Using a folder as a variable focal length?

[jay moussy]

A silly question, as I am new to pinhole and folders (just got an Ansco 1A):

It seems like a folder with locking rails allows the standard to move to any position.

Could it be used as a variable focal length pinhole?
Same body, used with specific focal lengths-optimized pinhole size combos?

There would be limits though, image circle issues, etc.

 

[DWThomas]

Sounds workable -- my first pinhole forays in this millennium were with a 4x5 press camera that allowed me to play around with the pinhole-to-film distance. If you try to go super wide angle, you may find 'stuff' interfering, that could also depend on what the actual pinhole mounting configuration is. A single pinhole would be enough to test out the coverage, even though it won't be optimum over a wide range, you should get useful information.

 

[Luckless]

I've seen a few press cameras with a pinhole 'lens turret' to make it a little easier to match a suitable pinhole with your current extension. Seemed like neat rigs.

One made use of some magnets on the backside of the lens board and a washer on the front to be dropped over your desired pinhole to ensure a solid seal. Another had a lovely laser cut holder for individual pinhole foils that would then seat against a light seal ring-ridge.

The size of your lens board may impact how easy such a project is to deal with.

 

[Donald Qualls]

I've got exactly this -- a 1927 Zeiss-Ikon Ideal, for which I have a spare bayonet-mount shutter that has no lens; I installed a pinhole in front of the shutter. In my case, I have a pinhole approximately .036", or about f/295 when the lens standard is at the camera's infinity stop (for the original 13.5 cm lens) -- but as noted, I can actually adjust the standard to any position from almost fully retracted (where I need to take advantage of the bed drop to avoid having the bed in the photo), to maximum close focus (for this camera, an extension of a couple inches, giving about a 185 mm projection distance).

The hole I have was optimized for the 135 mm distance when I made it, but moving it back toward the film plane won't make the image less sharp, it just won't gain sharpness as it would with a distance-optimized hole at the shorter distance (minimum is around 40 mm). Likewise, moving it away (by "focusing closer") won't hurt sharpness much; diffraction will overcome geometry a bit, but the end result won't be much less sharp than at the optimum distance.

 

[Grandpa Ron]

Jay,

If I understand your question correctly, you are going to vary the aperture by varying the bellows length. Well it will work.

Below are two 4x5 pictures shot from the same point, at two different bellows lengths.You will have to excuse the quality of negative processing as I was having a bad day in the dark room.

The first picture is with the bellows focal length set at 4.6" (11.7cm) and a .018" (.46mm) pinhole. giving and aperture of about f256.

The second image is with the bellows extended to 6.48" (16.5 cm) with the same .018 pinhole, for an aperture of about f360.

You can see that by moving the bellows out you spread the pinhole projection, thus slightly enlarging the image cast onto the 4x5 film.

The second image my be a tad sharper do to the smaller aperture but the quality of the negative leaves much to be desired. I only posted them to give some idea of what extending the bellows one f stop would do. While it is true that there is a optimum pinhole size for each aperture, I have no less than three sources giving me three different "correct" formulas.

I have several pinhole size/apertures to try when the weather get better here.

Good luck

 

[Donald Qualls]

I just realized I gave a wrong size for the pinhole in my Ideal setup -- it's about half a millimeter, more like .019" - .020" than .036"; the f/stop ratio above is correct, however, around f/270-f/295.