APO or apochromatic lenses came onto the scene roughly around when? I just bought a Linhof Technika III version 5 (looks like a IV, not a III here's a link to the same model as mine: http://www.cameraquest.com/tech3pic.htm). I found out the camera was manufactured in 1956 based on the serial number. I'm trying to figure out if the lens is an APO lens or not. No it's not marked APO, but neither is it marked Anastigmat and I think it's fair to assume that it is at least an anastigmat lens considering it's a Linhof version of Schneider Xenar 150mm f5.6 and seems rather obvious it's the original lens that came with the camera. The infinity stops on the Tech III's are filed right into the rail itself. And this lines up perfectly to the infinity stop and the engraved distance markers on the bed. Wikipedia says the Xenar is an anastigmat lens http://en.wikipedia.org/wiki/Schneider_Kreuznach#Xenar_2 but they do not say hardly anything about other lenses which I know are APO lenses, so it's hard for me to dig up the facts. And I'm curious to see what the great history buffs here on APUG have to say.