Originally posted by rdj92807 The Sigma 18-300 is a very sharp lens, and the above comments suggesting otherwise are really not accurate. I have tested the Sigma 18-300 versus many lenses at most settings, including the Pentax 16-85 & 60-250*. It performs surprisingly well against these lenses. For example, at 250, it's almost as sharp as the Pentax 60-250 when examining test pics at very high magnifications. Of course, you have to shoot 6.3 or above vs 4.0 for the Pentax. But this is a pretty darn good performance. The Sigma 18-300 has a very sophisticated optical formula, and it's biggest improvement in sharpness (vs the Sigma 18-250 Macro) is in the 200-300 range. The truth is, these latest superzooms have reached the point where they perform shockingly good for such a versatile lens. Frankly, I was surprised how good when I performed many comparison tests, as I was skeptical like everyone else.
Regarding the Pentax 16-85, it is a pretty good lens, but it's not nearly as good as the new Pentax DFA 28-105 IMO (which I love on my K-1). For full frame, the Pentax 28-105 DFA is a real winner. And FYI, I did not keep my copies of the Pentax 16-85.
Compared to the 16-85 it
is soft. Compared to the 60-250 (esp at the long end) it is very soft.
Sigma C 18-300 mm f/3.5-6.3 DC MACRO OS HSM review - Image resolution - LensTip.com
Besides only the center is sharp, and only stopped down to f/8. Once you get out of the center, the resolution drops off dramatically. Once you get to the long end, the resolution drops off the map. And this is only detailing resolution.
If it could rival the 16-85 or the 60-250 then why do you think those lenses exist and for high dollar pricing? They wouldn't warrant their costs, as no one would buy, if an 18-300 megazoom could satisfy the requirements of both lenses.
If you only care to share 1080p snapshots on facebook or twitter, then its fine. But then I'd question why are you using a DSLR? Just get a superzoom bridge camera. It will be much cheaper and none of that 'annoying' lens changing nonsense.
We won't even discuss the 28-105mm on the K-1 because that camera is using larger sensor with a much higher resolution. Apples and Oranges to a K-3 with a 16-85.