----- Original Message ----- From: "Michael Healy" <mjhe...@kcnet.com> > > I have tried using an old Cronos shutter on my 7x17" but it vignettes > something fierce. Its diameter is about 18mm, but it's also about 15mm deep. > I think it was designed for a 110 camera on monorail. I have a bigger > shutter on the way, so the problem is going away; but is there some formula > handy for figuring out how to anticipate the wall's thickness so it falls > outside the pinhole's (120 degree?) sweeping view? I barely passed geometry > to begin with, and now that half the brain cells have been killed off over > the years, there can't be any memory of it left anyway. > > Any thoughts?
There are many things to consider, one is: do you really need 120 degrees of view (DV)? the DV, BTW, depends on the diagonal of the format and the focal length of your camera. Now, assuming you do need 120 DV, the formula to anticipate the "wall's thickness" would depend on where inside the shutter you mount your pinhole. There are 3 cases: 1- installing the pinhole right midway inside the shutter: In this case the "wall thickness" has to be smaller than 1.732 times the diameter of the shutter's opening (in case you ask 1.732 = Tan[120/2]). In your example, with a shutter 18mm in diameter, the "wall thickness" should not be more than 18 / 1.732 , in other words it can not be thicker than 10.39mm 2- installing the pinhole anywhere from flush with the rear of the shutter to just before midway inside the it. In this case, if we call "I" the distance from the rear of the shutter to where the pinhole is installed and "W" the diameter of the shutter, the "wall thickness" has to be less than: [W / (2 * Tan(120/2))] + I 3- installing the pinhole anywhere from flush with the front of the shutter to just before midway inside the it. In this case, if we call "F" the distance from the front of the shutter to where the pinhole is installed and "W" the diameter of the shutter, the "wall thickness" has to be less than: [W / (2 * Tan(120/2))] + F Case 1 is a particular case of both Case 2 and 3. The 3 Cases could probably be combined into 1 single formula, but at this point of the night my brain cells are not functioning quite well, so I better hit the pillow. > Mike Healy > Tempe (110 degrees in the shade), AZ Guillermo (my tomato plants almost die last nite when temp went around freezing mark) (It will be warmer tonite 42F.........life in the great white north :-)
