How wide is feasible?

Discussion in 'Pinhole Photography' started by BetterSense, Jun 8, 2010.

  1. BetterSense

    BetterSense Member

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    I'm making a 4x5 pinhole camera and I want to go as wide as possible. I'm looking for suggestions on what's the shortest focal length that will cover to the corners and also what size to make the pinhole to optimize sharpness out to the corners. At this point I think making the pinhole slightly bigger than the Rayleigh formula recommends might be a good start, but I'm unsure just how short of a focal length is practical. I think if I use Tmax and expose properly I will be able to burn in the center enough to conquer tonality from falloff, but I am more worried about sharpness.
     
  2. Vaughn

    Vaughn Subscriber

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    Eventually it will be the differences in the distance from the pinhole to the film at the center vs the corners that will limit you. If it is twice the distance to the corners than to the center, that will be two stops (4x) less light hitting the corners. I have made 8x10 pinhole cameras using 250sht boxes of photopaper. Very wide angle, but the corners can go unexposed relative to the center. Size of the hole will make no difference -- more light will hit the corners, but totally overexpose the center. So you need to know how much dropoff of light in the corners you are willing to put up with. A "focal length" of 3 inches will give you about 1 stop of drop off in the extreme corners of a 4x5, if my math is anything close to right (no promises!LOL!).

    I'll shut up and let someone who knows what they are talking about post.
     
  3. DLM

    DLM Member

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    How thin your pinhole material is is also a factor (yes that looks weird to me too). I made a pinhole out of a small coffee can, where the 5x7 photo paper was oriented horizontally and curved along the backside of the can, and was able to get coverage corner to corner. I used aluminum from a red bull can, and sanded it down really thin. The focal length of the camera was right at 100mm to the center of the paper, but obviously the sides were much closer.
     
  4. keithwms

    keithwms Member

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    You might consider making a pinhole aperture for a fisheye lens.
     
  5. totalamateur

    totalamateur Member

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    Is it cheating to bend the film plane? If you cut a piece of 6" PVC in half and put an 8x10 sheet against the curved side, you'd have a 3" focal length, and I think that's pretty wide for 8x10 - 75mm, I think that's the equivalent of 11mm on a 35mm. Pretty sure that'l get you close to 180 degrees coverage.
     
  6. DLM

    DLM Member

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    That's basically what I did w/ the coffee can pinhole. It was made with the smaller cans, not the big Folger's type cans, but the smaller ones that only hold about 12-14 oz of beans.
     
  7. BetterSense

    BetterSense Member

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    I don't know; I'm not sure how that would affect the image and how lines are drawn. I definitely do not want lines to bend, the way fisheyes do. I'm specifically building a wide-angle pinhole because I want a very wide angle but cannot tolerate bent lines, nor can I afford a very wide angle lens for 4x5 anyway.
     
  8. DLM

    DLM Member

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    Yea, bending the film plane does distort the image. I wish I had a good image to post, but I've only done 3 shots with it and they were just to see if I could get the right amount of coverage. The last one I took got messed up because it was a windy day and I didn't hold the can down well, so it's blurry.
     
  9. Mark Fisher

    Mark Fisher Subscriber

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    Another way to think about it is that the "ideal" pinhole size really only applies at the center of the image. If you curve the film plane along one axis to maintain a constant distance from the centerline of the film to the pinhole, then the pinhole can be the "ideal" size along the centerline. Making the hole bigger will just make it less sharp. If you really wanted the sharpest, most even exposure, then maybe a spherical film plane would be the answer......I'm not sure how that would work though! (or how you'd print it).
     
  10. Mal

    Mal Member

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    Hmmm? If the film was forced into a spherical shape rather than a semi-circular shape. The film would all be the same distance from the pinhole and you could make the focal length as short as was practical with the equipment you had. It would also give you a fisheye view, I think? I imagine the challenge is getting the film into this shape.
    I agree with Vaughn: 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. Vignetting on a B&W image is a nice effect but it doesn't look as good with colour film... IMHO...
     
  11. BetterSense

    BetterSense Member

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    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'.
     
  12. Mal

    Mal Member

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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.
     
  13. BetterSense

    BetterSense Member

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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.
     
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  15. Mal

    Mal Member

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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...
     
  16. Rich Ullsmith

    Rich Ullsmith Subscriber

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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.
     
  17. Vaughn

    Vaughn Subscriber

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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!
     

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  18. bowzart

    bowzart Member

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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.
     
  19. Joe VanCleave

    Joe VanCleave Member

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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
     
  20. bowzart

    bowzart Member

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  21. Ric Johnson

    Ric Johnson Member

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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.
     
  22. Vaughn

    Vaughn Subscriber

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    Should only be slight less than 7" (sq root of 47) from the pinhole to the corner -- there will be some falling off of the light, but probably less than people usually burn down the corners/sides when enlarging standard negatives. Six inches should give about a normal view on 4x5.

    I did hear about a fellow who use to sell pinholes -- the edges of the holes were actually micro-filed to a knife edge. No other info, I am afraid.
     
  23. richard ide

    richard ide Member

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    If you make your own pinholes by dimpling a sheet of thin brass and use something like 600 grit sandpaper, you will get a knife edge on the hole. BTW I made an 8 x 10 pinhole camera where the film was curved in a semicircle in the box and the angle of view was close to 160 degrees. Interesting distortion as well.
     
  24. bowzart

    bowzart Member

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    Might have been me, Vaughn. I'm no longer interested in actively selling them because I don't really enjoy working for $5/hour, especially because my eyesight isn't what it used to be, but I do make one now and then for somebody.

    However, somewhere on APUG is a detailed description of my current method, which is the best yet, if anyone is crazy enough to try it.
     
  25. 2F/2F

    2F/2F Member

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    The rule of thumb I have been taught is that the image circle is approximately 3.5x the distance from the pinhole to the film plane, so divide the diagonal of your format by 3.5 and you should have the shortest distance you can use without losing the edges completely.
     
  26. 2F/2F

    2F/2F Member

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    6 inches (about 150 mm) is only slightly "widish" on 4x5 format (I call 180mm "normal" for 4x5), so I would not expect any serious falloff.

    It is not too hard to make a "center filter" dodging mask for printing pinhole pix. Expose a piece of film in the camera to an evenly-toned subject (sky, grey card, etc.). Then make a contact positive from that negative, exposed and developed to the proper density to mask ("dodge') the corners of the frame.
     
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