Originally posted by Shanti the samples at lenstip at 500mm are not so impressive
By all accounts the Sigma 50-500, particularly the OS version, does well at 500mm, and even the Lenstip test shows it performing quite decently at 500mm @ f8.
I'm pretty sure that the DA*300 is a much sharper lens than the Sigma 50-500. But add a 1.5 or 1.7 teleconvertor to the f4 DA*300, and it may perform a lot like the Sigma.
However, of course, when you are getting out to 500mm, while lens sharpness of course should be a priority, good technique becomes more important than ever, since even the slightest bump or vibration can make even the highest quality lens at that focal length look soft.