I have two lenses that have rear filters; one is a Canon and the other is the P67 600mm f/4. I remember reading that a plane parallel plate(filter)will affect image quality when a converging light beam passes through it. This is because the filter changes the final focus point differentially depending on the angle of the beam striking it. Rays passing through the filter at a steep angle will focus further back than rays passing through the filter near the optical axis. I called Pentax corp office in Colorado and spoke with a technical person to see if the 600mm was corrected for filter use. It is not, so the addition of the rear filter will make a slight difference in sharpness due to spherical aberration. For maximum sharpness, shoot this lens without filters.

-- Steve Rasmussen (srasmuss@flash.net), March 24, 1998

Answers

Bill: The situation I described will cause some spherical aberration and therefore will not focus to a point. Agreed, there will be chromatic(longitudinal)and that too will cause slight image degradation. SR

-- Steve Rasmussen (srasmuss@flash.net), November 02, 1998.

Though I've never played around with rear filters, I always have one on the back of my 16mm AFD Nikkor fisheye (just for rear element protection). I also always keep the sky filter in my 300/4 AF Nikkor filter drawer since that was the manual recommendation though I seem to remember that it made no difference in test shots...

=chow

-- chow (chow@cyberdude.com), June 11, 1998.

Steve No, a flat piece of glass in the converging rays will not mess up the focus---all the rays still converge to a point. This point will be a little farther back than without the glass. It will introduce a small amount of chromatic aberration, since the refractive index of the glass won't be the same for all colors--amount will depend on the f/no. Actually, filters are less likely to mess up the image on long lenses if they are somewhat close to the focus instead of at the entrance pupil. Bill

-- Bill Kelly (gklent@ix.netcom.com), September 17, 1998.