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  1. #11
    BetterSense's Avatar
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    I have a Zero Image 4x5 camera and at the shortest "focal length" of approximately 25mm (from pinhole to film) the light drop off at the corners is significant.
    Significant, but somewhat correctable?

    I'm making a foamcore cam right now and I went with 50mm, reasoning that that's shorter than the shortest lens I've ever seen for 4x5. Maybe I should make interchangeable 'pinhole cones'.
    f/22 and be there.

  2. #12
    Mal
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    If you have a look at my post in the pinhole forum (bottom of the first page of posts) titled "Zero Image" there is a photo on the second page of posts that gives an idea... The Zero Image camera has three x 25mm extensions (the base and two extras) that give 25mm, 50mm, and 75mm rough equivalents in 35mm... There was a post somewhere here by a guy in Holland (I think) who had worked out a process to reduce the vignetting during printing but I can't find the post. He has some very good images and examples of the distortions and effects that can be had with a pinhole camera.

  3. #13
    BetterSense's Avatar
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    What I figure, is that I know Tmax can take many stops of overexposure without changing tonality too much, because it has so much straight-line. So I figure that if I expose enough that the corners get enough exposure, I will hopefully be able to dodge back the center and have it look at least somewhat normal tonality from the center outwards, if I can get the dodging right. I might make a dodging mask by shooting a sheet of film in the pinhole camera pointed at a white wall, and sandwich the resulting 'center filter' with the negative at printing time. I'm still not sure if I should use the Rayleigh formula without modification or if I should make the hole somewhat bigger. Making it bigger would seem to help corner sharpness at the expense of center sharpness, at least in theory.
    f/22 and be there.

  4. #14
    Mal
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    Look up www.zeroimage.com and find the link on the left to Zero 45. There is a specifications sheet that lists the pinhole sizes and dimensions at 25mm, 50mm and 75mm equivalents. It may not be exactly what you are looking for but it will give you a start...

  5. #15

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    f250 hole gives me almost 90 degree coverage with a little fall-off at the corners. It seems to be a good aperture for decent sharpness, exposure times and coverage.

  6. #16
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    Here is an example of a pinhole image (cyanotype). "Focal length" was 7cm (pinhole to the center of the film) and it was 14cm from the pinhole to the corner of the film. Exposed onto an 8x10 piece of litho film. My nose is about 6 inches from the camera. Exposure was about 5 minutes (that is why my chin is resting on my hands.)

    Image fall-off not noticable because the my face was the darkest part of the scene. The corners received relatively 2 stops less light than the center -- but the cyanotype is a straight print. Pick your scene carefully and use the fall-off to your advantage!
    Attached Thumbnails Attached Thumbnails Self portrait.jpg  
    At least with LF landscape, a bad day of photography can still be a good day of exercise.

  7. #17
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    There are three factors, all of which contribute to losing light at the corners.

    The first is the difference in distance between the center and edge or corners. This has been pretty well covered here so far, and I think is fairly easily understood if one has a practical understanding of the inverse square. Practically, you can visualize the appearance of the hole at different distances. The hole will appear to be the largest when it's near (along the axis). As the hole is moved away, it will appear smaller. At 2x the "f" distance, the hole will appear to be half the diameter it seemed to be at "f". Because the quantity of light is dependent upon the AREA of the hole, this is 1/4 the light, as Vaughn said.

    The second is change in the aspect of the hole as seen from points which are away from the axis. The hole becomes apparently elliptical, rather than circular. The effect can be easily demonstrated by punching a circular hole in a note card and looking through it both on the central axis and tilted to approximate the view of the hole at the corner of the camera you are modeling. The difference in AREA between that ellipse and and a circle of the same size would account for the loss of light. This effect is very substantial, so the amount of light available at the 2*f is actually a great deal less than the 1/4 the simple inverse square would suggest.

    Third, pinholes such as the common laser drilled examples available on commercially made pinhole cameras are actually TUBES, and this is where the question regarding the thickness of the material mentioned above by DLM comes in. In lens terminology, this is "cutoff". This can be visualized by holding a tube, such as the ones on which toilet paper is served. Look directly down the center. The opening is round. Tilt it a bit, and you will see how the front end of the tube on one side, and the back end of the tube on the other, begin to encroach upon the circle. Again, the difference in AREA of the opening between the obstructed hole and the unobstructed hole gives an idea of the amount of loss, which also can be substantial. Of course, the toilet paper tube is greatly exaggerated, but I hope you get the idea. ANY drilled hole, regardless of the fantastic technology employed to drill it, will have this tubular character. For this reason, it is to our advantage to use the thinnest possible material and make it even thinner if we can. The best pinhole would be one made of material with no thickness at all, which isn't possible. The next best would be a hole made with a sharpened edge all around, like a knife. This probably isn't possible either.

    So, the OP's question is not one that can satisfied with a simple answer.

  8. #18
    Joe VanCleave's Avatar
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    There's a thread over on F295 where someone posted a formula that quantifies the off-axis light loss from all three sources, including the "tube" effect.

    ~Joe

  9. #19
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    You sent me the url for it sometime back, Joe, thanks. Here it is for those interested: http://www.f295.org/Pinholeforum/for...7200960/s-all/

  10. #20

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    On my 4x5 camera, the film back is 6 inches from the pinhole (f400). No drop off at all corner to corner.
    Ric Johnson
    Proud member of the League of Upper Midwest Pinholers & f295

    "I think, therefore, I photograph."

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