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Question: Please tell me if this lens is a good option for indoor (low light) sports.  I am considering this one vs the Tamron 70-200 f2.8. please compare the lenses if you can. Thanks in advance!
Document: I am a fairly new convert to Pentax, having switched from Olympus to get better low light, high ISO performance for shooting indoor sports. When looking for lenses for the new system, I considered several including the Sigma 70-200 f2.8, the Pentax 50-135 f2.8 and this one, the Pentax 60-250 f4. I rented all three to try before committing my hard earned money.
I was lacking a quality tele-zoom so I shopped around for a long time debating the merits between the DA* 60-250 vs the flagship 70-200 lenses from Sigma/Tamron. Basically, the trade-off is one F-stop (DA* is F/4, the others are F/2.8) against the smaller size and superb weather sealing of the DA* lens.
Especially if you go on an Africa safari or something where there is plenty of light interplay available, man this lens simply talks. As good as its relatives from Canon and Nikon in the similar range (70-200 from the others).Cons - PricingThe cons for this category has just arrived. It used be a pro before but this just became a cons category. For its price you can buy A Pentax DSLR and A HALF.
I loved my 50-135 f2.8 Pentax for my K-5 Pentax camera. It really grabs a good image amazingly easily. So I thought I needed a bigger zoom, read the positive reviews and ordered a ("like new") 60-250. For several months I have been disappointed with the results from this lens. Then today I decided to check the auto-focus and found it is not quite accurate.
But if you want virtually a professional lens in your hand that can reach over 150mm, it will be big and heavy! In fact, the two 70-200mm lenses that I was considering were both heavier and larger. But this is not a lens you will know you have around your neck! For me, the lens on the k-5 just feels right. Perfect balance and a feeling of confidence whenever I pick it up.Second, now I need a wide angle zoom!
The lens is actually not large if you are used to lenses like the Sigma 100-300mm f/4. It is small next to that lens. It is fairly close to a 70-200mm f/2.8, and maybe even a bit smaller.Most of my minor complaints with this lens are mechanical. Don't get me wrong, it feels great, but it lacks a lock or anything to hold the extending barrel in place while you hike around.
With lenses that have lower contrast and resolution, the image quality would fall apart by ISO 1600. If paired with the K-5, K-x or K-r, I expect one could go to ISO 3200 or even 6400 easily. For outdoor sports, I typically prefer to stop down to get better depth of field, so F/4 is not a limiting factor unless trying to capture very fast moving objects.
Zooming in to 100% view and the pixels are still sharp, even on my K-7 which is 14.6 MP. It seems to peak about f/5.6, but is really very good from wide open. This behaves a lot like 70-200mm f/4 lenses in this regard. Since this is internal focus, the focal length is measured at infinity. It seems to be significantly less than that at close focus as typical of all IF lenses.
The Pentax 50-135 was very nice for indoor, very sharp and with a large maximum aperture. It was also weather sealed for outdoor use, but way too short for baseball, soccer and softball although sufficient for track and field. The Pentax 60-250 f4 was, like in a fairy tale, just right. The range works for both indoor and outdoor sports, at least the ones I shoot, and it is weather sealed.
I have used the SMC Pentax DA 1:4-5.8, 55-300mm ED for a couple of years - with great success. When stopped down, this lens delivers good image quality over most of its zoom range.
Answer: I am using this lens primarily for sports - synchronized swimming. So far only outdoors (wish you would ask it a few months out -the indoor meet is coming up then). It is a beautiful lens for landscaping or nature shots, but so far i cannot say i am 100% satisfied with it for sports. Based on what i see, i can imagine it might be problematic with low-light conditions and sports as even outdoors it still hunts that focus point. But off course reflections can be the cause, at least for me. It probably as good as we can go at this price point, but definitely do not expect flawless  photos. As to comparing it against Tamron - sorry, no experience there.