Originally posted by Maksat Tutan @DSims;
Since I red a threat about Pentax SDM failure, I am afraid of 17-70 f4.
the thread I red was meaning that there is 34% chance of SMD failure on the 17-70 that requires +$200 repair cost, and in 2012, 50% of users reported the SDM failure.
So I took a look at the Sigma 17-70 f2.8-4 (older ones have f2.8-4.5). But I think that I must sacrifice at least 1 f stop to get full sharpness. So I am considering the f2.8s.
Am I wrong?
I don't think you're wrong about the Sigma; I think it must be treated as an f/4 lens at best. This is why I liked the DA17-70, which took nice photos at f/4.5 throughout its range.
I prioritize IQ and functionality over fears of failure. I didn't find lenses I liked better than the 3 SDM lenses I've had, including the DA17-70 (I still have the 16-50 and 50-135 * zooms). Fortunately I haven't had an SDM failure on any of them. I won't be happy if I have a failure, but already I would have missed out on
so many good photos that other lenses wouldn't have given me. I have to tell you, I've had to deal with so many worse things in my life that this just isn't a big risk. Missed photos would be the greater loss.
If you really need the speed, and the DA*16-50's too expensive, than the Tamron is probably your best bet. From what I've seen and heard the 17-50 and 28-75 give you almost equivalent IQ to each other; you just need to decide which range is better for you.