Last updated: 12 March 2003

Subject:	Vignetting check
Sent:	Sunday, March 9, 2003 15:18:54
I checked the film plane illumination manually. Here's the technique I
used, borrowed form years of large-format photography. Pass it on to
those who may wish to do the same.

Here's what you need:

1.) Wax paper or a spare piece of ground glass cut to the 35mm format
size or even tracing paper cut to size.
2.) Locking cable release.
3.) Your T-mount adapter, telescope mount gear (Meade #64 T-adapter, in
my case).
4.) Tripod or base, something which holds the scope and camera pretty

Cut the wax paper (etc.) into a "T" shaped rectangular shape which will
cover the film plane opening of your 35mm camera, I leave a slight
overlap of 3-4 mm on each edge. I hold the bottom of the "T" and have
the top of the "T" cover the entire film plane with the slight overlap
mentioned above.

Be very careful not to touch the shutter curtain or blades of your
camera (I assume no responsibility here). You want some translucent
material so you can see the area of the film plane opening which is
being illuminated, this is why you use wax paper or ground glass or
whatever you have handy (clear Plexiglas roughed up with fine sandpaper
works also).

Mount the scope and camera to tripod or stable base and point the
scope/camera at a brightly, evenly illuminated source (DO NOT POINT YOUR
opening which is bright, but not unduly so.

Put your camera's shutter speed on Bulb and lock the shutter open with
the locking cable release (you can do this without a cable release with
a long exposure time, but I find the cable release gives me ample time
so I don't feel rushed and also don't have to worry about the shutter
closing while I have the wax paper in place.

WARNING - I should mention that you place the wax paper in the film
plane, but DO NOT let anything, including the wax paper, protrude
through the shutter opening in the camera body or you really risk
damaging the shutter should the shutter trip/close unexpectedly.

Open the camera's film back and gently/carefully place the wax paper in
the film plane and inspect each edge and each corner to see that the
edges and corners are properly and evenly illuminated.

Here are the results of my tests using both a Canon EOS 630 and an EOS 5

With the #64 T-adapter, T-mount with the #64 T short tube, there is
vignetting in the extreme corners of the 35mm frame. All four edges are
illuminated properly, but the extreme corners show vignetting by approx
2 mm each.

With the #64 T-adapter, T-mount with the #64 T short+long tube, there is
NO vignetting whatsoever at the edges or corners of the 35mm frame.

The vignetting in my camera's viewfinder was indeed the mirror issue, as
the film plane tells the truth.

Using the #64 T with short tube only, with some slight cropping of the
extreme edges of the long dimension of the 35mm negative/positive
(chrome) would yield acceptable results and even coverage.

Just thought I'd pass this on.


S Ratzlaff

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