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[OM] Re: OT: optics

Subject: [OM] Re: OT: optics
From: Ralf Loi <ralf_loi@xxxxxxxxxx>
Date: Tue, 7 Jun 2005 17:28:22 +0200
Olli, here is an answer to your questions. Warning: my english is poor so
maybe the answer is criptic...

1) A modern lens is made of more pieces of glass. The focal lenght of the
21/3.5 zuiko is 21 mm, equivalent of a simple lens that converge parallel
rays into a point 21 mm away from that lens. The aperture is 21/3.5=6mm so
that ideal lens is 6 mm in diameter. Since the register distance (from
mount to the plane) is greater than 21 mm (roughly 46mm or like that), that
means that every lens with a focal lenght less than the register distance
must be a retrofocus lens, a special design that allows the rays to
converge from a (nodal) plane that is beyond the lens itself.

2) If you want to connect a telescope to the camera, you don't need
anything else, the telescope is nothing else than a long telephoto. When
you look through a telescope you have 3 lenses: the telescope, the
eyepiece, and the eye. If you want to match this case with the camera you
have to add another lens: the telescope (longer focal lens), the eyepiece
(the shorter focal lens), and another lens attached to the camera. If you
try you will find that also with only 2 lenses you can focus; this method
is called eyepiece projection (telescope, eyepiece and camera). Probably
with your choice of "telescope" and "eyepiece" you cannot focus with a SLR
(too less distance from the eyepiece to the focus plane).

Ralf Loi



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