I have a Sigma 50-500mm f/4-6.3 EX DG HSM (Bigma) for my Canon. I do know they make this lens for a Pentax mount with no HSM.
I cannot sing high enough praise for this lens. It is heavy, but heavy enough that it actually stabilizes you so that you can easily hand hold this monster maxed out. Something that could not be done with a much lighter lens. For example, I have a light 500mm f/8 Cat that I could never hand hold in a slight breeze without getting image blur.
this lens is about 90% always on my 20D during nice weather when I like to wander around and take photos of nature.
Just an image to show you the relative size of the lens. It is almost twice the length when zoomed out.
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