P3D Re: On Film Deviation

[aifxtony@xxxxxxxxxxxxx (Tony Alderson)]

>David Tiede wrote (digest 2937):
>In the Bercovitz paper "A Comparison of Camera Base Calculation Methods"
>he states "The practical maximum allowable value of d when using the 35mm
>format about 1.2mm. For medium format a value closer to 2.5mm is
>possible". Since the OFD depends on film size, what feature of the film
>size does it depend upon? The height, width or diagonal? Since the OFD is
>basically measuring a change in the horizontal direction, is it 1.2mm for
>a frame 36mm wide and somewhat less for the frame size of a Realist
>camera? What, for example, should the OFD be for a Nimslo half-frame
>camera?<

You have misinterpreted the situation. The maximum allowable OFD does not
depend on film size, per se. It depends on the viewing conditions. The
1.2mm deviation of the Realist format assumes small screen projection, and
limits the ON SCREEN infinity parallax to the average interocular, to avoid
divergent viewing. (A lot of assumptions here...) In other words, the
significant factor is the magnification of the film frame on projection.
Since a medium format slide projected on the same size screen will be
magnified less, the maximum OFD can be greater. If projected to a larger
screen, the maximum might be less. If we allow divergent viewing, the
maximum can be greater.

This is why I made the Reel 3-D 2x2x2 mounting gauge with 1.5 mm instead of
1.2 parallax: I conjectured that the full frame would be magnified slightly
less than a five perf image. This may not be an accurate assumption...

If you are filling the screen with half-frame images, you would be
magnifying the frame MORE, so your max. OFD would be less than 1.2 mm. But
in reality, these shots are mixed with Realist slides, so are magnified the
same amount. But since the Nimslo has fixed lenses, this only matters when
using it for macro work (e.g., Teco-Nimslo, Burdlo, etc.)

The whole subject of deviation requires some math, so unless prodded
mightily, I'll drop it for now. Besides, I gotta get off the computer and
get some food.

Tony "Primal needs before theory" Alderson
aifxtony@xxxxxxxxxxxxx





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