Weird pinhole results...

[pryan9]

So I ran a roll of film through my camera and the angle of view was perfect. The only problem is that the image is about the size of a quarter on a medium format frame. The image is circular right in the center of the frame.

I converted an old brownie box camera to make my pinhole cam. Could the focal distance have caused this issue?

Also, I developed the negs using caffenol for the first time but I highly doubt that had any part in the circular images.

*I posted this in my original thread discussing angle of view but it was burried on the second page so I started a new thread in hopes of getting some responses

 

[tjaded]

Where did you place the pinhole itself...on the outside of the camera body or inside?

 

[SMBooth]

Sound like its vignetting on something.

 

[tjaded]

I'm thinking the aperture plate behind the shutter...

 

[maarten m]

distance from pinhole to film?
how thick is the plate you made the pinhole from?

mm

 

[pryan9]

The distance is 3.25 inches from the pinhole to the film. I placed the pinhole behind the aperture slider thing so I don't think that is the issue. The pinhole is however about .75" inside where the lens used to be (which I removed). I thought originally that this would cause a little vignetting but not as extreme as my results. Im pretty frustrated because the little bit of an image I did get looks beautiful. Anyone have a suggestion on how I could possibly fix this?

 

[DWThomas]

I got full coverage on 4x5 (inch) film with a pinhole about 63 mm (approx 2.5 inches) from the film, so I'd say there is something in the way. If the pinhole plate is truly 3/4 inch behind the original lens mount, that could be it. It is surprising what odds and ends can get in the way. My first SQ-A body cap pinhole adapter had serious vignetting and it was an issue of what seemed like large clearance holes being far enough before or behind the pinhole plate to impinge on the intended cone of rays. In that case, I did an accurate scale drawing -- and "oops!" Used the scaled layout to more accurately determine the necessary dimensions and the second round was fine.

 

[erikg]

I think you've answered your own question, it's the distance to the opening. you should make the opening bigger. It's amazing how wide the angle of view can be.

 

[DWThomas]

(OK, I'm crazy ....)

I did a quick construction in TurboCAD, working from the film frame and film-to-pinhole distance. An aperture 3/4 inch in front of (or behind) the pinhole would have to have a diameter of 1-5/16 inches to avoid clipping the corners!