Last updated: 12 March 2003
Subject: Vignetting check Sent: Sunday, March 9, 2003 15:18:54 From: Stephen.Ratzlaff@nokia.com 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 stable. 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 TELESCOPE AT THE SUN AS YOUR LIGHT SOURCE). I used a frosted skylight 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 body. 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. Regards S Ratzlaff
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