I've bought the DA 50-135 f2.8 for doing playhouse images for actor head shots and for billboards. I used it for one outdoor play that got into dusk in August and will be using it again in October and December. I can't imagiine a better lens for this work and i think it will work equally well for concerts.
Its 1 1/2 lb weight is a good deal lighter than any f2.8 that goes to 200mm.
And i don't believe they are very much more expensive than the longer primes you are talking about. I got this zoom for $795 for Adorama.
For concert work, i would think you would find its quiet SDM drive to be a plus.
When i focus it, i've used it both on the half shutter focus, but more recently have disabled the half shutter focus and have been using the AF button to focus on a portion of the stage where i want to catch the actors as they pass through. that way, there's no camera delay in AF when snapping the actors. During dress rehearsals when we take a lot of our photos, we're allowed to be anywhere in the audience seats that we wish. I like to stand a few rows back because the DOF increases the farther you are back. With the 135mm end of the zoom, there is more than adquate enlargement 6 or 7 rows back.
I also disable SR and rely on a faster shutter to steady the image as the faster shutter is needed to stop actor motion in many cases. I like to shoot at 800 to 1600 iso, but in the outdoor play last summer, i went to 3200 for the poor lighting. The K20 did a creditable job and I don't feel like i had any disadvantage to the Canon guy who's shooting with me. He and I bought our new zooms at the same time, he got a 70-200 and i, the Pentax 50-135. I've seen him change out from his new zoom because of the disadvantage of not being able to go down to 50mm at times, while i easily did the whole play with the 50-135
As to SDM problems, i'm convinced that issue is WAY overblown. I'd had my DA300 for over a year and used it a lot. The DA 50-135 is so sharp with great IQ that i'm quite willing to buy a new motor every year if necessary; i use it a few times every week. I have a clock on the wall with a tiny electric motor thats been running for over 20 years, so this nonsense about electric motors being unreliable is exaggerated, IMO. I also should mention that the 50-135 is quite easy to manual focus if necessary.
Best of luck in your concert shooting.
Last edited by philbaum; 09-27-2009 at 11:24 PM.