on 11/1/02 5:27 PM, Mark Rollins at mark_at_mrollins.com wrote:
>> Nope. If my eye is fixed on the center of something and I slide a
>> magnifying glass into my field of view, I don't have to move my eye to still
>
> Look thru a simple, convex magnifying glass - as you move it to the left,
> the objects DO appear to move toward the right as the center is magnified.
> This is obviously how strong the lens is.
??? That's got nothing to do with what I said. If you move a ring to the
left, the things in the field of view pass in from the left and off to the
right. Whether I'm using a magnifier or a ring, if my eye is fixed on the
period at the end of the previous sentence, once I get the magnifier or ring
centered on that period, my eye will be looking along the same line of sight
in space. I guess you could argue that the magnifier is sliding along the
bottom of the screen, but since the eye naturally focuses on the *center* of
an icon, this is the wrong line to be moving it along. However this is a
minor point compared to the next one.
>> Secondly, a magnifying lens doesn't work anywhere close to the way you
>> describe. The magnification doesn't change depending on where in the focal
>> plane you are. Magnification is fixed. (Very near the edges, depending on
>> the quality and prescription of the lens, there is some region of
>
> Nope. Talking about a simple, convex magnifier, a la Sherlock Holmes. The
> center IS the greatest magnification, and it falls off proportionally
> (sometimes dramatically) towards the edges. Hold a straight object across
> the center of the lens, and as you move it up and down, you see it as
> (artificially) curved, this is due to the different amount of magnification
> from center to edge.
Sorry, but what you are saying is equivalent to saying that the magnifying
lens has a VARIABLE focal length, depending on radial position. If you have
one like that, I hope you didn't pay much money for it, because whoever
ground it was monumentally incompetent. (Actually, come to think about it,
that might be tough to do, but I can't think of any practical use for such a
lens at the moment anyway, so I can't imagine anyone trying it.) A real
magnifier has a single focal length and therefore a single magnification.
Typically, as you get to the edges of the field of view, you'll see a
'rainbow' due to chromatic aberration, i.e. red light has a slightly
different focal length than blue, but this is only a small distance and has
no noticeable impact on the magnification. Bottom line: this idea of
variable magnification as something crosses the field of view is completely
non-physical. Also, if you've got a lens that distorts a straight line into
a curve as you move off-center, that's also a symptom of a REALLY bad lens.
- Eric.
--Eric Strobel (fyzycyst_at_NOSPAM^mailaps.org)
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