There are test shots at each ISO setting here: https://www.ephotozine.com/article/pentax-k-1-expert-review-28884/performance - if we download the High Res images then we can see for ourselves where the quality gives up.

Having said that, in extremis and with an important enough image then any ISO can do and make a successful image. Or the noise can be used creatively.

Just enjoy it and buy whilst the prices are low?

The alternative to volatile prices is Resale Price Maintenance, where the price was the price, by law. That was abolished many years ago, but some companies took the attitude, "Whatever the rewards, we will never stoop to discounting!" So cutting prices was clearly immoral in their minds, although they didn't survive ultimately, so there must be a lesson in that somewhere.

Absolutely spot on Kris.

Well a 500mm f/2.8 would indeed be a very fast lens, but a 24mm f/2.8 would be quite normal. It is definitely something that is relative to the focal length.

I think this is a pity in many ways. In any show there are defining moments scattered throughout the performance and these just won't be seen and recorded if only one two or three songs are available for photographers. The audience I guess has the advantage in this. For example, one of my favourite shots from last year's Frankie Valli tour was a really relaxed shot at the end of the concert, with the group happy that it had all gone really well, earpieces discarded and that was it. The look of satisfaction of a job well done is a defining moment for me.

An MX-1 at ISO1600 could cope quite happily, so I'm sure a K-1 would do even better.

From the K-5 onwards, there has been a sea change in the ISO performance of Pentax DSLRs. With the K-1 I would just use whatever ISO is necessary to ensure lack of camera shake. Yes, the higher values will show more noise, but only like a fast film would have been expected to show more grain. It is so much better though than film ever was that we are almost totally freed in normal circumstances from it even being a concern.

As for f/2.8, this seems to have become a fast lens, probably because of the ascendancy of zoom lenses, but for prime lenses it is not at all fast. Lenses of f/2 and faster are fast lenses as I know them, so we are in the realms of the original three Limited lenses, the 50mm f/1.4, the upcoming 85mm f/1.8 and so on.

Your histogram is heavily to the right, which might indicate a bit of over exposure. Certainly I think your sample image could be a little darker.

I would suggest using centre weighted metering and making sure that the exposure compensation is set o -0.3EV or -0.7EV.

As regards CA, branches against bright sky are a demanding test for a lens. If you shoot JPEG and make sure CA correction is switched on in the menu that should take care of it.

Probably the two new lenses already announced?

Please see info further up the thread, but these days I tend to use 1500 pixels wide for the Gallery and still use 1000 pixels wide for the forums.

I think a step at a time. The viewfinder image is a purely optical one, so it can't stop working as such, but if the AF isn't functioning properly then I'd suggest checking the AF/MF switch is clicked fully home and then wiping the electrical contacts on lens and camera body with a soft, dry cotton cloth. Finally, new or fully charged batteries and we should be good to go.

To make sure we are focusing properly using the viewfinder, it's also essential to set the dioptre for our eyesight. Point the camera at a uniform, light coloured wall or similar and adjust till the etched lines in the viewfinder are sharpest. The image does not have to be in focus to do this, in fact better if it isn't. Then your eye is relaxed when looking at the focusing screen, which has a virtual distance of about 1 metre.

SD cards can become corrupted, but this can be avoided by making sure that images are regularly transferred to the computer and that the card is formatted in the camera it's going to be used in, not in the computer. Only change cards when the camera is switched off. Don't resave from the computer back to the card.

If all else fails, then a faulty camera needs to be returned, and hopefully with only 500 shots it isn't very old so will be under warranty. And cameras do fail sometimes I'm afraid, although to be fair it does seem they are generally very reliable.

Hope that helps!