Re: construction question

----- Original Message ----- From: "peter" <peter@peterpinhole.com>

> I am about to make a camera from an old 120-film bellows camera.
> The image size is 6x9 cm, and I want the pinholes to be somewhat angled towards each other. This is a rough sketch:
>
> http://www.flickr.com/photos/79021423@N00/485124809/
>
> Sort of a Pinhole Blender, but way much simpler.
> My question is: which are the preferred distances (A and B on the sketch), in order to make images that blend into each other?
> I don't want the middle section to be too light, nor too underexposed.

Peter:

More information than B and A (on your sketch) is needed to calculate image brightness on the film plane. I just made a very quick EXCEL spreadsheet that it is based on a similar sketch as yours but that closes the triangles:
http://ca.geocities.com/pinhole@rogers.com/calculation.xls
In it, "p"are the pinholes, "L" the distance vertices 1 to 4, "C" distance 1 to 2, "B" distance 2 to 4, "X" distance from vertex 4 to the pinhole and "F" would be the "focal length" of the pinhole (if we wanted to consider one exist), this "F" line is a perpendicular to the pinhole until it reaches the film plane (where "C" is). Delta in the spreadsheet is the angle measured from the line "F" to other lines joining "p" and film plane. MaxAngle is given by the angle between "F" and a line joining the pinhole "p" and the point 2, this point 2 being the middle of the film plane.
The percentage sign (%) column in the spreadsheet is actually the portion in tenths of the distance "L" at which point the pinhole is located, for instance, to locate the pinhole half way between points 1 and 2, the cell E2 should be set to 0.5 To position the pinhole 2 tenths of the distance "L" away from point 4, cell E2 should be set to 0.2.

Cell in red are the only ones where input is allowed, B being the height of the triangles, C would be on the film plane and contains half the film plane plus any length needed to connect to the "lens/pinhole" plane L and in this way close the triangle, it should also include any space needed to accommodate the film spool. L by the way would be calculated for you based on values B and C in cells A2 and B2 respectively. Again, cell E2 is the distance from vertex 4 to pinhole in tenths of length L, that distance translate to the value X (column F). The rows should only be read until the value of "delta" is equal to "MaxAng", when this is so, the value of f/stop is the difference in f/stops between the middle of the film plane and the point on "C" where the line "F" lands. In the case of the values already in the spreadsheet, the MaxAngle is 41 and it matches "delta" in row 43 giving an f/stop difference of 2.357 stops.
You could try different values of B, C and % and see what is best for you, based on the f/stop difference.

The spreadsheet will give you invalid (negative) values of MaxAng and/or f/stops if the pinhole is located close to point 4 and the line F lands beyonf the middle of the film plane and into the other triangle.

Spreadsheet is set to "protected" (except the cells in RED). You could unprotect it and unhide formulas and hidden columns. Feel free to make any modification, corrections and/or ask questions.
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Received on Wed May 9 16:55:26 2007