This is where the DFA 100 has a slight advantage (though the differences between these three are a toss-up really): it isn't as sharp wide open as the Tamron is (it's slightly sharper stopped down instead).

What is the serial number of the lens in the second picture? Looks like it begins 69 to me (compared to 72 for the other one) - maybe they are from a few years apart (not suggesting that the first two digits are year of manufacturer, but rather than these are seemingly several batches apart) and Pentax changed the precise composition of the coating between times?

The Tamron 90mm is a very good lens as are the Pentax DFA 100mm and Sigma 105mm macros - the differences between them are so slight that they are often referred to collectively as '100mm-ish macros' (i.e. it doesnt matter for image quality which you get - buy whichever is on the best deal or 'feels right' for you). I quite often find my Pentax DFA 100 too long - ie I can't get far enough away from the subject - but I like the images it produces on the occasions where I can. A 100mm-ish macro lens also has the advantage that it does double duty (ie you gain a good portrait lens, AND the best macro focal length).

If I were buying a dedicated portrait lens, it would likely be the DA70 Limited: having tested 70mm as a focal length (on my DA 50-200) it gives a more comfortable/natural distance than either 50mm or 100mm (I have lenses of both - see signature), it also has a faster aperture (f2.4 vs f2. and faster autofocus (due to no macro) than either of my DFA primes.

{EDIT: This got a bit longer than I planned. If you just want the recommendation feel free to skip it }

As for a landscape lens, I might go left-field and suggest the DA18-55 kit lens. It isn't the fastest not the pixel-peepiest, but the images it can produce (especially stopped down, where a lot of landscape photography takes place) are lovely.

On the assumption that 'use the kit lens' is not a useful recommendation to you, the two main alternatives are the Tamron 17-50 and the Pentax DA 17-70 I mentioned earlier.

The Tamron has f2.8 (compared to f4 on the DA), and is cheaper (by about £120 or so).

The DA 17-70 still has constant aperture which is the real important factor for landscapes (you don't need to change the shutter speed when you zoom in) - it is also likely to be much better built than the Tamron (Tamron's 28-75 and 17-50 have bad reputations for reliability, the 90mm macro an excellent reputation). It also has that extra bit of reach which might be just enough to mean you don't need to take a second lens with you for walkabout - always nice.

The DA*16-50 mentioned above has the capacity to produce some very nice images - it is also weather sealed (the only lens on this list which is) which will be useful for outdoor work if you have a K20D or K7. However the 16-50 has a dark soul: there are reports various failures all of which (autofocus, poor image quality on one side, etc.) seem to be caused by misalignment (ie the lens being built squint). These problems tend to be magnified by internet forums (when was the last time someone posted "I bought a lens a year ago, and nothing has gone wrong. Well done manufacturer!" ?), but it seems a safe bet to say that the DA*16-50 is the least reliable lens (ie most likely to go wrong) in Pentax's current lineup. It's also less sharp wide open at the wide end (ie f2.8 at 16/17mm respectively) than the Tamron is.

Recommendation

Landscape: DA 17-70 f4

Portrait: DA 70mm f2.4

Thanks, but for that price...

Given that Canon have more than ten times the market share of Pentax, that isn't actually an acquittal!

Having a single lens for both portrait and landscape photography is tricky, because landscapes traditionally require quite a wide lens whereas portraits require a moderate telephoto. The closest you're likely to get to it is the Pentax DA 17-70 f4 (or alternatively the Sigma 17-70 f2.8-4 OS HSM, which is cheaper). This is wide enough for most landscapes, while being long enough for portraits. Its only challenge is that it is only f4, which is fine for landscapes (you'll be shooting at smaller apertures for depth-of-field anyway) but not necessarily the best for portraits (where larger apertures - f2.8 or faster - are normally preferred).

Bump! Still looking.

Lightroom is an absolute godsend - not only does it RAW process very well (I must confess that I cheat and use a set of presets from the Other Forum), it also combines this with a Picasa-like* album system which is much easier than remembering what you shot on which day. It's also a lot more responsive than I remember the Organizer in Photoshop Elements being - though that was a couple of versions ago.

*I won't say Picasa-beating, because it still will not apply keywords to a selection of images in one go

As an example: if I'm taking a sunset photo (let's assume the sun has already gone down to avoid reverse grad issues ), I might well want to have the darkest part of the grad over the sky in order to counteract the ground being entirely unlit. If using a 100mm grad on a 52mm-element lens, I will not be able to do that without having the dark area also covering the bottom, since on grad filters the top half of the filter (ie 50mm of it on a 100mm filter) is dark to varying levels - correct?

Stefan, we are still waiting for the Tokina 11-16mm more than two years after its release. I'd suggest that this will be lucky to make it to Pentax by 2014