I use a K100D and sometimes use A Sigma 400 with it. This is good enough for Motor Sport and some Railway shoots but no good out on the local nature reserve for bird shoots. It seems I have a choice of buying a 30' step ladder, a boat or an improvement in the lens department. Boats and ladders are not practicable and a lens like an Apollo rocket and as wide as a dustbin lid are not in my price world. I've read a number of articles on the Samyang (and its other names) which leave me thinking it is probably not a good buy as its images sound poor. I dont want to spend £300 on something that is going to disappoint. So I have to compromise. I'm thinking along the lines of a Teleconverter for the Sigma. I am not sure if they are available in a form that retains the automatic functions of this lens. Manual operation would not be the end of the world if it had to be so. I realise I would loose F stops (I'm used to that from film days so understand that side). My line of thought is I am better to spend X pounds on a half decent quality converter than the same on a poor quality underperforming super flop.

Thank you all above for your helpful comments.I have a sort of top spend of about £500. Taking that a bit loosely I have seen Sigma 120-400 & 150-500. I believe in both cases the K100d would not run the Auto focus, and why pay for a bit I cant use? I want the quality of image that will make a Sparrow look like it has feathers not fluff at postcard size of print with the bird being Sparrow size type of image. I have looked into using a scope but its not the way I want to go. Similarly from my reading I dont think mirror lenses are for me. There is quite a lot of info on the Pentax User on this subject and as pointed out above and in the Forums it is a bit of get some idea of shots from various lenses, find one at the right price and try it out and post some results here to help others.

Hi,
my first post to the forum so I hope I dont sound a bit clueless! I've not long upgraded from a Fuji 5600 to a K100d as my photography interests have moved on to bird photography. I want a 500mm telephoto for this as the Pentax FAJ 75-300 is often not big enough and also I think the image quality is not quite good enough at the 300 end. I am on a constrained budget and dont mind using older equipment if the quality is there albeit if things are not as automatic as the newest lenses. The manual is written in a slightly confusing manor on the subject, so using my past knowledge I thought a quality lens of some vintage would be a cheaper way to go and if it didnt automatically do things I would be able to by running the K100d in its various manual modes. (As I used to with my old Zenith E). Camera shops and Ebay have various makes of lenses but I dont know which are good and which to avoid.