Originally posted by Lumberjack93 How sharp is the HD PENTAX DA 20-40mm F2.8-4 ED Limited DC WR as compared to the Pentax 15 & 21 lenses? I know that fixed focal length lenses are generally sharper than zoom lenses but this one may be close enough.
Without having done definitive testing, since getting my HD DA 20-40mm, I have not used my DA 21mm nearly as often. Its versatility, excellent WR construction, and compactness make it a fine walk-around option. The DA 15mm is a great companion to the DA 20-40mm and is incredibly small to pack along. It lives in the front accessory compartment of my holster-type camera case.
That said, with things/people that move, especially in less than outdoor daylight conditions, a larger aperture of at least f/2.8 capability is generally the standard. And the flexibility of a zoom lens when dealing with walking around and children is undeniable. The DA 20-40mm does offer f/2.8 at the shorter end. If you are shooting with a camera having outstanding higher ISO performance, like the KP or K-70, this can compensate, making f/4 or even f/5.6 as effective as f/2.8 or larger on a less-capable camera body. Depending on the degree of fast action, and how low the light will be, if the situation is quite extreme and I need a zoom lens, it will be the Sigma 17-50mm f/2.8 EX DC HSM on my KP. It is very good, but they are getting harder to find lately. If I need longer, I mate it with the DA* 50-135mm f/2.8 but with these f/2.8 lenses you are getting into a bit of bulk and weight factors.