sigma 50-500

i am lead to belive that you can convert a canon lens to pentax via a converter but only works on mamaul focus any body know if this is possible i ask because i can have the loan of the above lens

No. You can convert a Pentax lens to work on a Canon with an adapter but not the other way around - sorry!

Well, if by convert you mean professionally strip the lens down, file off a few 1000th's of a millimetre here and there, calibrate it, and attach a suitable mount to the bottom of the newly shaved lens after drilling new holes .... then yes, it's possible. I really want to do it myself, but I got a Pentax mount lens of what I wanted for less that I had to pay for the Canon variant of the lens.

You're better off selling the canon lens and buying a Pentax one - unless it's for the love of doing something crazy. If that's the case, go for it!

Edit: Just as a disclaimer - I by no means think that the resulting images will be great out of the converted lens, so you quite possibly would be flushing cash down the drain. So at your own risk!

Whelmed you mean file off 1,46mm
Canon EOS register is 44mm while the K-Mount is 45,46mm

Also you would lose all aperture control.

Depending on the mount it's not too bad. On the old 50mm F1.2's, for instance, you actually have to add to the base. That's because when you strip off the Canon FD mount, the overlaid Pentax one is actually thinner. I think for that lens it's 0.5mm you have to add. I'm sure there is TONS of sample variation though

I saw the website somewhere for the FD mount conversion, but apparently I've lost the bookmark. Alas. I really wanted to do that conversion but the lenses are so expensive that it just made more sense for me to get a PK mount. Got a great deal on a Porst 55mm f1.2 which I'll be showing off in the next few days when my camera gets back from HK!