Just purchased a secondhand SMC Pentax 75mm shift lens to complement my existing 55mm/75mm/105mm outfit, mainly aimed at exterior architectural shots.

Not yet had time to shoot a test film, but holding the lens up to a white light source (well, white flourescent) and comparing with my other lenses, the 75mm lens seems to have a very slight colour cast - maybe towards yellow/green, compared to the 75mm and 105mm lenses, which are completely neutral as far as I can see. I also notice that the AR coating is brown, as opposed to green on the 55mm lens.

Is the very slight cast normal for this lens, and is it likely to be detectable when shooting?

-- Mark Brown (mark@enri.go.jp), August 06, 2000

Answers

Most optical glass has some color but it is usually very slight. Many times, optical designers will compromise when it comes to using a glass that has slight color in that they will use it if it reduces an aberration significantly over a glass that has no color. The 75mm shift is designed to be shot off axis and so it must be better corrected for off axis aberrations than a standard 75mm lens. This requires more elements and could easily require exotic glass that has slight color.

From what I've heard, all SMC lenses have seven layers, all of which are the same seven compounds used on all SMC lenses. The sequence of deposition could be different and yield a different color to the eye. The SMC should not affect the color cast at all on a lens.

-- Steve Rasmussen (srasmuss@flash.net), August 07, 2000.