Re: Slit apertures - an experiment and conclusions

> This is merely a resend of an earlier response about slit widths and
> distances. I had not provided the correct subject lines. Sorry.
> Giorgio Piccagli
> Not Yet Holier Than Many

> I've read in the "E.Renner-Bible", that you have to cross two slits for
> getting a disorted image.

I made some very rough and somewhat wide slits (about 1mm wide) and was
playing around with them. No pictures where actually made (I just compared
the image on the GG). Here are some thoughts on that:

I mounted a slit horizontally on a Super Graphics lensboard and aimed it to
one of these halogen patio lamps fixtures. Needles to say, when only 1 slit
is used, the image projected on the GG is elongated in the direction of the
slit and the amount of elongation is directly related to the length of the
slit and the distance slit-GG. I then mounted another slit (of about the
same width) on the bottom of a 35mm film canister and this canister mounted
on top of the lensboard, so both slits would be "crossed". The camera was
again aimed to the lamp and the image was not elongated or not as elongated
as before, actually, it appeared to me that the image ratio W/H was very
close to the actual ratio W/H of the lamp itself, in other words, no
distortion or very minimal. I then proceeded to unmount the slit from the
canister and "cross tape" it right on top of the other slit on the
lensboard, effectively making a "rectangular pinhole" having the width of
the front slit's width and the height of the rear slit's width. Aiming this
new aperture gave me an image that appeared to have the same W/H ratio as
when using the 2 slits separated by the film canister. My conclusion based
on these experiment is: 1) when using 2 separated and crossed slits, the
distortion or elongation, will be clearly noticeable only if the 2 slits
have, substantial, different width 2) closer objects would appear to be more
distorted that farther ones. Not a surprise, I must say 3) to made
distorted slit images it may not be necessary to use 2 slits but just a
rectangular aperture with dimensions equal to the "cross superposition" of
the 2 slits we intended to use, in the first place. 4) and "oval" shaped
pinhole would work as well as a "rectangular" pinhole.

I'd be very much interested in opinions, corrections, refutations to my
conclusions.

Another thing I did was to make a long "rectangular" pinhole-slit aperture
using the same 0.002" thick brass material I use to make pinholes, it is
0.018" wide. I would have used razor blades but I wanted to use the old
style Gillette (SP) blades but couldn't find any (are they still made?).
I'd describe what I did, but I think it is better if a just let you see 2
images (front and rear) of this pinhole-slit aperture.

http://members.home.com/penate/slit1.jpg
http://members.home.com/penate/slit2.jpg

Guillermo
Received on Mon Sep 4 18:30:53 2000