"Ter McDonagh" <[EMAIL PROTECTED]> wrote:
> Howdy folks, I'm new to the forum, but have "lurked" for a couple
> months. I just obtained a 16" Cooke Process Anastigmat.
> Unfortunately, none of my cameras have that much bellows. I'm
> thinking of building a very basic box (maybe 3/8 plywood)about 15
> 1/2" long (by 5"x5"), and already have a spare 4x5 spring-back. I
> don't plan to have any lensboard movements on the first
> prototype, and maybe a little front rise later on, and want to
> focus from infinity down to around 15-20 feet. I will be
> mounting it to a tripod, but wood still like it to be light
> enough to hike with. My question is, what do I do for front
> focusing, keeping in mind that the lens is about 2 1/2" in
> diameter and weighs about 4 pounds? Do I mount a sort of
> monorail on the bottom (or top) of the box and put the lensboard
> on a standard, and fit a square bellow between the standard and
> box? Is there a simple type of helical focus I can make that
> will be tight enough not to "unfocus" when I close down the
> aperture? Do I just use two bolts (or four) and use shims for
> some preset focus distances, and if so, how do I do it so they're
> light-tight? Also, is there an easy formula to figure out how
> much "throw" I need to focus from infinity down to my goal of
> 15-20'? Thanks for any help.
Some formulas are:
1/focal_length = 1/subject_distance + 1/bellows_extension
subject_distance = (1 + 1/magnification) * focal_length
bellows_extension = (1 + magnification) * focal_length
To focus at 10 feet (120"), the extension required is
approximately 18.5 inches for a 16 inch lens. If your camera has
a 10" draw, you'd have to add 8.5 inches extension at the front
or rear - more to be safe.
To use long lenses with small view cameras, the usual approach
employed was to mount the lens on the end of a tube which was
supported on a bed (board) to which a flatbed view camera or SLR
was attached. The tube is fitted with a lensboard to provide a
light-tight connection, and is supported by V-blocks so the
lens/tube/front standard/bellows/center of the film plane are all
on the same axis. The camera bellows is used for focusing. The
disadvantages are size, and vignetting by the front standard
opening if it is smaller than the film format. Vignetting is
minimized or eliminated by using the shortest tube which permits
close focusing.
Another alternative is to make an extension back if you are using
a flatbed camera which has a removable GG back which is rotated
for horizontal or vertical frames. One simply makes a plate to
fit the camera in place of the GG back, a simple rear standard to
hold the removed GG back, a bellows from the plate to the new
rear standard, and a bed to align everything. The lens is mounted
on the front standard which provides focusing and movements.
Vignetting isn't a problem using the same format size. Since your
lens should cover 8x10, you could build an 8x10 extension back.
The camera's position on the bed can be altered if different
focal length lenses are used. For convenience, a folding bed
may be used, and tilt added to the rear extension standard.
Or you can build a box camera as you suggested. If you don't want
to use a bellows and make a focusing track, you can mount the
lens on a box which fits into another box to which the GG is
attached. The second box should have a double wall to act as a
light baffle.
----------------------|
|--------------- |
| ----------------------|
| |GG
Lens | |Back
| |
| ----------------------|
|--------------- |
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