I have used the 300 for several years and loved the quality of the images. I also have the manual focus example which I purchased in 1977. which was a superb lens(optics look the same as the autofocus lens). I purchased the 150-450 whilst I was at the K1 demo day in my nearest camera shop, The first thing I noticed was the serious increase in weight! The focus limiter is a definite advantage. The 300 had a habit of going to the shortest focus point if I failed to find the right spot.
So far I have been pleased with my images. I have tried it on aircraft, small birds and red squirrels. and it performs well.
I use a Bushawk Rifle grip which I find helps with supporting the weight.
prior to buying the 150-450 I used the 300+1.2 converter on my K3 and the 50-135 also with a 1.2 converter on my K5 II at airshows. This year I plan to use the 150-450 on the K1 and a 24-70 on my K3 at the shows.

Thanks! I will check immediately!

I have read many reviews about the K1 and I was impressed. I was lucky enough to attend a presentation at my nearest dealer (40 miles away at Cambrian Photography in North Wales) and I am now even more impressed!!!
All that I really wanted from full frame was a bigger viewfinder, But the K1 has so many features that I will find useful. and it felt just like I was holding my K3! I have placed a pre order for a camera and grip and bought the 150-450 to play with until the camera arrives! According to the rep. Pre orders have outstripped supply already! Apparently the 70-200 will be released end of March.

I had a similar problem with my 50-135 about a year ago. I was taking part in a photoshoot at a steam railway event. I switched the lens from my K3 to the K5 II and it worked fine. But back on the K3 it failed to operate! Other lenses worked perfectly on the K3 so I was baffled! I later cleaned the contacts on my lenses and bodies with isopropyl alcohol and have never experienced a recurrence so far! My lenses are used regularly.

Superb images! I was near the white tower just out of your frame of the red`s shot. I used the K3 with 300mm f4 and 1.4 converter plus K5II with 50-135 but I was unable to fill the frame so I left early! So well done on your great shots!

I have the screen problem too,
How do you revert to 1.11?

.
. My 300mm f4 has been tried on my 35mm MX and KM camera`s and looks fine so that should be a safe bet for FF (it looks optically the same as my old 300.f4 that I bought in 1977) (SRS Microsystems very kindly tried it on a 35mm film Pentax for me before I purchased it too!)
.I have a 100mm Macro
Two 1.4x converters
.My collection of M series manual focus lenses include -20mm f4, 35mm f2.8, 50mm f1.7.100mm f2.8 (non macro), 200mm and the 300mm f4 all purchased in the 1970`s. I am looking forward to using the 20 mm on full frame as it`s a stunner, the rest will be replaced with FF, There should be a 24-70 f2.8 in the pipeline which I will buy.
I have pre-ordered the 70-200mm f2.8 from my local dealer.

The only thing that I have (from a trustworthy source) is that there will be a Full Frame camera early next year. Nothing more than that.

I have also been attending a lot of airshows this year, At least one and usually two per week, So I had the opportunity to try out a variety of techniques to improve my success rate with the K3 but to be honest I was unable to match the K5II. The mirror flapping is neither here nor there as it is only a minor inconvenience. (If actual images were recorded then I would have been annoyed as it would have meant waiting for the writing to the card to finish before continuing to shoot.)

My own experience has been somewhat mixed. Using the selective spot focusing has been superb on many subjects with my 300mm DA*, Including. surfing, Motor racing and Airshows. But this requires you to lock onto something of contrast which of course is not always possible.If the subject is small and fast moving you cannot always get the spot in exactly the right place so the 300mm just zooms out all the way and stays there!
Some subjects require auto 27 (K3) and this rarely results in acceptably sharp images yet the K5 II can be relied upon to come up with the goods almost every time.