[Leica] Re: Ultron decentring, was Re: 35mm Summicron, version differences

["Erwin Puts" <imxputs@knoware.nl>]

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