Fortunately, in practice it's not a problem and you'll get crisp shots at f/16. They won't be as crisp as the optimum aperture maybe, but still absolutely good enough. f/22 and beyond is another story, but if you need the DOF then, as always, there's a trade off.

What you are referring to is Depth of Field. Depth of Focus is something else entirely and refers to the amount of leeway available in the actual location of the sensor plane.

There are depth of field tables in books and no doubt on the web, but if you want a simple "rule of thumb" for landscapes then just use a small aperture, such as f/16, and focus on a distance about a third of the way in to a scene.

A thorough understanding of depth of field is useful, so a suitable book on lenses would be the way to go IMHO.

18-135mm is excellent and adds WR. The 18-270mm is even more versatile for travel, but lacks WR.

More than likely you haven't set the aperture on the lens to "A". As soon as you do that, all should be well.

These were all shot on the K-5 with the 18-135mm WR lens.

On Saturday we had an ADAPS Manchester Stroll:

SRS have a terrific offer on the K-5 at the moment, so I'd be tempted to go for that. The K-5 series are more rugged than the K30-K500-K50 series, and smoother operating as well. The shutter is quite a bit quieter, for example.

All the Pentax DSLRs are excellent though, so it's a tough call.

I'm just back after this event, where we hired six models for the day and arranged with a local mill owner to use the disused floors of his building. There are many images to process, but in the meantime here's a glimpse of our models. This also gives a clue that there were some pretty exciting props to use!

I'm glad you picked up the Lowry-esque aspect. The image has been stretched intentionally for that reason.