5 by 4 Pinhole

Discussion in 'Pinhole Photography' started by Norman Hart, Jul 5, 2006.

  1. Norman Hart

    Norman Hart Member

    Messages:
    18
    Joined:
    Mar 24, 2006
    Shooter:
    Large Format
    Hi.
    Can anyone inform me if it is possible to get full coverage on 5 by 4
    if you only have a focal length of only two centimeters.I am in the process of
    making a pinhole camera which will be using fidelity dark slides. I think
    I may need to increase the focal length.

    Thanks Norman :confused:
     
  2. reellis67

    reellis67 Subscriber

    Messages:
    1,887
    Joined:
    Mar 10, 2005
    Location:
    Central Flor
    Shooter:
    4x5 Format
  3. Norman Hart

    Norman Hart Member

    Messages:
    18
    Joined:
    Mar 24, 2006
    Shooter:
    Large Format
    Thanks Randy

    With the info provided at mrpinhole.com I will certainly not get the coverage .It confirms that I will have to up the focal length of my project and settle for not such a wide view from the camera. It does make me wonder why the Zero 75 camera claims to get full coverage with only one section at 25 mm focal length though.

    Norman
     
  4. Ole

    Ole Moderator Staff Member Moderator

    Messages:
    9,281
    Joined:
    Sep 9, 2002
    Location:
    Bergen, Norw
    Shooter:
    Large Format
    "Coverage" is a relative term. If your hole is thin enough, you will have "coverage" is that you can get an image with acceptable "sharpness" all the way into the corners. However the light loss in the corners with a 20mm "focal length" on 4x5" film will be tremendous! That's a 75 degree angle of incidence - assuming cos^4(theta), the light loss is about eight stops. This is assuming your hole material is thin enough that the edges don't overlap at that shallow an angle.

    So it may be sharp enough to call it "coverage" - just a pity there won't be enough light to make an image...
     
  5. smieglitz

    smieglitz Member

    Messages:
    1,941
    Joined:
    Oct 18, 2002
    Location:
    Climax, Michigan
    Shooter:
    Large Format
    Here is a pic taken with a camera of focal length ~25mm onto a 120 roll film holder. The camera was a 3/4" thick piece of plywood with a Graflock back attached to it making the focal length approximately 1" or 25 mm. You can see the vignetting in the corners already with this film.

    [​IMG]

    To see the coverage of this camera on a full 4x5 sheet (Fuji Acros Quickload) check out the following URL. Be forewarned the image is a nude:

    http://my.net-link.net/~jsmigiel/images/pinhole/woodpile01.72.jpg

    Joe
     
  6. Dave Wooten

    Dave Wooten Subscriber

    Messages:
    2,720
    Joined:
    Nov 1, 2004
    Location:
    Vegas/myster
    Shooter:
    ULarge Format
    5 cm will cover (50 mm) @ f/154 -- covers 4 x 5 nicely...and is quite wide...

    figure 3.5 x's focal length for diagonal coverage....i.e. 25 mm x 3.5 = 87.5 mm diagonal,,,subtract the vignetting on Joe's photos above and it should be pretty close...(nice photos Joe)
     
  7. Jim Jones

    Jim Jones Subscriber

    Messages:
    2,428
    Joined:
    Jan 16, 2006
    Location:
    Rural NW MO
    Shooter:
    Multi Format
    In addition to the cos^4(theta) fall-of in the corners that Ole mentioned, there will be reduced sharpness due to astigmatism. Yes, wide angle pinhole cameras do suffer from astigmatism, and all pinhole cameras have slight chromatic abberation, too. A center filter (expensive!) like those used on wide angle lenses might help. Another possibility is photographing an evenly illuminated blank white subject with enough exposure to place the corners of the film on the toe of the characteristic curve and develop it to a contrast index of about 1. Either sandwich this negative in front of the unexposed film in the film holder, or mount it in front of the holder. This should fairly well compensate for the vignetting, but will increase exposure time considerably. Undercompensation is probably desirable. Another option is prefogging the corners of the negatives slightly to increase response to the subject in the corners. This should help the corner shadows at the cost of the highlights.
     
  8. Greg_E

    Greg_E Member

    Messages:
    775
    Joined:
    May 17, 2006
    Shooter:
    Medium Format
    Couldn't you make up for the loss of light be curving the film? Then the distance would be the same for all parts. I would think that the curve would be based on the distance between the center of the pinhole to the film plane at center of film. Then draw a sphere around the pinhole with that radius. Obviously this works better for wide strips of roll film than it will for 4x5, since you can almost ignore the verticle component and just deal with a curve in the horizontal component.

    The exposed (developed) white frame is an interesting idea, I would have never thought of that. If you made it small enough, could you put in in front of the "lens" to get the same effect? Then it would be just like any other graduated filter. Yes building it that much smaller would be more difficult, but it would be really handy to add or remove.
     
  9. Norman Hart

    Norman Hart Member

    Messages:
    18
    Joined:
    Mar 24, 2006
    Shooter:
    Large Format
    Decision made 5 by 4

    Thanks Guys
    It is nice to know that there is all knowledge and advice is out there to be taped.I have decided to stick with the 5 by 4 format and settle for a
    focal length of four inches this should give me around 60 degree angle of view
    with a pinhole of .018 thou.I am going to stick to the dark slide holders
    for convenience.If this all works out I will try to post some pictures.I also like Gregs suggestion about curving the film plane. Perhaps a mark 2 project
    may be attempted later. Who knows I may need further advice.

    Rgards Norman
     
  10. Jim Jones

    Jim Jones Subscriber

    Messages:
    2,428
    Joined:
    Jan 16, 2006
    Location:
    Rural NW MO
    Shooter:
    Multi Format
    Greg -- Curving the film nearly eliminates the fall-off due to the corners of the film being further from the pinhole as it is with flat film. However, the light passes obliquely through the pinhole to reach the corners. Thus, the pinhole is effectively an ellipse with a minor axis less than the optimum diminsion.
     
    Last edited by a moderator: Jul 7, 2006