least an instance of it.

Wide-angle lenses have the opposite effect.

There are some images demonstrating this effect on this page:
http://www.vale-photographic-club.co.uk/VPC/Hints_1_foreshortening.html

There is house and an orange. The distance between the two is the same
in all the photos.

Consider the imnages in the rightmost column: they have been taken with
lenses of diferrent focal lengths. The camera has been positioned so
that the orange is approximately the same size in all the images.

A person looking at an image naturally infers the distance between the
viewpoint (camera) and the orange from the size of the orange in the
frame.

The orange appears to be same distance from the camera in each of the
shots.

The person will then infer the distance between orange and house
from their relative dimensions in the image.

In the bottom image the orange "covers" the house from roof to ground.

In the top image the orange covers only part of the glazed door of the
house. With the orange apparently close to the viewer the natural
interpretation of this picture is that the orange is much closer to the
house than it actually is. That is the foreshortening effect of a long
focus lens.

I was tempted to explain this earlier but found it difficult to put clearly. I think the above is very helpful, but leaves out something important.

In the sample images on that web page, the camera position relative to the foreground object, the orange, is changed, as the lens focal length is changed. I don't think that the results should be surprising. If the camera is placed very near the orange in the foreground, the orange will appear proportionately larger than the background object. The opposite applies if the camera is moved back from the orange.

But if the camera position is not changed, the relative sizes of orange and house (foreground/near and background/distant objects) remain constant, no matter how the lens focal length is changed. The only effect of changing the lens focal length is to change the size of the image on the recording medium (film or other device). If the medium size is constant, the effect is of zooming in or out. Zooming in means shifting to a long focal length lens and putting only the centre of the previous field of view on the film, magnified. The relative sizes of the images of near and distant objects don't change at all.

But there is still an illusion of foreshortening when, to take an example many have seen, a photo is taken from a great distance with a long focal length lens of a rowing eight, from astern. The rowers seem very close to one another. The effect is an illusion, as can be understood when an equivalent photograph of the same scene from the same viewpoint is taken with a 50mm lens on 35mm film (which is normally taken to be representative of how a scene will look to the naked eye) and the picture is enlarged to show exactly the same view of the eight, in the same proportion; this time the crew spacing looks normal.

The effect of the long focal length lens is no more than the effect of viewing the subject down a long tube which cuts out the surrounding picture, and then magnifying it to bring it apparently closer. The brain interprets the image from the eye in the absence of any further information with reference to the angles subtended by the respective figures of bow and stroke, which are nearly identical, and the knowledge that they are reasonably likely to be normal human beings, and therefore nearly the same size. The conclusion is that they are at nearly the same distance. This is of course correct, though the distance may be a quarter of a mile; but lacking any of the information about the distance of the boat that the surrounding view would have provided, the eye/mind assesses the distance as being the distance at which a seated oarsman would typically subtend the observed angle, and concludes (as a result of the magnification) that it is quite close, and nearly the same for bow as for stroke; from which it follows that the boat length is very short.

If anyone is still reading, that explanation was probably more for myself than for you. I think I understand it now.
--
Paul
.

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