Re: "focal length" of SLR cameras

klaus writes:
> I just plan to built a pinhole bodycap for my NIKON F3 and CANON AE1. Does
> somebody have experience in that and what is the distance form body cap to
> film for computing the pinhole diameter?

I don't have my bodycap at hand, but I've done this to my beloved
AE1. And it's an exercise that my students have the opportunity
to do in my workshops. The good news about many of the 35mm SLRs
is that they have a mark that shows the film plane: a circle with
a line through it. On my AE1, it's on the top of the camera, between
the viewfinder and the film rewind knob. Looks like this: -0-

(My 16mm Bolex has that mark as well.)

Figure out where your pinhole will be relative to the bodycap, screw
the cap onto the camera, and use a nice scale to estimate the focal
length. In my case, the bodycap is drilled out to 3/8", and a sheet
of brass is gaffer-taped in from behind. I estimated the distance
to the front of the bodycap when in place on the camera, then I
subtracted the thickness of the plastic.

It is an extremely easy way to get pinhole results fast, with a
nicely controlled exposure, and without the need for a darkroom or
darkroom skills (take it to the nearest lab). I found that the
images were diffuse, with great depth of field, compared to images
made with a lens (as you'd expect), but had no special qualities
that a casual viewer would associate with a pinhole camera. On the
other hand, one of my students a few years back found that there
seemed to be a shift in color that made his images tend toward
primary colors. I have no explanation for that, but his reds,
yellows, and blues were remarkably pure. Sorry; I don't know what
film he was using other than that it was a color print film.

Good luck!

--Eric
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Received on Thu Jan 6 06:57:16 2005