hello all, well i'm looking to finally break down and get a new lens but to be honest i have not a clue what means what. i've been looking on ritz and calumet's web sites and am thoroughly confused. the lens i have now is 18-70mm (the stadard D70 outfit lens) so i'm trying to find one out of that focal range for more variety. one of the questions i have is about a 50mm "normal" lens, would that be the same as the 50mm setting on my existing lens (told you not to laugh)? also i see on these two sites lenses with similare focal ranges but one would be like 100 bucks or so and the other would be in the 300+ range, i notice the f/numbers to be different but what exactly does that mean? i know that's the aperture settings but does the f/number really make that much of a difference to drive the price up that high? and my final question, what's the advantage/disadvantage of a mirror lens? i saw a 500mm mirror in my price range, i don't think i need something that big but i was just curious is all. thanks in advance for any assistance!!! eb