VERY light - important for outdoors.
Very sharp at short and mid range
Decent on long end.
Un-sharp artifacts on the long end are ... hmmm... they look nice (not harsh type I've seen on 80-320mm sample images).
Relatively fast autofocus despite long range. No humming.
Popular 58mm filter thread

Plastic mount. I've used it heavily for 4 years now and don't see any major wear on the mount though. So is it really a weakness for outdoor lens?
It doesn't go down to 50mm leaving a gap between most walk-about lenses that go to 50mm.

I am yet to find a zoom lens of better value for the money. I used this lens on my Pentax ZX-5n years ago. Then I went digital with K100D. I've shot tons of beautifull and sharp pictures of my kids, including playing soccer.
I mostly shoot wide open to F8.
The lens is VERY sharp at short end and mid range, and very decent on the long end. It also has beautifull, dreamy bokeh, especially when object is at about 10m away and the background at infinity.
I was considering upgrade to Pentax DA 55-300mm. I went to camera show in Toronto with this lens, tripod and my K100d. I shoot several images with both lenses for direct comparison. I wanted to check how much better the new 55-300mm lens is.
It was WORSE. Or should I say mine old 100-300mm was better. On each shot. Also 55-300mm lens was having more trouble focusing in dim, indoor light of the show(humming back and forth).
I will never sell it.

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Pentax DA 55-300mm