Pentax-DA* 300mm F4 Review
The DA* 300mm F4.0 lens has an internal SDM autofocus system. The SDM is very quiet compared to the cameras screw drive. For this section, we will examine the relative speed of the autofocus capability of the lens. The lens was timed for its response time from minimum focus distance to infinity and infinity to minimum focus distance. The test was completed three times and the times were averaged to minimize errors. The results were then charted as seen below.
Note that if you mount this lens on a Pentax camera older than the K10D (or on a K10D with firmware lower than version 1.30), it will fall back to the traditional screwdrive AF system, which we did not test.
Overall the DA* 300mm did fairly well, considering the long distance that the focus ring has to travel (the long throw). The lens was slightly faster from infinity to minimum focus distance. The overall response time for the lens was slightly less than one and half seconds. For action photography, better AF performance may be desirable, but this lens will certainly get the job done for you.