Originally posted by m42geo I just purchased my DA* 50-135 F2.8 3 days ago and done some test shot vs my Sigma 70-300 F4-5.6
I use a K110D and I found that my sigma can focus much better than the pentax, the DA* temp to focus a little behind where it suppose to focus to. and in term of image quilty I find that they are the same at 135mm set at F4
I do not know if anybody out there have the sigma 70-300 F4-5.6 as well but I can either say the sigma is a super good deal or the DA* is super expensive.
Give give me some idea
All zooms are compromises, even good ones. They can impossibly be optimised so that they offer the same quality for all focal lengths. As a almost perfect rule, they are best in the middle of the focal length range, and are suffering most from compromises in the short and long ends. I've shot the DA*50-135 against some of the best prime lenses Pentax made in this focal range: A50/1.7, A*85/1.4, A100/2.8, A*135/1.8. When I say that I shot them against each other I mean identical settings, multiple motives, brick walls on tripod with SR off and remote controle to remove all random differences... Not just one focal length, multiple appertures and iso's etc.
In the middle of the focal length range at f4 or more, it is comparable to the A100/2.8 and close to the legendary A*85/1.4. That a zoom rival excellent primes in this way is rare. In the 50mm end, it is OK, but not as sharp as the A50/1.7, and it has some corner softness. The 135mm end is its worse end: more corner softness, some distorsion. This is not strange: correcting this would have required more glass and made the lens more expensive and heavy and perhaps not as good in some other aspect. It get's noticeably better quite soon when you zoom away from 135mm towards 100mm.
Your consumer sigma zoom on the other hand has 135mm somewhere in the middle of its focal length range. Usually these ~70 to ~300mm lenses are quite bad above 200mm and quite good below 200mm. I don't know this lens first hand, but it is reasonable to assume that you are somewhere near its best range. So you have compared the worst part of an excellent zoom with the best part of another zoom. Not the best way to evaluate the lens.
As for autofocus speed. I don't know your sigma, but what I can say about the DA* lens is that is has a rather long ...searching for the right word in English...focus ring turning (?), and a lot of glass, which means that it takes some time to turn it, especially with the week screwdrive AF engine of the K110 and its old focus algorithms. If the sigma is faster it may be because of a shorter focus ring range and lighter construction(?).
Some times I'd wish my DA*50-135 would focus faster even on the K20D, but on the other hand it is a dream to focus manually, and I hope it will be a bit faster on the K-7.