There�s a contradiction here. The description by William of how lenses work is correct but the conclusion about JC�s description is... well... All the best! Raimo K Personal photography homepage at: http://www.uusikaupunki.fi/~raikorho
----- Original Message ----- From: "William Robb" <[EMAIL PROTECTED]> To: "Pentax Discuss" <[EMAIL PROTECTED]> Sent: Thursday, July 29, 2004 4:44 AM Subject: Re: Scratch on rear element > Correct me if I am wrong, but do lenses not use the entire rear > element, independent of aperture? > > I know with large format lenses, stopping one down will increase the > size of the image circle enough to allow more movements. > My 65mm doesn't even cover 4x5 until around f/22. > > When checking to see if I am free from vignetting, I was taught to > sight from the corners of the screen, and check to see if I can see > the entire iris through the edge of the rear element. This angle > increases as the lens stops down. > My view camera has corner cutouts on the ground glass to allow this > procedure as part of the stock feature set. > > What this effect has on a fixed lens is to push small flaws near the > edge of the rear element out of the image area. Whether the flaw is > an optical aberation or a physical defect doesn't matter. > > Stopping down increases the image circle. > > JC, you are correct, though your wording isn't quite right. > > I can't believe I wrote this. > > William Robb > > > > J. C. O'Connell wrote: > > > > > You don't seem to have a clue as to how lenses work > > > especially with regard to the diaphram. As you stop > > > down the diameter of the optical path is reduced > > > proportinally. If there are scratches , etc on the > > > ***perimeter*** of the rear element and the lens is stopped down > enuff > > > taking > > > the angle of view into account, the "crud" on the > > > perimeter is often COMPLETELY OUT OF THE OPTICAL PATH, I.E. IT IS > > > IRELLAVANT AND HAS ZERO AFFECT ON THE IMAGE FORMED > > > BY THE LENS! > > > > >
