There has been no FA55mm 645 lens, however, the mock ups of the forthcoming 645D wear a new 55mm lens. I don't know what name the new lenses will be given (FA55mm 645D or something?) but they'll certainly have the old 645 bayonet mount. Depends if you're in a hurry because it may still be some while off...

Those clouds have an almost liquid feel - lovely stuff

Quite. Unfortunately the laws of physics dictate that a 300mm lens with an aperture of f/4 is going to be rather on the large and heavy side.

Actually, when Amateur Photographer tested the K-7 and D300S back-to-back a couple of weeks ago they awarded higher marks to the Pentax in the "Image Quality/Noise" section. It was the only area the Pentax won in, with Nikon taking the honours in both AF and Metering categories and the rest being a tie. The Nikon, therefore, scraped through overall as their recommendation. I tend to trust my own eyes before that of any reviewer but I find AP more reliable than most.

Nice idea, but Leica don't have to worry about little things like mirror boxes in a rangefinder - nor whether their customers have deep enough pockets to sustain such a model.

Yep. Vivitar 400/5.6 is probably about the cheapest you'll find in K mount. I sold a mint-ish one for £30 on eBay and that was about £25 more than it was worth to be honest...

The problem with lenses like that is that you'll get better results from a (comparatively) cheap 70-300 like the Tamron and cropping them. To get better results than the Tamron on a tight budget there are far fewer options (the Tair or Takumar I mentioned being probably the best bets).

Both the Takumar and the Tair lenses I linked to are sharp at larger apertures - to the point where they're perfectly useable wide open (although with all old lenses there's always a possibility that it's been mistreated at some point in its life which could lead to misaligned elements, etc - especially if it's been dropped so caveat emptor!).

On the minus side both will show greater CA's than modern designs like the DA*300, but this is easily fixed in PP these days so it's not such a big deal.

I've also had old 300/400mm lenses from Sigma, Vivitar and Pentacon pass through my hands and I wouldn't recommend any of them - performance being no better than the current Tamron 70-300, but without the modern conveniences that lens offers.

In terms of bang-for-buck you wouldn't go far wrong with a Tair (if you can find a nice clean non-abused example) and the same goes for the Tak (which gives you an extra half-stop of speed but will cost you slightly more). Happy hunting!

Better images? No. They use the same sensor so all else being equal the images should be too.

Better handling? Yes. Twin control wheels, more external controls and brighter pentaprism viewfinder means the K10 should be a more pleasant and intuitive shooting experience - which may, of course, lead to you getting the better images you mention...

Used values will depend very much on condition but expect £250+ for a half decent one.

There are reasons and they centre around reflective surfaces, but rather than re-invent the wheel I'll point you to this thread from our resident P-TTL expert (although his tests were with a K10D and AF540FGZ the results are the same if you substitute the flash for a 360 model):

Link

See if that explains your results. It probably will but if not feel free to ask again.

Me? Near gym equipment? Can't think of a less likely scenario...

No.

Focal length is focal length and a 300mm lens on film is still a 300mm lens on digital.

The difference is that your digital sensor "sees" a cropped part of the image projected by the lens (because it's smaller than a 35mm film area). Hence a 300mm lens fitted to a digital camera will "see" an area that's roughly the same as that "seen" by a 450mm lens on 35mm film.

A 300mm lens designed for digital will give exactly the same view as a 300mm lens designed for film on your GX10.

Does that make it clearer?