FULL FRAME OR APS-C
It's nice to have the K-1 because it offers better dynamic range: K-1 14.6 vs K-3ii 13.6 and high resolution for large format prints provided it is coupled with good lenses. It is also a feature rich camera and offers flexibility because of this. The downside is the substantial hike in cost, size and weight of lenses in comparison with the APS-C equivalents.
If the new K-3 iii came with fully articulated rear screen and built in gps, then I might well have sold the K-1 and it's lenses because I do not print sufficiently large to justify the extra resolution.
In the end, there are quite a few variables on any such decision, that would depends on each individual. I keep thinking about getting a K-1, but I end-up spending the money on film and developing instead.
K.
Kris Lockyear
It is an illusion that photos are made with the camera… they are made with the eye, heart and head. Henri Cartier-Bresson
Lots of film bodies, a couple of digital ones, too many lenses (mainly older glass) and a Horseman LE 5x4.
My website
The single biggest factor in going Full Frame was unlocking the potential of my legacy wide and ultra wide angle lenses. The other factor was the greater resolution and ability to print larger.
After using the K1 for a while I realised a few other things - firstly, the articulated rear screen (which I had dismissed as a gimmick) is incredibly useful. Secondly, the ability to crop quite severely without loss of quality is notable and makes the FF option more flexible. Finally, the new generation of lenses are truly superb on the K1 series - yes, they are expensive and heavy, but they are the amongst the best you can buy (probably of any brand). For me the K1 series are the right option 95% of the time - there are some times when I have dug out the K5iis for its size and weight advantage, but very rarely nowadays.
Hope that builds on Ronnies comments and helps a little

LennyBloke
Whatever price it starts off at within 6 months it wil be £150-£200 cheaper anyway and if the AF is the best Pentax AF yet as hinted at, then given your lens investment already maybe the best way forward for you.
I had the same dilemma as you, I had a K-3, a K-5iis and then recently bought a KP and sold the K-5iis. However, I need several cameras as I will be running photography tuition and walks in Corfu this year and will need to loan out equipment. So, I also bought a K-1 a month or so back and a cheap (£60) K-5 with 100,000 shutter clicks on it to act as a 'loaner'.
My biggest issue with the K-1 was lenses. I had many APS-C lenses, but very few full-frame ones and none with AF. I took the decision to hunt for bargain basement AF lenses for the K-1 and have managed to find some real bargains that are more than good enough to satisfy me and have good enough IQ to make prints. Among them are a Tamron 70-300 LD Di for £36, a Pentax-FA 80-320 for £20 and a Cosina 19-35 for £33.
I absolutely love everything about the K-1, the IQ, the feel, handling and weight of it, the way that images look on the full frame sensor, the dynamic range, the GPS and Astrotracer, the list goes on. My Mk1 K-1 with less than 3,000 clicks was bought for £870.
However, there is a 'BUT', I also have the KP and this is a superb camera, I would highly recommend it. The only thing I don't like is the smaller batteries which run out too quickly, but apart from that it's excellent. Fantastic IQ, better than the K-3 and not a million miles from the K-1, superb ergonomics, an articulated rear screen and one killer feature (for me at least) is that it has an additional focus-peaking feature that is not on the K-1. This is called 'edge peaking' and presents a flat grey image with only in-focus highlighted edges visible. This is highly accurate and so much easier to use than the normal focus peaking.
If you don't want to wait for the K-3iii and are unsure about making the jump to full frame, I would recommend you consider the KP. Careful looking around will land you a brand new one for under £600, I got the silver one and it cost me £579.00. To me, that's the bargain of the century in camera terms.
Regards
David
Flickr
Nicola's Apartments, Kassiopi, Corfu
Some cameras, some lenses, some bits 'n' bobs

A question is only easy when you know the answer ..
At the end of the pondering it is about self knowing, then you can make your choice in confidence. Good luck and enjoy!
Nigel
I wonder why anyone would not move to full frame if thinking of upgrading.
Do you print ? If printing at a size of 24"x16" or larger the 36MP sensor has distinct advantages.
If you have a large collection of FF older lenses it is nice to use them on a format for which they were intended, otherwise there is little to choose between aps-c and FF.
I say this as a K1 user since 2016. I would not now go back to aps-c, but if I did, it would not be a retrograde step.
There is not much difference between the two formats when one considers modern software tools such as HDR merges
Peter
My Flickr page
if u don't mind me asking where on earth did u get a new KP for £579/600

Hi Maranatha
If you search ebay you will find them. Mine is the silver edition. There is one on there now with only 5 shutter actuations at £585, see link:
link
Regards
David
Flickr
Nicola's Apartments, Kassiopi, Corfu
Some cameras, some lenses, some bits 'n' bobs
A question is only easy when you know the answer ..
Do you print ? If printing at a size of 24"x16" or larger the 36MP sensor has distinct advantages.
If you have a large collection of FF older lenses it is nice to use them on a format for which they were intended, otherwise there is little to choose between aps-c and FF.
The largest print I have ever had made in 39 years is 20 x 14. I really don't want any more magicpixies than my K-3 as it is just yet more hard disk space and back-up to facilitate. Just as the vast majority of my slides were never anything more than a slide (although they did get projected), the vast majority of my pictures never end-up anything more than on-screen or in a book. I have a couple of boxes of prints from my film images that I am steeling myself to throw-away. I have the negs, I have the scans, I never look at the prints.
As to the older glass, that is one of two reasons I sometimes consider buying a K1. I then I remember I have dozens of "full frame" cameras that I use regularly. (The other reason is the in-viewfinder levels on the K1.)
This is a classic "each to their own" sort of question.
Kris Lockyear
It is an illusion that photos are made with the camera… they are made with the eye, heart and head. Henri Cartier-Bresson
Lots of film bodies, a couple of digital ones, too many lenses (mainly older glass) and a Horseman LE 5x4.
My website
Philip
(*If the K-3 Mark III had included a movable LCD screen, I would have been willing to pay for that new APS-C camera.)
Personally, I love my K-3 and would love to get a K-3 III if it ever becomes available at a reasonable cost (not guaranteed with the current market). I have all of the DA Limiteds and many other APS-C lenses which I couldn't replace on FF, either because they simply don't exist or because I wouldn't be able to afford them. These days with kids, a smaller kit is advantageous too and I suspect there would be too many situations where I would leave a K-1 behind and take my much smaller and lighter Fujifilm kit.
The K-3 III will almost certainly have the 26MP sensor that's in most modern APS-C cameras and it's been shown to be superb.
It's all down to what you prioritise.
Pentax hybrid user - Digital K3, film 645 and 35mm SLR and Pentax (&other) lenses adapted to Fuji X and Panasonic L digital
Fan of DA limited and old manual lenses
Regards
David
Flickr
Nicola's Apartments, Kassiopi, Corfu
Some cameras, some lenses, some bits 'n' bobs
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With the imminent? release of the K3 lll (APS-C) and a supposed cost nudging close to £2000 I wonder why anyone would not move to full frame if thinking of upgrading.
Any thoughts and observations would be appreciated.
Andy G.