Avoid those second hand camera shops in the back streets of Rio. Don't want you picking up a Pentax Z1 with KA mount. I'll get my coat...

Thank you Jonathan for your detailed reply.

When I bought my K5 in 2011 as an upgrade from the K10, I read on here about how much better the AF performance was. I suppose I found out that any change was relative and (tracking) AF still seems to be the usual weakness in Pentax performance. I like the idea of a further Pentax upgrade but I don't want to shell out £1600 and still be disappointed. Hence my ultra-scepticism.

He wrote, "I haven't really had much opportunity to try motion tracking yet although if it matches the K3 I will be happy as that proved a very capable camera for fast moving subjects including running dogs, children and birds in flight."

That's a shame as it is what I want to know about. Makes me suspicious as to why he avoided it.

In order to try and see detail on the goose, I was looking at the darker plumage around the neck. If the eyes were the focal point then I would be looking in the wrong place.

In the 'Scotsman' shot, I thought there seemed to be some softness in the detail of the ballast and foliage and also the number-plate didn't jump out. However, I accept the point about focal length and conversely I am impressed by the noise performance. It must have been fading evening light to have necessitated ISO12800.

As I said before, pictures such as the goose and 'Flying Scotsman' in particular fail to give me confidence that the K1 and/or new lens tracking autofocus performance is up to scratch (though I do note that Scotsman was visiting the Fife Circle well into the evening when light levels were declining).

Have you found out yet whether it can track a moving subject and keep it in focus?

See: http://www.tipa.com/english/XXVI_tipa_awards_2016.html

They seem to have invented a category so that each brand can win a prize!

Check the adjustment of the eyesight correction slider on the viewfinder. Looking at a plain background (well lit interior wall, sky), adjust slider until the in-viewfinder markings are at their boldest. You should try to relax your eyes to focus on the far distance while doing this.

EDIT: Oops. On re-reading I see you have mentioned the eyepiece.

great hi-fi...

By all means choose Pentax for landscape or architecture but I suggest you search for posts referring to tracking autofocus. For moving railway photography I find more consistent results by pre-focussing on a certain point instead of letting continuous-AF do its own thing.