Ok, it's a fairly common Pentaxian question but I want to put some limitations on it. Firstly I'm talking about birding and sharpness really. Has to be under £1000 and ending up with at least 400mm and preferably autofocus. The options I can think of (for me) are:-
1. Sigma 150 - 500mm
2. Sigma 120-400mm
3. Pentax 300mm DA* + Kenko 1.5x teleconverter
4. Pentax 65-200mm + teleconverter

I would want the final quality to be at least a bit better than my 55-300mm DAL.
All views welcome.
Dave

I've got the 60-300mm and it's ok but I would say my DAL 55-300mm is better and obviously has autofocus.
Dave

Yes, they do kind of have that cuddly appeal don't they.

Thanks for the quick and useful replies. I think I will adopt the following tactics with the older lenses:- buy, try, keep/resell, until I can afford a more expensive prime. These older lenses will sell for the purchase price or more if I get a bargain so not much to lose and as you have alluded, fun to try. I have already some acceptable results with the Tamron 500mm mirror lens which I would not have thought possible.
Cheers, Dave.

I guess there is an element with this that is similar to people always advocating the car they bought is "The best". The problem I'm having is discerning whether a particular lens will give me noticably better results than my 55-300mm DAL on a Pentax K-x. This lens seems pretty good so will I only see clear improvements by spending mega bucks. I'm really talking sharpness/resolution performance at the 300mm end for bird photography. Although the lens database is extremely helpful the user ratings are subjective. How do I know if, say, a Tamron Adaptall SP300mm prime will be better than a modern digital ED zoom such as the one I have. All a bit confusing really, especially if one is looking at older manual primes to keep costs down.
Dave

Thanks for that Dan. Hopefully later they will give the exact model and maybe add some longer Sigmas. Typical that I want to try the most expensive - the 300mm prime.
Dave

This is great news. Lenses-for-Hire have said they have no intention in the future due to low demand. All things are relative but as a sole player in the UK market I was seriously thinking of setting up my own cottage industry. At standard market rates the payback time is pretty good and I think SRS will be overwhelmed with demand. Hopefully it will include a telephoto prime (e.g 300mm DA ED SDM). In fact I think anything is possible right back to the older favourites if they can get them as well as Sigma.
I will be a definite customer if prices and terms are similar or better than equiv Canon, Nikon.
Thanks SRS,
Dave

I should have been a bit more precise. I guess I mean the cheapest lenses that will give an improved IQ over a zoom. I currently have the pretty good 55-300 Pentax DAL but doesn't seem too good with a tele-converter so would like a straight 400mm that might give at least as good or better quality than the 55-300mm. I should add that since writing this I have become more aware of good earlier 300mm primes that could achieve this focal length with a tele-converter (Tamron/Kenko) and still give as good as quality as the 55-300mm.

I always think it's difficult to answer "the which one shall I get" questions without knowing a person's exact priorities.I wouldn't agree that the K-x is overpriced for example. I thought £595 to be quite reasonable for a DSLR including standard lens and a quality 55-300mm zoom that sells separately for £200-300 ! Personally while the k20D is probably better overall I found it too weighty for me and wanted the brilliant K-x high Iso performance. Others will have different priorities.

What are the best value Pentax-k mount options available for a 400mm prime lens? I would consider 3rd party or legacy lenses provided the quality was good.