Thanks for that, but i have to use autofocus these days my eyes are not as good as they used to be i just can't seem to get sharp focus without it. I intend to buy the 1.7 adapter and use the mirror lens as a stop gap until i can get hold of a good 400mm f4 telephoto. John.

Has anybody tried using a Pentax SMC 1.7 X AF Adapter and an f8 pk mirror lens. I know by rights it shouldn't work but i was wondering how sharp the focus would be if it did work correctly.

Thanks for your quick reply you have answered my question perfectly. What i was hoping that buy buying a 300mm manual focus lense ie: without the DG (Digital) suffix i would gain some extra milimeters due to the cropping effect on the camera but that is not the case and i certainly can't afford the sigma 170-400mm at around 450 pounds.

Having recently bought a K110D (an upgrade from a panasonic lumix FZ7). The lenses that came with it are the standard package 55-200 Sigma DC and Pentax 18-55 DA and i have also bought a Sigma 70-300 DG and a 1.5x AF converter.
I intend to take wildlife (bird shots), Is the Sigma 70-300 a true 300mm or is it 450mm to take account of the 1.5 cropping on the camera, I am assuming it is 300mm because of the DG suffix.
If that is the case I am thinking of getting a 70-300mm Pka manual and a 2x Pka converter to take advantage of the 1.5 cropping. Is this a good idea ? Also will the 1.5 AF converter work okay with a Pka lense (obviously not the AF function). I want to stick to Pka rather than Pk because all the manual stuff is a bit daunting at the moment. Any help or comments would be most appreciated.