Murray,
I hope I understood your description (sometimes my ESL comprehension fails
me miserably).
You are considering (first) the 4 side of the bellows frame opening: Since
the bellows do not end at the center of the lens (or pinhole for than
matter) but beside the shutter, the angle between them and a perpendicular
to either side of the 4 opening is irrelevant, the bellows will not
vignette your image unless they sag beyond a line from the end sides of the
4 opening and the pinhole. If the aim is to get a 4x5 image by building
an extension box, you should analyze the 3 size first, as the ratio 4
intended image size to 3 opening is greater than 5 image side to 4
opening. If you lower a perpendicular from the pinhole to the film plane,
you'll have a triangle (let's call it A) with corners at: pinhole -
middle of the 3 opening - end side of the 3 opening, you also have
another triangle (let's call it B) with corners at: pinhole - middle of
the 4 image size - end side of the 4 image size, since this 2 triangles
have equal angles, we can write that:
Base of A divided by Base of B = height of A divided by height of
B
Base of A is half the 3 opening = 1.5 = 38.1mm
Base of B is half the image size width of 4 = 2 = 50.8mm
height of A = bellows extension = 130mm
height of B = height of A + extension box deepness EBD = 130 + EBD
So you have:
Base of A / Base of B = height of A / height of B
38.1 / 50.8 = 130 / (130 + EBD)
EBD = (130 * 50.8 / 38.1) - 130
EBD = 43.33mm
An extension box of 43.33mm will give you an image width of just 4, since
my skills and tools to make cameras or modifications to existing ones aren't
that great, I'd make it at least 46mm deep instead.
A similar analysis can be made for the other side and the result would be
1.28 (close to your finding of 1.25), but since it this result is smaller,
you have to use the larger one of 43.33mm, which is equal to just above 1.7
All the above could have been explained fast and nice with a simple drawing,
but this is just an ascii posts only list.
Guillermo
- Original Message -
From: Uptown Gallery mur...@uptowngallery.org
Gutted some Polaroid rollfilm cameras with 130 mm lens/bellows. Extends a
little farther than that due to 3.5 feet to infinity focus adjustment.
Pulled lens, kept shutter for experimentation on one.
Rear opening of bellows frame is a hair larger that 4 x 3. I took some
measurements along 4 axis and calculated a 22 degree angle for the
bellows.
From this I figured I need a 1.25 entension box to get a 5 image. I
didn't
calculate or measure the short side yet...might be different due to aspect
ratio of 4:3 differing from 5:4.
Gonna try ground acrylic instead of glass.
Question is, with a pinhole I should have adequate coverage, but do you
think my anticipation of angle will prevent 'vignetting' by the bellows or
rear of the camera (existing opening?)
Thanks
Murray
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