P3D cure--beamsplitter overlap

[northws1@xxxxxxxxxxxxx (Mark J. Shields)]

Hi!

A long time ago a list member posted the speculation that a black card
placed over the center of a beamsplitter and partially blocking light
rays from the inner edges of the outer mirrors would cure image overlap
at wider apertures. I thought it sounded interesting, but never tried
it.

When I got my Stitz, I found, as I had just been told, that the
"diaphragm" was actually not a diaphragm at all, but a pair of metal
curtains extending from the center to partially block light rays from
the inner edges of the outer mirrors. It is of course completely
adjustable and is just a sophisticated version of the black card.

I just tried the black card on my Pentax, and it works very well.
This means that all apertures down to f/1.8 can be used (f/1.4 if
your lens opens that wide). It is necessary to use a separately-sized
card for each aperture, but this shouls amount to no more than two
or three cards--easily labeled. Since the "black band" effects of the
card and the lens diaphragm are cumulative, the card has to be removed
at smaller apertures.

The good news is that this will work with _any_ beamsplitter. I
anticipate that it will be most useful for family groups indoors
(I don't like to use flash). I will likely pick one "indoor" aperture
and stick with it. The card can be tucked inside the rectangular
Pentax "lens cover."

I realize that there are several valid reasons for not using these
wider apertures, but that is up to the user. Overlap can be completely
eliminated.

Mark Shields

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