> "Tek"<tek### [at] evilsuperbraincom>  wrote:
>
>>
>> The only squashing effect I'm aware of is when viewing through a telescope,
>> which is the same flattening effect you get with a zoom lens, this image has
>> a non-zoomed field of view so you wouldn't see squashing.
>>
>> The squashing is because the more you zoom in, the narrower your field of
>> view, the closer to parallel the edges of your view become, so the less
>> perspective you have in the image. With a wide field of view objects further
>> away get a lot more perspective, so get smaller quicker, a zoomed lens keeps
>> things much more similar sizes.
>
> Hmm, that *would* seem to be the case, even in the situation I'm thinking of.
> Maybe I'm wrong about the whole idea. But I'm imagining, say, the planet Mars:
> If it were perhaps far larger in actual size (to take up an area in the sky two
> or three times the visible diameter of the Moon, for example) while still being
> so far away, it seems to me that the 'perspective' of the 'closer' and 'farther'
> parts of the disc would be foreshortened. Based on the idea that, relative to me
> as the observer at such a distance, the 'distance difference' between the closer
> and farther parts would be quite small, relatively speaking.  I guess I just
> need to do an experiment, to prove or disprove it to myself. (I have an image in
> my mind of what I think it should look like--but maybe that's a false image!)
If Mars was that big, you'd be able to see it's curvature, and Earth 
would orbit around Mars, not the Sun... In that case, mars would be 2 or 
3 times bigger than the Sun, and MUCH, MUCH more massive. 8 to 27 times 
the volume, and a larger density.
>
> There's actually a 'down-to-earth' example that I've looked at, which *seems* to
> show this effect:  It's a giant crane hoist (a *really* big one) at a Navy yard
> near me. It has an elevated 'track' (that the crane runs on) which is probably a
> good 300-feet long. When that track is rotated to be 'in line' with my line of
> sight, it's near and far ends appear to be just about the same size
> visually--almost no perspective distortion, in other words. (And I can only view
> it from far away--not close enough to show the normal perspective distortion--so
> an unfortunate side effect is that it 'looks' quite small.)
>
> Time to experiment! :-)
>
> Ken
>
>

Our perseption of perspective depends heavily on the distance that 
separate us from what we observe, and the relative depth compared to 
that distance.

In a perfectly transparent medium, any sphere whose apparent diameter is 
the same will look the same. Be it a 1 cm marble at 1 m or a 1 000 000 
km thing at 100 000 000 km.  In both cases, the diameter is 1% of the 
distance, and both will have the same apparent curvature.

In the example of the crane, you may be at about 5000 feet (a little 
under a mile) fom it.
The close part is at 5000 feet, and the far point is at 5300 feet. A 
difference of about 6%.
If you are near it, say at around 100 feet and on some scafolding, the 
close point is at 110 feet and the far end at 400 feet. A diference of 
72.5%.


Alain

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