lenses.....

I own several lenses:
- SMC-DA 18-55 kit. It's not used much.
- Sigma 24-70/2.8 - the 2.8 means it lets lots of light in. Payoff is that it weighs 800g. Front end is also 82mm in diameter.
- Pentax D-FA50mm /2.8 macro - very close focus distances are possible here and that's what I tend to use it for.
- SMC-M 28mm /3.5 - old manual lens I use for video
- SMC-M 135/3.5 - old manual lens I use when I want something small and light and reasonably telephoto. Don't use this much, either.
- Sigma 100-300/4 - most used lens at the moment, but it's a monster, over 8" long and 1.5kg. Needs a monopod and with its hood is a seriously imposing beastie. Does lovely things, though....

Lenses I used to have:
Tamron 10-24 3.5 - 4.5 - wide. Very wide.
Tokina 80-400 f4.5-5.6 - compact tele.
Pentax SMC-M 85/2 - compact portrait lens
Pentax 28-70/4 - very small, very light zoom

What I'd like in addition to these and why:
- 10-17 fisheye. Why? Because you get 180 degrees crushed into one picture. I don't think I'd use it much - I've had a 10-24 before and didn't get on with it. Alternative might be the 16-45/4 as this is convenient - either that or a 17-50/2.8.
- 70-200 / 2.8: sometimes the 100-300 is simply too long.
- 50-135/2.8: indoor, conference / wedding photography: this is *the* lens range for people. Flattering portraits without having to get in their face. And 2.8 means it's easy to isolate away from a background.

Until recently I also had an 85/2 and a 28-70. The 28-70 was replaced by the 24-70 a while back - it's a solid 2.8 all the way through and though it's a *lot* heavier, it's still balanced. The 85 is a true manual lens - like the 135 and 28, you need to set aperture by hand - but I have no problem with that, I just found I was never using the 85. And it's a lovely lens and deserves to be used.

The 85 was bought with portraits in mind, and the 50 does that almost as well. Traditionally, 85 was "the" portrait length, as it's flattering to the face - and f2 is really nice to stop down to f4, get the nose and one eye in focus and stay away from the wrinkles - but on digital, it's a bit 'longer' than that (closer to 120). So 50 is a decent length on crop format and I find mine works great.

The thing about lenses is that they change the way things look depending on both the distance and the focal length of the lens. So a 200 will compress things, a 10 will broaden them. Older lenses tend to be less forgiving here, but that's just something they do.

My personal advice is this: buy a lens - or keep it - because you like what it produces. You like the way it renders colours, skin, textures, shapes, whatever. All lenses have character and it's a good idea to understand what that character is so you can use it when the opportunity presents itself.
The 85 was fantastic for dreamy, floaty portraits. The 50 is great for accurate repro. The 135 gives a lovely seventies, faded colour almost sepia feel; the 24-70 is superb on green but needs to be f14-f16 for best results when further away. The 10 is extreme and sharper at 13 - but it sharpens up beautifully on screen and composition needs to be thought about a *lot* more. The 100-300 simply rocks but is silly-heavy and requires 1/320+.

You can buy and sell pretty much any decent lens for reasonable prices. You'll probably lose twenty, thirty of your favourite currency on the deal - but that's life. You'll be richer in experience as a result.
Don't be afraid of older lenses, too; MF is a skill and it's a difficult one to do well. Practice and you'll understand far more about why certain things work with AF....and if you get to set your aperture yourself, suddenly things get a *lot* more difficult (ha, I have a green button for autometering even with fully manual lenses, because I use a Pentax!) - but again, you learn. No difficulty, no learning....