Originally posted by GordonZA
The Sigma 500mm is sharper than the Pentax 250-600, based on the copies I own (the Sigma) and owned (the 250-600). But in any case the 250-600 is rare as hens teeth so its performance relative to the new Pentax lens isn't actionable. The Sigma 500 to Pentax 560 is the comparison I want to see. They are similar weights and I expect the Pentax price will drop to the $5k level of the Sigma after a while. So the questions are resolution and useability. I expect the minimum focus distance of the Sigma is going to be much better than the Pentax just based on the design parameters, which will argue for the Sigma if you want birds at a feeder, for example. And the Sigma is shorter than the Pentax, which may affect the ability to carry the lens. But this is one of those things you have to experience to judge. So carryability is a big issue I would like the review to address. And then resolution and focus speed are the elephants in the room. I'll pay more and put up with more hassle in ease of use for better image quality and better focus speed. I hope the review will provide a comparison on these dimensions for the only two super telephoto lenses presently produced for the K mount.