| Like others have said "It depends"
But one of the very nice things about Pentax lenses is that they don't lose much value, so you can try out different ones and re-sell them for not much loss (or even some profit sometimes) and try something else.
Personally I think everyone should try:
Macro photography...as another poster said a true 1:1 macro lens in the 90 to 125mm range
Low light photography...a fast fifty or something like that. My personal choice would be the 43 ltd.
Wide or ultra wide...something wider than 16mm. I've got the pentax DA12-24
A good "walk around zoom" my personal choice is the Tamron 28-75 F2.8, but any in that range, 16-50, 17-70 etc even the not so lowly kit 18-55 I or II
mid tele photo my choice is the K 135 F2.5
Long range tele from 250 onwards.
You don't need to keep any of these lenses, but I think you should give them a tryout for at least a month or so. Then, if you don't like it, sell it. It's doubtful you'll lose much money, especially if you bought it second hand. A for instance is in order. After looking at a bunch of shots from the Zenitar fisheye, I said "that looks like fun, but I'm not a terrible big fan of FE, and I doubt I'd use the lens much. But people kept saying "Try it you'll like it" and after a year of hearing that I gave in and got one. Now I use it a lot. And have great fun doing it.
Maybe there is no ONE "must have" lens, but from where I sit anyway, there are a whole bunch of "must try" lenses
NaCl(try it, the worst that can happen is that you sell it)H2O |