The difference between IR focus and visible wavelength focus is not constant from lens to lens. Different designs of the same focal length can have different IR focus corrections, and the correction is not necessarily proportional when comparing lenses of different focal lengths.
UV transmission in any significant degree requires exotic glass so isn't normally a consideration. I know nothing about the spectral characteristics of Pentax's AF sensors; perhaps you have guessed that IR contamination is the cause of the problem. It sounds feasible. Regards, Anthony Farr > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of > Bob Sullivan > Sent: Monday, 25 February 2008 4:21 PM > To: PDML > Subject: K20D & back focus > > Reading the promo literature on the K20D, I noticed a new feature. > The camera has the ability to store adjustments for backfocus (or > forward) for 20 lenses. > The claim is that all the DA lenses are fine, but others may need adjusting. > > Any ideas on why this might be? > Do some lenses transmit more of the right wavelength light for focusing? > I am puzzled as to the physics/mechanics of this. > > Regards, Bob S. > -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
