Michael, Pinholes are "afocal", meaning: they don't have focal length, per se. You can use them to whatever distance pinhole-film you wish, knowing that if you don't use them to their "optimum" distance, results will be "less-sharp", which depending on the photographer intentions could be good or bad for your resulting image.
Knowing that a pinhole can be used at any distance, again, they are afocal, the fall-off is not caused by its size, but by 3 different factors: first: light on the film plane decreases as the distance pinhole-film increases, distance at the center is shorter than distance at the edges of the film, therefore you'll have fall-off as you go off film center. secondly: seen from the center of the film the pinhole looks round (assuming you made it round), but seen from the edges, it would look oval, that translate into less area for light to go thru, that too causes fall-off. Lastly: light falling at the edges, falls at an angle on the film, therefore covers a bigger area of the film, that too, causes fall off. Geekly speaking :) the fall off at the edges will be a factor of the function COSINE to the 4th power of half the total angle of view of your camera, in your case that translate to just over 3 stops. BTW, glass lenses are not exempt of this COSINE^4 condition, that is why for wide angle lenses manufacturers also make special filters to go with them and that correct the fall off at the edges by causing fall off at the center (forgot their name, center filters I think). If you were to use a pinhole optimum for 73mm, your fall off will be the same, the fall off is only a function of the angle of view, in other words, the wider the angle of your camera, the more fall off. Your camera has 109 degrees angle of view, the format diagonal is just 2.8 times the distance pinhole-film, BTW. Optimum does not mean longest, it means "if you want the sharpest images possible use this distance", longer or shorter that optimum and the resolution suffers. The only way to get less fall-off is to make the film or paper to conform to a cylindrical shaped film plane and position the pinhole at the center of the circle describing the half cylinder, there are some examples here: http://www.cyberbeach.net/~dbardell/pinhole.html The reason why such cameras have less fall off is that the distance pinhole-film is the same from center to edge of film and also the light always falls perpendicular to the film so it doesn't have to cover more area (as explained above). Something else that contributes to a fall-off is the thickness of the material the pinhole is made of and whether it has burrs around its edges. Guillermo ----- Original Message ----- From: "Michael Healy" <mjhe...@kcnet.com> > I bought the 12-"lens" set from Pinhole Resourses, and I've been using the > .0102"/.259mm pinhole on my 4x5. This one's claimed optimal focal length is > supposed to be 48mm. I am using it with a recessed lensboard to a length of > about 50mm. That's the closest I can get even w/ a bag bellows. > > The results are fantastic. I love the focal length. Unfortunately, its > fall-off is quite noticable. No vignetting, but the fall-off is about 1.5-2 > stops from center to edge. So I think I have to try the next one of the > bunch, the one whose optimal focal length is reported to be 73mm. That seems > like quite a jump, though. > > Before I try this at a shorter bellows length than 73mm, has anybody done > this? Will I have problems just shooting this at, say, my 50mm (w/ bellows > compensation)? Does their "optimal" mean "longest"? That's what the 48mm > seems to mean, that if you go even to 50mm, you'll have fall-off. Anybody w/ > experience of this? If this works, should I consider ordering a 50-55mm > pinhole to fit my 4x5? I really like the weird, extreme result from the > short one -- it's just that the light fall-off makes cyanotypes very > difficult to print. I'm anticipating that albumen won't work much better. > > Thoughts? > > Mike Healy
