[shardulm]

I have used a DA 55-300 on K-1. It is very usable at 300mm on K-1 in FF mode.Guess when reach is needed you need 300mm or more. Ands it is indeed a long reach lens on APSC especially on a 24mpix APSC.

[surfar]

I have used a DA 55-300 on K-1. It is very usable at 300mm on K-1 in FF mode

As well as being fine with a 1.4x TC.

---------- Post added 01-12-17 at 04:09 AM ----------

My bad - forgot to mention but I'm also considering second hand lenses.

Theres many to be considered, just read the reviews when you spot an available one....plenty of value if you search.

[DeadJohn]

Suggestion: Get the 28-105, use it, then reevaluate what your 2nd lens should be.

Standard zoom
Pentax 28-105mm 3.5/5.6
I'm a bit lacking of options here so would welcome any recommendations. I enjoyed my kit Olympus 40-150mm f4/5.6. The Pentax28-105 apparently is pretty good for the price but I'm concerned the zoom might not be enough.

40-150 on micro 4/3 Olympus is like 80-300 on full frame K-1. This is "telephoto" rather than "standard" zoom.

However, you say portraits and street. Street means different things to different people, but a standard zoom like 28-105 should work okay for that. You can also use the lens for portraits but may eventually want to upgrade to a wider aperture lens for thinner depth of field on portraits. To use the 28-105 for portraits compose with ample distance between your subject and background to get more background blur.

If you need telephoto for wildlife or other subjects, keep the Olympus as a 2nd body until you can afford a longer lens for the K-1.

Landscape/milky way
Samyang 14mm 2.8 (read it has some bad distortion issues though)
Laowa 12mm 2.8
Irix 15mm 2.4 It looks like the Laowa and Irix have very little distortion however I'm not sure how they would perform for astrophotography.

The Samyang/Rokinon/Bower 14 2.8 excels at astrophotography because it has very little coma distortion. It does have other distortion issues but that's generally not noticed in sky photos and landscapes, being more of a problem for architecture where curved lines are more obvious.

IMO 14mm is exceptionally wide on full frame. Before committing to a 14mm lens, use the 28-105 at 28mm for some landscapes and Milky Way. If 28mm seems like a good approximate focal length, all those lenses on your short list above are too wide. Consider the Samyang 24/1.4 as an alternative. It's 2 stops faster. Even if you stop down to f2.0 for sharpness you'll only need half the exposure time.

[surfar]

IMO 14mm is exceptionally wide on full frame

Its ULTRAwide!

Consider the Samyang 24/1.4 as an alternative

Yes, probably wide enough for most people....

[Paul the Sunman]

I have used a DA 55-300 on K-1. It is very usable at 300mm on K-1 in FF mode.

Well, it actually vignettes unrecoverably in the extreme corners, so a little cropping in pp is usually required. It's completely fine shooting in 1:1 crop mode (4912x4912 pixels). For comparison, APS-C crop mode is 4800x3200, so it's larger in both directions.

[shardulm]

Well, it actually vignettes unrecoverably in the extreme corners, so a little cropping in pp is usually required. It's completely fine shooting in 1:1 crop mode (4912x4912 pixels). For comparison, APS-C crop mode is 4800x3200, so it's larger in both directions.

As much as I agree in most cases shots @300mm need cropping and vignetting becomes less critical.