[StigVidar]

If I’d known more about the 70-200 f2.8 and how it would work out for me before I bought it, I’d likely not have got it.

The OP mention that he/she have the 55-300 telezoom and should be able to figure out if 50-135 or 70-200 is more useful than the other.

I started with the old FA*80-200/2.8 and got tired of it because of weight and not that good optical performance. So I sold it and bought 50-135 which has been very good. But I missed 200mm and found out I could buy DA*200/2.8 as an addition to 50-135. That has served me well for years, but I used the longer focal lengths most of the time and bought DFA*70-200/2.8 last year and sold my DA*200/2.8.

DFA*70-200/2.8 is heavy, but at the same time not so difficult to carry as the old FA*80-200/2.8. The new lens has a nice tripod foot that can be used as a handle. That was not possible with the old one and it makes all the difference for me.

[gatorguy]

I use a K-3 III, have both lenses, but prefer the 70-200. The 50-135 is little used, almost none the last year. The new 70-200 is sharper and has a lot faster AF. But it is heavy. A good shoulder strap for the lens is essential if you are carrying it for hours

The converter works fine on the 70-200 and usable with wide open aperture. I don't think the converter works very well on the 50-135, but it depends on how much loss of sharpness you can accept. But it is almost always better to crop the image from the 50-135 instead of using the converter.

We have the same camera and lenses, and I agree on the Pentax 70-200 being uncomfortably heavy to use for hours. I've done it as one of my two cameras setups (the other a KP with 16-85 or 10-17) for a day-long multi-band concert event I was hired for, 10am-8pm, so that's from experience. My arm remembers well

Other than agreeing on that point it's not unusual for different owners of the same gear to have different opinions about it. I'm not wanting for 70-200's, with the other two significantly lighter than the Pentax. The Marketplace has been whispering in my ear.

[Fila]

Thank you for such great discussion, it really helps not to just have my thoughts bouncing around in my head.

HD 1.4x teleconverter is really nice suggestion, so far I did not have any lenses to consider it, and it seems very compact way to "add another lens".

[StigVidar]

HD 1.4x teleconverter is really nice suggestion, so far I did not have any lenses to consider it, and it seems very compact way to "add another lens".

It is, but it does require a lens with good optical performance. An alternative if you use raw format, is to upscale your images with what some software call "super resolution". The result could be better than what you get with a teleconverter.