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Paralax and zooming



Brian was correct (see below). Let me show how this was demonstrated.

Ludwik was guessing (wrongly):

... Strong zooming, to cover as many pixels as possible, is equivalent
to filming from a short distance. It may be associated with sizable
paralax errors.

Brian responded:

This is factually incorrect. A prime method to REDUCE parallax errors
is to place a camera at as large a distance as possible and use the
largest magnification which a telephoto lens can offer - i.e. by zooming.

A good experiment for some of us; I just tried it and got a confirmation.

Two vertical sticks in the plane parallel to the wall. They are separated
by about 20 cm from each other. Their distance from the wall also about
20 cm. And three thick lines on the wall (electric tape), each about as
wide as the diameter of one stick. The wall lines are also separated from
each other by 20 cm. Looking along three lines perpendicular to the wall
I see each stick hiding the line on the wall. Suppose my eye is in the
central position and the central stick is exactly in front of the central
line on the wall. Without moving my head I see that the other two sticks
are now separated from the lines on the wall. The angular difference
(line on the wall and the stick) is what I call a paralax.

I place the camcorder close to the sticks (its lens is 60 cm from them),
focus it manually and minimize zooming. The paralax is ALPHA (the width
of one stick) on each side. About 60% of my viewer (horizontally) is
occupied by the sticks. I move the camcorder to a distance of about six
meters from the sticks, zoom it to have 60% of my viewer occupied by
sticks and refocus. The paralax is four to five times smaller, a small
fraction of stick's width.

I suppose that this would be obvious if I had an optical diagram in
front of me, and if I understood its functioning. But for the time being
I am happy to know that filming with the maximum zoom from a distance is
better than filming without zooming and being very close. At least from
the point of view of parallax errors. A useful piece of information.

Thanks Brian. How did you figure this out? Ludwik Kowalski

P.S.
Other replies. I agree with all comments about incline plane experiments.
They are very instructive and easier to perform. Our students perform
them each Fall using Pasco carts and Vernier ultrasonic rangers.
But the topic of our thread was free fall and I did not want to be
distructed. I was deliberately focussing on what is difficult.