[Leica] Erwin on de-centeringing:Why care???

[goldman@math.umn.edu]

Erwin, 
	I am really puzzled about decentering.  If it takes labs test or the
very elaborate setup you describe to detect, then why should I have any
concern about it for real picture taking? Will I see any reduced quality
in the pictures.
Jay


Date: Mon, 14 Aug 2000 20:45:59 +0200
From: "Erwin Puts" <imxputs@knoware.nl>
Subject: [Leica] Re: Ultron decentring, was Re: 35mm Summicron, version
differen
ces
Message-ID: <000c01c00620$3c167a00$ac2340c3@pbncomputer>
MORE (you've seen 34%):
References:
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puter>
<398F4F43.89C4708@2alpha.net><000001c0012c$4b7dd360$7d3140c3@pbncomputer>
 <39948331.3A022060@2alpha.net>

Some Luggers expressed interest in detecting decentered elements with a
practical test. In fact only an optical bench or an interferogram can
detect
decentering, but there is one field test, that will show decentering, if
done well. You need to take a night shot of a scene with a row of lights
spanning the whole image from a reasonable distance, preferably 10 or more
meters. The filmplane and the row of lights (a parking area or an
industrial
complex that has lights all over a building) must be aligned of course and
that is the hard part, at least with an M. But you can use the frame lines
to make sure the camera and line of lights are fully parallel to each
other.
If the lens has decentring, the left or right part of the picture should
show the lights with a higher flare and a larger halo and a lower
contrast.
You need to expose correctly and enlarge a bit, but this test will show
decentering if it is severe enough.

Erwin

- ----- End of forwarded message from goldman@math.umn.edu -----