Answer to Question #9751 in Optics for Chris

Question

I am a Technology teacher and we need help! We are trying to match up a small lens that a local lens grinding company told me was a +2. Internet research tells me that a +2 lens has an approx 500mm focal length ( a doctor at the lens place told me the same) I bought a lens with a 334mm focal distance I figured it might be close, but knew we would have to adjust for the difference. It almost works, but give us a bit too large of a focused image. I have found another lens with a 476mm focal length that is the right diameter. Question is, I am really looking for as close to a 500mm focal length lens as I can find, and disregard the +2 right? I assume that a true +2 IS 500mm and 500mm only?? or can a +2 have other focal lengths? I told the girl from Anchor I needed a plus two and what size, and she rec. the 334mm fl. Thanks for any input you can offer us!!! - Chris