Lens Upgrade Choices
a sigma 10/20
a sigma 150 /500
or a nice little macro
If you want a fisheye then the Pentax 10-17mm is lovely and compact too.
If you want quality and size doesn't matter, then the 16-50mm and 50-135mm are the perfect pairing.
If you want primes then the Limited lenses may be for you.
Best regards, John
Pentax DA* 300 f/4
Pentax 1.4x Teleconverter
Pentax DA* 50-135 f/2.8
Maybe trade your Tammy 17-50 for the Sigma equivalent....
Those will keep you happy for a good while and will knock spots off nearly everthing else out there for IQ
Andy
A few bits & Bobs
The 16-85mm is tempting though, I'll give you that!
[link=https://500px.com/will_brealey/[/link]
Given my relative lack of use of the long telephoto end of the range I suspect that the DA 55-300 is probably fine for my needs. I also love the FA 50 1.7 so that's a keeper. I've never been able to make up my mind on primes v zooms so I was thinking of keeping my options open by getting a wide and short telephoto prime to complement the DA35mm and FA50, a decent all-in-one (I don't trust the Tamron 18-250 - its pitiable in anything other than bright light and my copy is unusably soft at the wider focal lengths) and one or perhaps two high quality zooms. I like the Tamron 17-50 and would consider maybe pairing this with the DA 50-135 but I do experience an unusually high proportion of focusing mishaps with it so would also be open to an alternative good quality short zoom (the 16-85 looks interesting but then if I had that could I really justify the 50-135 given the overlap in focal length? This gets complicated!)
It all depends on your needs (what you feel is lacking either in FL or IQ).
First thought would be a UWA zoom (DA12-24, Sigma 10-20) or a DA15Ltd or Samyang 16/2 to expand the range.
For a prime inside your current range, consider one of the FA Limiteds (FA77 Ltd for portraits comes to mind, FA31 Ltd as a walkaround / full body portrait).
Other random thoughts:
Don't worry about overlap of 16-85 and 50-135; it just gives the convenience of a little less swapping of lenses.
I don't know what other equipment you have; a decent flash or a solid tripod are always useful additions to any setup. Maybe a panhead for stitching panoramas.
Pentax K10D + Vivitar 55/2.8 macro + Super Takumar 55/1.8 + SuperMultiCoated Takumar 85/1.8 + SuperMultiCoated Takumar 135/3.5 + SuperMultiCoated Takumar 200/4 + Super Takumar 300/4
Pentax K100D + DA18-55ALII + DA55-300
Pentax K5 + FA31Ltd + M50/1.7 + DFA100WR + M120/2.8 (+ DA18-55WR at occasion)
Keywords: Charming, polite, and generally agreeable.
Sell (or PX) what you have and buy a 16-50mm, 50-135mm, 60-250mm
Sorted.

+1
OT good to see you pop up again George

Sell (or PX) what you have and buy a 16-50mm, 50-135mm, 60-250mm
Sorted.

+1
OT good to see you pop up again George

Nice to see you, too!
G
Keywords: Charming, polite, and generally agreeable.
The FA 31 is just outstanding. I don't have the FA 77 but it is exceptional too.
Then as mentioned already, pretty much any DA* lens... the 50-135mm, 300mm, 55mm in particular are well thought of. The 16-50mm is a harder one to recommend given that there is now the DA HD 20-40mm and DA HD 16-85mm and it also has compete with a lot of primes and wide angles in that range.
Don't forget the 100mm DFA WR... devastatingly sharp for both macro and tele work (though it can be hard to find non-macro subjects at 100mm FOV, its well worth it when you do).
If you look around right now there's some good prices on the DA* 55 and DFA 100mm WR in particular.
-------
Here's what I'd do, sell it all (except 55-300mm) and acquire:
20-40mm (WR, small light, solid and very good IQ)
FA 31 (low light capable, versatile FL and the best IQ/rendering)
DA* 55 or FA 77 (choose based on whether you need WR, which FL you prefer, either will probably make you sell the FA 50mm)
+
your existing 55-300mm
and possibly add a wide at some point if you will use it enough
K-3ii
One other question on some of the suggestions, would the DA 55 be likely to offer a significant improvement in IQ over the FA 50 1.7?
I'm interested in the number of people advocating the DA 16-50. I know that it's quality control problems are probably behind it but everything I've read suggests that optically it's little or no better than the Tamron 17-50 (I'm not too worried about weather sealing).
You can add the Sigma in that mix as well (to make it more complicated

Pentax K10D + Vivitar 55/2.8 macro + Super Takumar 55/1.8 + SuperMultiCoated Takumar 85/1.8 + SuperMultiCoated Takumar 135/3.5 + SuperMultiCoated Takumar 200/4 + Super Takumar 300/4
Pentax K100D + DA18-55ALII + DA55-300
Pentax K5 + FA31Ltd + M50/1.7 + DFA100WR + M120/2.8 (+ DA18-55WR at occasion)
Garybaldbee
Member
Kingston Upon Thames
I will shortly be receiving a lump sum of money and have a serious need to treat myself to something nice from part of it. As a consequence I'm looking at options to upgrade my lens collection. My main lenses (used on a K5 II) are;
Tamron 17-50 f2.8
DA 18-50 WR kit lens
DA35mm f2.4
FA 50mm f1.7
Tamron 18-250mm
DA 55-300
I was wondering what your thoughts where on how I could improve on the IQ this lot gives me (within reason my considerations will be based on IQ rather than price). I tend to shoot mostly portraits, landscapes and cityscapes and holiday walk around shots.
Given my relative lack of use of the long telephoto end of the range I suspect that the DA 55-300 is probably fine for my needs. I also love the FA 50 1.7 so that's a keeper. I've never been able to make up my mind on primes v zooms so I was thinking of keeping my options open by getting a wide and short telephoto prime to complement the DA35mm and FA50, a decent all-in-one (I don't trust the Tamron 18-250 - its pitiable in anything other than bright light and my copy is unusably soft at the wider focal lengths) and one or perhaps two high quality zooms. I like the Tamron 17-50 and would consider maybe pairing this with the DA 50-135 but I do experience an unusually high proportion of focusing mishaps with it so would also be open to an alternative good quality short zoom (the 16-85 looks interesting but then if I had that could I really justify the 50-135 given the overlap in focal length? This gets complicated!)
Oh, and a fisheye might be fun too - I had thought about the Samyang 8mm for starters.
Your thoughts and suggestions are welcome!