I know I am not alone in my appreciation of the tremendous resources available on this list, Guillermo being a prime example. I always learn something from his posts. Thank you Guillermo, thank you all for your intelligence, artistry and civility. It is always an inspiration.
I've already altered an ancient Leonardo to be a close-up camera. I've also been using a PhotoTechnicalGroup (f512) camera with good effect. Well-built, easy to use, only lacks a graphloc back. Does anyone have an email address for Dominique Stroobant? I had communicated via his son Ishmael's hotmail account years ago but would like to get in touch with him again. His 17mm pinhole for Leica thread mount is still my most beloved pinhole, especially for hand-held work. Thanks, Howard Wells http://home.earthlink.net/~sandwell/ Guillermo wrote: > > ----- Original Message ----- > From: "Gregg Kemp" <gregg.kemp@???????> > > > Guillermo, I don't remember ever seeing your correction formula before. > Is > > that theoretical, or have you tested it? > > Theoretical, Gregg, as I haven't done any close up work myself. > > For those (few) people interested in were that correction factor comes from, > here it is: > > ****************** > Pinhole is in most ways, not different than glass lens photography. > > The lens conjugate equation is: > > 1 / F = 1 / I + 1 / O > > where I = distance pinhole to Image plane; F = Focal length and O = > distance pinhole to Object being photographed. > > We can simplify that to: F = I x O / I + O > > When the Object being photographed is at infinity ( O = infinity ): > > I + O = O > > therefore the formula F = I x O / I + O becomes: > > F = I x O / O and that becomes > > F = I > > In other words, when the Object is far away (more than 10 times the distance > pinhole to film, in practical terms), the Focal length of the camera is > equal to the distance pinhole to film plane. > > Now, when the Object being photographed is close to the pinhole lens (less > than 10 times the distance pinhole to film, in practical terms), the Focal > length of the camera is given by (as I stated above): > > F = I x O / I + O > > For close up work, then, "F" has to be substituted (in any of the formulas > for optimum pinhole size) by: > > I x O / I + O > > For instance, the formula I use is: > > D = 0.0073 x SQRT( F ) > > where D = pinhole diameter in inches; F=pinhole camera focal length and SQRT > stands for square root of > > For close up work that formula becomes: > > D = 0.0073 x SQRT( I x O / I + O ) > > As I said before, F = I for infinity, therefore I can write the infinity > formula as > > D = 0.0073 x SQRT( I ) > > Base on that, I can also rewrite the close-up formula as: > > D = 0.0073 x SQRT( I ) x SQRT ( O / I + O ) > > There you have the correction factor: > > SQRT ( O / I + O ) > > BTW, there are many formulas for optimum pinhole size but all have within > them "SQRT(F)", therefore, the above correction factor should apply fine to > all of them. > > Let's see an example: > > Our pinhole camera has a distance pinhole to film I = 8" and the Object > being photographed is O = 12" away from the pinhole, what size of pinhole > is the optimum to use: > > The optimum pinhole size for infinity is: > > D = 0.0073 x SQRT ( 8 ) > D = 0.020" (aprox) > > Correction factor is: SQRT ( 12 / 8 + 12 ) > Correction factor is: 0.774 > > Pinhole size for close up work (object 12" from pinhole lens) > > D = 0.020" * 0.774 > D = 0.0155" > > BTW, all the above is nothing but a sort of "bellows" correction. > > For people with mathfobia but that have read this msg up to this point, here > are some corrections factor based on how many times the camera > "pinhole-film" distance the object is away from the pinhole lens: > > less than 10 times => correction factor = 0.95 > less than 9 times => correction factor = 0.94 > less than 8 times => correction factor = 0.94 > less than 7 times => correction factor = 0.93 > less than 6 times => correction factor = 0.92 > less than 5 times => correction factor = 0.91 > less than 4 times => correction factor = 0.89 > less than 3 times => correction factor = 0.86 > less than 2 times => correction factor = 0.81 > less than 1 times => correction factor = 0.70 > less than 0.5 times => correction factor = 0.57 > > Another couple of formulas than may help the original's question poster are: > > Magnification M = I / O > > Therefore the Correction factor can also be written as: > > Correction factor = SQRT [ 1 / (M+1) ] > > So for instance, if she wanted to photograph an object 2" tall with a > camera having 8" between pinhole and film and get a magnification of 2X, > she will need to position the object at a distance: > > O = I / M = 8 / 2 = 4" > O = 4 inches > > and the pinhole size (diameter) should be: > > D = 0.0073 SQRT(8) * SQRT[ 1 / (2+1)] > D = 0.012" > > The image size will be 4" and therefore the minimum format size needed is > 4x5 (portrait) or perhaps -better- 5x7" to allow for some cropping. > > Guillermo > > _______________________________________________ > Pinhole-Discussion mailing list > Pinhole-Discussion@p at ??????? > unsubscribe or change your account at > http://www.???????/discussion/
