Originally posted by loco The 55-300 is the only "long" lens I own (I rented the DA* 300), so I've used it quite a bit. While it can be a very competent lens in terms of IQ, mine has always had AF accuracy issues. Plus, the AF is very slow and tends to rack all the way through on occasion which has made me lose numerous shots. If I had had to use that lens for my eagle tour, I'm not sure I'd have gotten any decent shots.
So, for birds in flight, it's probably not a great choice. At least it wouldn't be for me. However, if you are planning to photograph more stationary animals, it will work well. It's certainly lightweight and very hand-holdable.
With the DA* 300 on my K-5 I do have occasional slow focus issues in low light but mostly, and nearly always in good light, it's OK. I would expect a K-3 to offer some improvement in that respect, can anyone confirm?
Comparing the DA* 300 with my 55-300DA, it's effectively much longer, because of the better image quality. Perhaps moreso on a K-3 (my next likely purchase). In handheld tests, I have found the DA*300 to produce a similar image quality to the 55-300 (@ 300mm f/l) at roughly between 1.5 and 2 times the distance, depending on conditions and whether you are interested only in the frame centre or more towards the edge. The darker the conditions, the greater the difference seems to be in practice.