Originally posted by Landscape_with_K1 the only test series of images at different focal lengths I could find, shows perfectly sharp images up to 500mm.
Perhaps that was my set in the samples thread:
Sigma 170-500mm F5-6.3 APO - PentaxForums.com The distance to subject in that sequence wasn't far - less than 5 metres from memory. As other reviewers have said, the sharpness at the long end is OK when the subject is close, but the limitations were exposed at greater distances (many images were throwaways). The same is true of other consumer tele zooms, but it was quite marked with this lens.
To be fair, I should say that I was using it with the K-30 - I didn't have the K-3 then. But I found the Sigma 400 f5.6 tele macro on the K-30 a big improvement from the 170-500. The FA*300 f4.5 with 1.5x TC is better again. Either of those is a FF option too and the weight is comparable to the 170-500.
Given that the 170-500 was a FF lens, I was surprised by the vignetting I saw on a crop camera. That would be more pronounced on K-1.
I can't compare with the DG version, but you will notice that one reviewer ( @Alan C; ) has had both and found the DG to be sharper at longer focal lengths. Each has 13 elements in 11 groups and 9 aperture blades. Presumably the difference is in the coatings.
Personally I'd be a little skeptical about the high ratings for the DG version. I'd like to see a comparison with a Bigma (let alone a DFA 150-450) in controlled conditions on a high-resolution sensor before being satisfied that it really is worth a 9.