The image is excellent, but does need correcting so the sky is black.

The 12-24mm doesn't really lend itself to PL filters for a couple of reasons. The first, as Steve has pointed out, is that you will get different polarisation across the field of view. This causes banding in the sky and looks quite odd sometimes.

The second is that ultra wides have natural vignetting and this can be quite enough without needing to darken skies further.

The third is that filters will reduce quality, especially when the light is hitting a two part PL at angles from the edge of the field. You will need a very expensive PL if you decide to go ahead, if you want to get the best out of your lens.

That's very odd. The only thought I have is to ask if you are sure that you have bayoneted the lens hood firmly into place? I know when I first got the 12-24mm I found I wasn't being firm enough locking it in. Just a slight difference meant that there was some vignetting, similar to what we see in your shots.

The sizing of the petal lens hoods seems to be very precise.

It looks that way.

I suppose that seeing not everyone uses the leads and they are more often than not left out when cameras are sold second hand, there must be quite a waste in leads.

Some manufacturers don't include a proper charger either, which is something I do find essential.

In the studio, using flash, probably at f/8 or f/11, you don't need a fast lens. The 18-135mm would be a good choice as it's very versatile for other purposes as well.

The standard answer would be a fast 50mm. The DA 50mm f/1.8 would be a good choice. The classic "85mm" on APS-C would be the DA* 55mm f/1.4, pricey but quality costs money.

Why does there have to be a "personal limit" on MP? If you asked the question when pixel counts were 3MP then why would you ask it then, and why ask it now? Whatever MP are offered will define what's available.

There will no doubt come a point when further development in quantity wouldn't yield any image improvement that the eye could see, but no doubt then manufacturers will find something that needs the extra pixels.

How about the 24 hour holographic 360 degree camera that you wear all the time, recording your entire life as you go? Then you can shoot your stills from the record, after the event, at any focal length you like. That might need quite a few pixels.

I've just moved it for you, after all it's only the first post that isn't connected to the actual image posts.

They might get more attention in Your Photos or the Gallery. I think the idea is splendid and the results you have are very good as they are.

Whether it would happen here or not is something else, but I agree there's nothing demanding about making normal, sharp images with the K-3. When people say the best technique is neded to get the best out of it, it's no different to anything else in that respect,

If you want the sharpest images, use a tripod, use the 2 second self timer to avoid shake and use the lowest ISO setting. Much of the time that's not necessary of course.

I don't reallt see it as such a conundrum. If I can afford it, K-3. If I can't run to that, then K-5 IIs. If it needs to be lower cost still, then K-50, and so on.

I don't think we can ignore cost when saying "which is the one of choice?"