Hasselblad used to sell a copying devise designed to make the film plane and the subject plane precisely parallel. It was done with two mirrors. One was placed at the center of the item you were copying and the other was mounted on a lens flange in place of a lens and had a hole in its center. The 1966 Hasselblad catalog has a picture of the unit with the following description:
#40185 Linear Mirror Unit. "The Hasselblad linear mirror unit is used in photo-copying when extremely accurate parallel alignment between film plane and subject is essential. Using the linear mirror unit, parallelism can be adjusted to closer than two minutes of arc. The linear mirror unit consists of a lens flange mirror 40193 attached in place of the lens on the Hasselblad 500C body and a reflecting mirror 40207 placed on the subject plane."
I believe you simply center the mirror on the copy plane in the center or the mirror mounted in place of the camera's lens by viewing the image in the ground glass or the reflex viewing eyepiece. The mirror to be hung on the copy job looks to be about 5 or 6 inches in diameter.
I have never used or seen this Hasselblad unit but it gives you food for thought. If you are willing to experiment, a skilled glass dealer could cut you a large round mirror to hang dead center over your copy job, and a round mirror with a hole in its exact center to be glued to the surface of an old screw-in lens filter. The mirror on your camera filter would need to fill the entire filter ring.
In your camera's finder you probably want to see a perfect bulls eye target created by the two mirrors reflecting back at each other. I'm just guessing this is how it works. If it did work, you could have different sized mirrors for large and small copy subjects and your parallel set ups would be quick and easy.
Walter
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On Thursday, February 19, 2004, at 04:53 PM, kpp@xxxxxxxxxx wrote:
well here are my experience and ideas on putting a painting squarely in the picture frame...
i had a case of photographing paintings for a friend and just yesterday i photographed some test charts, only to find that squaring the rectangular frame withing the pic frame is not that easy without instrument help...
i lacked a bubble lavel but still that i believe is half the solutuion, cos one should point the leveled camera to the painting center...
as often does i came with idea while showering (dont't you ever do?)
and here what it is.
find the painting center in the cross section of its diagonals. then point the camera there. but who says it is square too? in onother word, is the optical center as far away from all corners?
so enter the strings...
in order to find the vertical line (to the painting surface)that passes from the painting center one has to use isosceles traigles made from string. cut 4 equal lengths of string (2 for each axis and consisting from an actually one double length piece linking each diagonal points), fix them to the painting corners, and pull them until they all end in the same spot, which should be the camera lens center...
it seems good enough, but then i had another problem...i was to use 6 different focal distances for the test chart, so then i would have to make six sets (one from each position)...and the under the running water i remembered the chord fictures most fleece jackets and parkas have to keep the elastic chords shortened.
you know the spring loaded aperture in a cylidrical apparatus that keeps the elastic chord tout. in theory if one would pull the 2 strings length through the opening and tensioned, it is impossible to miss the painting center...
well actually it was too late to test the idea... what do you think about it? any better idea? there's got to be one..
thanks, kostas
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