[aslyfox]

. . . Bottom line: Anything that is over 300mm and produces good images will be expensive. What most do (including myself) is start buying less expensive lenses thinking that they will be 'good enough'. After doing that 3 or 4 times you will have spent as much as you would have spent by getting a good lens in the first place. The DFA 150-450 is a real treat but I wasted a lot of time and money with other cheaper lenses before I understood this.

keep an eye out in the marketplace

The Pentax Marketplace | Buy & Sell Pentax Cameras and Lenses (United States) - PentaxForums.com

it can be sorted by country

or place a " wanted " notice

Wanted Items - PentaxForums.com

you might get lucky on an " experienced " HD Pentax-D FA 150-450mm F4.5 - 5.6 ED DC AW

HD Pentax-D FA 150-450mm F4.5-5.6 ED DC AW Reviews - D FA Zoom Lenses - Pentax Lens Reviews & Lens Database

HD Pentax-D FA 150-450mm F4.5-5.6 (APS-C)
Introduction

HD Pentax-D FA 150-450mm F4.5-5.6 Review - Introduction | PentaxForums.com Reviews

yes, it is big

and heavy

but many use it without a tripod

and it is pricey

good gear often is

[MossyRocks]

My only advice on teleconverters is to stay away from the 1.4X-L and 2X-L converters as they only work with some lenses and I don't' believe that they would work with your setup. If you have a lens they will work with they are great and probably as good as you can find for teleconverters.

Good long lenses are expensive. Probably the cheapest you will find is a Pentax 500/4.5 which isn't a bad lens but is a very old design. I see a mention of getting a 300/4 and a converter for it so that is an option too. I got a steal of a deal on my Sigma 300/4 APO Macro and it has been a good performer. A 1.4x or the 1.7X converter would probably work well with it.

[Bob 256]

Referring to the previous comment, I have to say, it depends a lot on what you intend to shoot with a teleconverter. I've had good luck with a X1.4 and even a X2, but my subject was the sun which is pretty bright, and even with a higher equivalent f-stop, needed ND filtering to reduce its light. That said, I almost never use a teleconverter for poorly lighted subjects because of the light loss. The other caveat is that if you have a badly performing lens, the teleconverter will only make it that much worse, so if used, the primary lens should be a good performer on its own (and of course the teleconverter needs to be a good one and not a cheap knock-off). I have to agree with much of what has been said, and believe that in many cases, cropping will produce as good or better results though at the loss of some pixel resolution. In some cases however, like the case I gave, telextenders can have good use and if the lens and extender are chosen well, result in great shots. They can't be totally discredited, but use with some degree of "if there's a better way, use it" mentality.

Shot below: K-1 and 300mm main lens with X2 teleconverter (and cropping). Original shows fine details around the sun's rim.

[boriscleto]

I used the DA L 55-300 on a Vivitar Series 1 1.4x AF teleconverter before I got the Sigma 150-450. The only time I had trouble with AF was in poor light, deep woods, evening shadows etc.

It was basically a Tamron 1.4x PZ without the PZ contacts, so it didn't work with my lil'Bigma. They used to cost $99, but they stopped making them years ago in K-mount.

[ramseybuckeye]

I’m well aware of the recommendations. However a ton of plm users have been reporting success. I’m myself only use my da converter on my prime da* 200 and 300 typically and occasionally on the da* 60-250 or 50-135.

I would suggest comparing output against cropping as mentioned. But flat denial of the ability to focus as suggested by some is clearly not black and white.

Sorry Brad, worded that wrong, quoted you to agree with an example and meant the chart for the OP.

[Dutch mosquito]

Both the HD DA 1.4 x Teleconverter from Pentax and the Tamron-F 1.4X Pz-AF MC4 are optically very good, both transmit the aperture data and both enable autofocus (Pentax converter better than the Tamron converter). However, there is an important difference between these two converters and that is that the Pentax HD DA 1.4 x Teleconverter transmits the focal length of the lens times a factor of 1.4 to the camera and the Tamron-F 1.4X PZ-AF MC4 the focal length from only the lens. If you shoot with shake reduction on, then the shake reduction will only work correctly for the Pentax teleconverter, not for the Tamron teleconverter.

[UncleVanya]

Sorry Brad, worded that wrong, quoted you to agree with an example and meant the chart for the OP.

Ooops. No harm my friend. Speed reply on my part sorry if it came across cross.

[Papa_Joe]

Concerning a longer lens, I would suggest to read this thread and name a lens as well.

Help choosing longer lens, on a budget. - PentaxForums.com
Tokina 400mm f5.6 ATX AF Lens Reviews - Tokina Lenses - Pentax Lens Review Database

Price is reasonable. IQ is good. You just have to find one.
And you can use the 1.4 teleconverter with it.

What do you need the longer focal length for?

[Cerebum]

Before you throw your 55-300mm in the bin 😁 it is a good and very capable lens for the price (did you calibrate yours? Because that helps) but the ones listed above are excellent, if sadly waaaaaay out of my price range. I picked up a vivitar KAF 2x tc off a forum member for £20. When it nails focus, it's great, but in anything but excellent light it hunts like crazy. I read somewhere that AF is designed to work best up to middle apertures and that's why a tc on a lens that starts at f5.6 is an issue. If you can get a cheap KAF TC, buy it and have a play. It won't do any harm and, if the light is good, you may just get the image without breaking the bank 😁