Re: Covering Power of Pinholes

Covering Power of PinholesSimple answer is that covering power generally is 1 1/2 times the focal length either side of the axis. Erics book has both complex and simple formulae. The simple formula is the diameter of the aperture in thosandths of an inch (for example 0.010= 10) divided by 55 equals the optimal "focal length" in inches. On the other hand, the whole thing is very forgiving. You have to be off on the diamerter by 40% to get one stop exposure change. Sharpness is even more forgiving. there's a series of pictures somewher in erics book that illustrates this. Take it from a semi-reformed "critical standards" guy. You don't need critical standards.
  ----- Original Message -----
  From: Joe Tait
  To: pinhole-discussion@pinhole.com
  Sent: Thursday, May 30, 2002 10:49 PM
  Subject: [pinhole-discussion] Covering Power of Pinholes

  Hello!

  New to the list and this is my first post.

  A little background.....

  I have been doing photography for 6 years, and currently shoot 4"x5" w/ a Cambo Legend monorail and 6x7 with a Koni Rapid 200. Most of my experience has been with conventional silver printing, but I have tried cyanotype & gum bichromate in the last 6 months, and am going to plunge forward into various alt processes as I can.

  I really want to try pinhole next, and want larger negatives for contact printing. I've found some interesting info on the web and will build my own camera & pinhole very soon. One thing that I am confused about is determining how much covering power a given pinhole has. Is it just relative to the distance of the pinhole to film/pinhole dia., which then determines the "focal length"? Forgive me, I struggle with comprehending even the most basic concepts of optics generally, and am no better with pinhole.

  I'd like to try say an 11"x14" format, or perhaps a panoramic 8"x16". The wide-angle possibilities (both the really wide & moderate) are in my sights first and I like distortion, but not to the point of monotony. I am looking to use sheet Lith film developed in dilute developer, or pyro; and will be making my own film holders and back to be able to do multiple exposures.

  Could someone enlighten me how to figure out the proper pinhole size & lens-to-film distance for the aforementioned formats? I understand that a curved film plane is employed to compensate for light fall-off. Is the optimum curve determined by experience, or are there known combinations?

  I haven't found any books that focus on specifics. Eric Renner's book apparently focuses on the history more than construction, which is the opposite of what I want to learn first. Perhaps someone could recommend more literature because the web only seems to offer an overview of pinhole.

  Lastly, Larry Bullis' excellent article on pinhole construction mentioned using silver sheeting & a microscope to make pinholes. I'd actually like to try both of these methods. Does anyone work in this way? Where do you get the silver sheeting and what kind of microscope do you use? Precision pinholes seem to really effect the resolution, quite appropriate for certain shots (although the softness is perfect for others).

  Thanks.

  -Joe
Received on Fri May 31 09:36:04 2002