Let's see if the Inverse Square Law is important My rough calc is basically how much area (and hence light transmittance) of a circle seen from straight on and at 45 degrees.
There are two types of circular pinhole cameras. The half circle (like my Whatlux? and the Quaker Oats pinhole camera with the lens on the opposite side of the cylinder. For a QO camera the 'lens hole' to film plane distance is a chord of a circle. For the HCpinhole it is equidistant. for QO at 45 degrees a circular film plane is closer to being the same distance as the center--which is actually the farthest point from the lens. for the HC all points on the same horizontal line are equidistant, and FlatFilmPlane affected at points off that line. (A Deardorff, Nikon, Hasselblad are all FFP cameras) Now to calc for a point directly in line with the lens and a point on a flat film plane 45 degrees away: Lens to film plane distnace is FL FL and at 45 d FL* 1.414. [distance * 1.414 (sq root of 2 pythagorean therom for a 45 deg right triangle x^2 +x^2=hyp^2 or x*SQR RT2=hyp) ] Aplying the Inverse Square Law results in the light at the center being D and at 45 degrees being D45= .6 (the area of the ellipse) / 1.414 So there is more than one stop at 45 degrees. Or about about 43% of the light at the center. About a stop and a third. A Schneider Super Angulon 90 and my Reodenstock 65mm centering circles compensate almost 2 stops from center to edge. So even 'real' lenses that cost $$$ are affected yet usable. Of course you'd learn more simply building a box and testing. As the fella on the label says of empirical testing "Nothing is better for thee than me." But he might have been talking about Dektol. Will ---William Nettles [EMAIL PROTECTED] Nettles Photo / Imaging Site http://www.wgn.net/~nettles > From: [EMAIL PROTECTED] > Reply-To: [EMAIL PROTECTED] > Date: Sun, 30 Dec 2001 12:07:12 -0700 > To: [EMAIL PROTECTED] > Subject: Cameramakers digest, Vol 1 #295 - 2 msgs > > illiam: > Hmmm. Nice discussion. Just curious-- did your calculations take into > account loss du to the inverse square law? Maybe I'll do some of this today, > since I'm kinda laying around doing nothing. Should be easy math as we're > dealing with right triangles with flat film planes. My experience with > pinhole cameras is limited to cylindrical cameras mostly. It might also be > fun to calculate how curved, equidistant film non-planes would affect the > apparent focal length of the "lens". _______________________________________________ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers
