I've been thinking about getting some negative lenses to experiment with. I have this question: if you sandwich a -1 and a -2, is the result a -3? Thanks. Fred.

Yes, combining a lens of -1 diopter and a lens of -2 diopters produces a group with an effective focal length of -3 diopters -- more or less! For precise calculation, you need to take into account the distance separating the two elements.

t.r.....Thanks!! That's all I needed to know. Fred.