I know this is not focused strictly on Pentax 67, but it seems relevant - hope it's OK!

Just about to buy an enlarger lens to enlarge from my P67II negs, and would appreciate some guidance on a particular issue. My preference (as I am obsessive about image quality and optics) is to go for the Schneider APO-Componon 90mm. This is the closest to the appropriate focal length that exists in this lens range. However, I have also read that getting a longer lens (say 100 to 105mm) would be beneficial, because it would allow me to use only the area in the middle of the image circle, and hence get the most of the "sweet spot". For exmaple one posting on the web states that "lens performace suffers that far off axis as the MTF curves fall precipitously" even though they the lens would cover the image area. Now, there is no APO-Componon in this range, but there is a Componon-S.

I know that it is probably not necessary to get the APO, but I'd like to, as I am willing to spend more to get even small improvements in optical performance.

My question is: will I get better optical performance using the APO 90mm lens, or using a slightly longer non-APO lens, hence using the middle of the image circle. Or is there another lens range that would allow me to get APO optics at the right length (say 105mm)?

Many thanks in advance!

-- Ed Hurst (BullMoo@hotmail.com), April 23, 2001

Answers

Ed, when considering APO vs other levels of correction, remember that there are at least six aberrations that affect sharpness, with Longitudinal Chromatic only being one of those. APO refers to longitudinal chromatic. The color aberrations have only a slight affect on sharpness and usually manifest themselves as fringes. At 90 --105mm, lenses correct color very well while only being achromatic. Also, be aware that some companies will only use the term APO when they employ low dispersion glass. The Componon S could be APO but using conventional glass. The 600mm Takumar is APO but uses conventional glass. There is a huge difference in color separation between a 100mm lens and a 600mm however. I haven't used the lenses you describe but would guess that the Componon S will not fringe and would be sharper due to the fact that less off axis rays are used. SR

-- Steve Rasmussen (srasmuss@flash.net), April 23, 2001.

A 105mm APO should be slightly better than the Componon-S but my guess is that the difference will be hard to see. The lenses are probably six element symmetrical Plasmats and are very good at close up work, even off axis. SR

-- Steve Rasmussen (srasmuss@flash.net), April 24, 2001.

Mark: Color aberration is mostly a matter of distance traveled by the light between the optic and the paper or film. The longer the distance, the tougher it is to correct. If the light has to travel much longer than the focal length of an achromatic lens, then soft edges could result, even though you use B&W. Achromatic lenses do OK up to a 200mm distance. Beyond that, APO is required. Much depends on the distance between your lens and your paper.

-- Steve Rasmussen (srasmuss@flash.net), February 27, 2002.

Steve,

Thanks for your help once again. Would I be right in saying that finding a lens that is 105mm AND APO would be best, as it would have the advantage of avoiding off axis rays, whilst removing any very slight colour fringing that there might be? I ask because I have discovered that Rodenstock sell a 105 that is also APO...

-- Ed Hurst (BullMoo@hotmail.com), April 24, 2001.

My questiion is along similar lines. I do B&W enlargements(67 negs) currently on a Rodenstock 105 Rodagon 5.6 enlarger lens (20x24 max). I'm looking at an upgrade (enlarger as well) to the Schneider APO 90 5.6 (naturally more bucks). Would this be an unnecessary overkill, the APO component applying mainly to color? Thanks mark smith

-- mark smith (msmith@fayar.net), February 27, 2002.