Coverage of a 4x5 pinhole camera

[DirkP]

I built a pinhole 4x5 camera. With 3 pinholes by analogy with the Ondu 4x5 'Rise'.
The camera works fine, but I have, as expected, vignetting. I have a focal length of 60mm with a pinhole of 0.3mm.
In practice, the coverage is +/- 140mm.
I don't understand how Ondu manages to illuminate the entire 4x5 with a focal length of 58mm, especially when using the higher (or lower) pinhole.
I would like to modify my camera or build a second one, so if anyone has an answer, it would be very welcome.

grtz!

 

[Donald Qualls]

Film latitude is the key.

If you meter to overexpose the center by two stops, the corners will be underexposed by about one stop, which will allow ordinary burning and dodging in printing or post to bring everything into line (lots of printer intentionally burn down the corners of their prints anyway). You might gain by pulling development 20% to reduce the density of the overexposed center.

I have a homebuilt pinhole camera made from the cardboard roller that came inside a roll of RA-4 paper for a mini-lab. It perfectly holds two sheets of 9x12 cm film, end to end around the inside of the cylinder, with the pinhole in one end (and tripod mount on the other). I calculated the f stop for the average distance from pinhole to film (halfway along the cylinder) and I get good exposure over the full width of the two sheets with Fomapan 100 (emulsion choices in 9x12 are a bit limited in the US).

 

[Dan Fromm]

Something doesn't compute. See https://www.35mmc.com/26/10/2020/ma...art-3-by-sroyon/#Pinhole_8220focal_length8221 on pinhole focal length. The Ondu 4x5 Rise body is 87 mm deep and the pinhole isn't very recessed. Film-to-pinhole distance has to be >87 mm. 58 mm focal length is hard to understand.

cos^4 applies to pinholes. At the edge of a 150 mm circle, illumination will be down 2.8 stops from the center, should give noticeable exposure with most negative films, possibly not with reversal. Check your pinhole set up for mechanical vignetting.

 

[DirkP]

Thanks for the responses.
Ondu's website states:
Dimensions (W) 87mm x (H) 162mm (L) 142mm
Focal length:58mm
Pinhole diameter:0.3mm
The pinhole is indeed at the very front at Ondu, but I think they include the slats at the back and that (together with the thickness of the sheet holder) accounts for that 29mm.

But it could indeed be that I have a mechanical hurdle, Dan.

As Donals wrote, the exposure should reduce at the edge, at really stops with an edge without much vignetting. My pinholes do lie far in front but behind them is a 'tunnel' that is probably too narrow. I'm going to try to adjust that.

I've started to have doubts because Mr.Pinhole's calculator at a focal length 60mm and a pinhole of +/- 0.3mm comes out to a coverage with a daimeter of only 115mm.

 

[Donald Qualls]

the exposure should reduce at the edge, at really stops with an edge without much vignetting.

You've got something physical blocking light, then. Is the pinhole installed on the front of a thick part with a hole in it? Can you take the back off the camera and ensure you can see the pinhole from all four corners of the frame gate?

Solving this (at least the sharp cutoff) might be as simple as chamfering the rear side of the opening in the front of the body.

Mr. Pinhole probably has a cutoff at some level of light drop-off. My paper roll pinhole mentioned in my previous post exposes the film at nearly 90 degrees from the central axis (the mount for the pinhole is quite thin in this case).

 

[DirkP]

I think you're right Donald, thanks. In attach two pictures: the front = no problem, inside = probabely the problem, the 'tunnels' are to deep and to small.

 

[DWThomas]

Ah yes, one has to try a little ray tracing on a scaled cross-section. I have a home-built 4x5 pinhole camera that works pretty well with about a 62 mm film to pinhole spacing. That's about 103º diagonal angle of view.

When I made a pinhole body cap for my Bronica SQ-A, the first try got terrible vignetting, such that I did a scaled cross-section layout and redesigned the shutter & pinhole mount considerably. The various structural issues compound when the distance between any mounting rings and shutter parts get spread out.

It's been several years since I did any new designs, but my preference has been to use Pinhole Designer. The downside is it's a Windows executable, and hasn't been updated in about two decades. It still runs on my WIn10 system, and it's pretty transparent about the assumptions it makes. Mr Pinhole has the advantage of being system independent as it runs on a web browser, but in my opinion it's rather pessimistic, further exacerbated by mostly hiding what its assumptions are.

 

[Jim Jones]

From Donald Qualls, post #5: "Mr. Pinhole probably has a cutoff at some level of light drop-off. My paper roll pinhole mentioned in my previous post exposes the film at nearly 90 degrees from the central axis (the mount for the pinhole is quite thin in this case)."

Mr. Pinhole is quick and easy to use, but an advanced pinhole photographer may well prefer Pinhole Designer (https://www.pinhole.cz/en/pinholedesigner/). This program lets the photographer select the wavelength of light (useful when using blue sensitive or IR film) and the constant in the formula used for determining optimum pinhole diameter. Neither program mentions the occasional desirability of using a slightly larger pinhole than calculated for extreme wide angle pinhole photography to improve image edge sharpness at a cost of reduced center sharpness.
Several decades ago I improvised a camera and appropriate test targets to verify this. The camera and targets were donated to Eric Renner at Pinhole Resource long ago. The curves represent the smoothed average of two exposures made through four pinholes in a wide-angle test camera with a focal length of 3.5 inches. Solid lines represent radial target lines, broken lines represent tangential target lines. The subject was unevenly illuminated to compensate for the cos 4th power factor. Not graphed was the performance of the .008" pinhole, which was too small to perform as well as the higher pinholes at any off-axis angle. These tests were conducted with incandescent lighting (maybe 2800K , maybe 400K).
The graphs derived from these tests is reproduced below. Lord Kevin's constant in many pinhole diameter formulae seems to be popular, although based on my tests and photographs, a slightly smaller constant is better. I made the pinholes with very thin edges. Less careful fabrication might result in much worse image edge sharpness.
Two favorite sources used in my research are: Young, M. "Pinhole Optics," Applied Optics, Vol. 10, No. 12 (December 1971) 2763-2767) and Jon Grepstad: https://jongrepstad.com/pinhole-photography/pinhole-photography-history-images-cameras-formulas/

 

[DirkP]

Thanks to your posts and to reading a discussion on dpreview (https://www.dpreview.com/forums/thread/3930185), I finally understood that there is actually no image circle with a pinhole camera. It actually just depends on the thickness of the material in which the pinhole is made. I thought too much about lenses and confused that with the edges being less exposed on a pinhole camera. My three pinholes had too little space at the back to expose the entire 4x5'' sheet. I will rework the lens side of my camrea. tnx!

 

[Donald Qualls]

Good for you. Looking forward to seeing results.