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12-11-2008, 06:50 PM   #1
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few questions about 50-135 behaviour

I've managed to test my new 50-135 a lot more over the last two days and have a few questions about whats normal and what isnt.

Firstly I've had to dial in -2 AF correction and all in all it seems to be nailing focus, especially of objects in the distance (5meters up to infinity is typically what i tested, while on f/2.8). So I'm very happy there. Performance is amazing.

If I focus on something close up, well as close as the 50-135 comfortably gets, say in the 2 to 3 meter mark it will miss focus ever so slightly (might need to dial in something more like -4), manually focusing produces great results tho.

So far is this sounding normal-ish?

Lastly in low indoor light dialing in -9 seems to do the trick.. I'm not that concerned about the indoor lighting, its not where I plan to use the lens and I'm more than happy to manually focus in the rare occasion I do. But is this normal behaviour?

12-11-2008, 07:19 PM   #2
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I don't have that lens but I've been doing similar tests with three other zooms in the last few days. I have come to the same conclusion as you that differing amounts of AF correction is needed for different situations. But I believe it is more focal distance related rather than distance related. Following are my findings with three lenses that I have been playing with:

On K10D body:
Pentax 12-24 needs -40 AF correction at 12mm and -80 at 24mm

Tamron 17-50 needs -40 AF correction at 17mm and -90 at 50mm

Tamron 28-75 needs -40 AF correction at all focal distances (very good)

On K20D body:
Pentax 12-24 (will finish testing tomorrow) tentatively -4 at 12mm and -6 at 24mm

Tamron 17-50 needs -2 AF correction at 17mm and -6 at 50mm

Tamron 28-75 needs -2 AF correction at all focal distances (very good)


Both my bodies are spot on with my primes. So I think some zooms have this kind of behavior. My Tamron 28-75 is my best zoom in this regard and the other two need more fussing with to get aboslutely sharp pictures at wide open aperture. When stopped down things even out a little bit.

Last edited by Stratario; 12-11-2008 at 07:33 PM.
12-12-2008, 05:06 AM   #3
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Original Poster
QuoteOriginally posted by Stratario Quote
I don't have that lens but I've been doing similar tests with three other zooms in the last few days. I have come to the same conclusion as you that differing amounts of AF correction is needed for different situations. But I believe it is more focal distance related rather than distance related. Following are my findings with three lenses that I have been playing with.
Hey thats pretty interesting, looks like we're both equally anal I didnt even think of changing focal lengths, I did all mine at 135mm. I guess its time to test a little more, but would be nice to know if this kinda thing is normal on a 50-135..
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