Hye.
My take on K mount lenseses [as opposed to Pentax lenses].
Lenses to consider for landscape:
DA 15/4 Limited
DA 12/24 4 [rectilinear wide zoom]
DA 14 2.8 [which is truly good, and smaller than expected]
Cosina-Voightlander 20.
The Cosina Zeiss don´t really add anything to what Pentax has to offer [in fact, they do perform worse than the equivalent Pentax Primes, specially when compared to the Limited series].
The 16-45 4 is a very good lens, and now can be had for a laugh and some coins.
Were you going for stitching:
Sorry to say, but AVOID the A series but for some examples mainly regarding the 50 1.7, or some more exotic lenses regarding short tele with slower apertures. The Wide Angle 15/3.5 A lens is very, very expensive [very few of them manufactured].
A must is the 35 Macro Limited 2.8. Bit on the slow side, but spectacular performance from 2.8 on [my experience, not Photozone backed]. Unless speed needed, the 35 Macro is preferable to the 31 Limited, for its more neutral rendering [ie: lack of character].
FA 50 1.4 preferable to A 50 1.4. Pretty much the very same lens, though, but better coatings on the latter series.
FA 43 1.9: stellar performance and resolution, but almost as "femenine" as Leica R series: little bit on the unpredictable side. Unbeatable and almost obscene resolution from 2.4 on. [which means: you WILL see the defect on the lackering or car paint on THAT car which is always at the back of any frame].
FA645 55/2.8 [remember to put an original adapter to it].
Short Tele:
DA 70 2.4 over the FA 77 1.8.
FA 100 2.8 Macro "AKA The Tank" over DFA 100 Macro or DFA 100 WR Macro. Optically they are very similar [out of focus transition slightly better on the WR], but mechanically the FA is the best of them all: clutch and focus limiter, a must for tube creeping.
Vivitar Series 1 105 2.5 Macro [Komine made]
The 85 1.4 is a very good lens, but it is a fairly expensive lens in any of its iterations [M, A, FA].
Some stuff to remember:Any system is as weatherproof as the combo you put on it:
No Pentax is truly Weather Resistant unless paired with sealed lenses.
Those lenses in the Pentax line:
The DA* Series [three zooms, and 4 primes].
The WR series [two zooms and a prime]
No other lens in the K mount is weather sealed unless clearly stated.
If you buy a Ricoh lens compatible with K mount, beware that you need to hack it due to the exceeding protrusion of the aperture controlling lever´s flap.
If you crave any modern Tokina ATX Pro lens on Sonicapus camp, remember that most of them are rebranded and simplyfied Pentax lenses [not all].
Pentax Lenses compatible with current bodies metering AND focusing confirmation:
M42 or Pentax Mount
Takumar
Super-Takumar
SMC Takumar
K series
M series
A series
A* series
F series
F* series
FA series
FA* series
FAJ series
DA series
DA*series
DFA series
645 series
67 series
More informatio for the mount:
http://www.bdimitrov.de/kmp/Other lens brands you might want to check with K-mount on Ebay or garage sales:
Cosina
Chinon
Miranda
Vivitar [until 1985]
Russian Exotica [Zenith, Kiev]
Tokina
Angenieux [and this is one of the best optic-brands EVER, so if you come across one, just buy it, almost regardless of the mount].