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Old Apr 22, 2009, 4:10 PM   #1
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i read on photozone that this lens has a very slow autofocus speed
i know it is considered a slow lens because of it widest aperature setting
can any users out there, either with the Pentax or Tamron tell me if in fact it does suffer from a slow autofocus speed in daylight outdoor use ,
i really like this lens for outdoor use only


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Old Apr 22, 2009, 7:50 PM   #2
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I don't have this particular lens, but I do have both a Sigma 18-200 3.5-6.5 for my Pentax K100D and a Quantaray (Tamron) 18-200 3.5-6.5 for my Nikon D70s and . . .

My experience with them, is that once you get smaller than f5.6, the cameras have a hard time finding focus in any light darker than a well lit room / or outdoors when focusing during darker days or near sunrise or sunset.(I did a bunch of non-scientific experiments.)

Just the nature of the beast. The cameras need light to focus. And past 5.6 they are starving. I've noticed that the Nikon 18-200 is 3.5-5.6 as well as the Canon 18-200 is 3.5-5.6. So they would probably have a better chance of maintaining useable focus performance in slightly less than ideal lighting.

My Nikon D70s, with its built-in white light auto-focus assist, comes to the rescue and is able to acquire focus quickly, but only because it is providing it's own light to focus by.

Once I put my Pentax 28-80 3.5-5.6 on my Pentax K100D, it is able to acquire focus fairly reliably and not so bad on speed in rooms that are more poorly lit.

I've been considering getting one of the split focus-screen for my K100D for situations like this, along with when I am shooting my kids running around and my camera has difficulting "knowing" what I want to focus on.

Then again, if you get the 18-250, when you are at 200mm focal length, you won't be at f6.5 like me (although my camera reads f6.7 for some reason), you'll be closer to f5.6 so I believe at 200mm you'd have better autofocus performance than my lens.

Take care,

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Old Apr 23, 2009, 5:52 AM   #3
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Dave -

I have the Tamron 18-250 on my new K-m, and it has worked fine for me - mostly outdoor shots, but some at very low light (dawn and dusk). Even for some rather fast-moving subjects (eg butterflies) have had very few focus problems.
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Old Apr 23, 2009, 5:59 AM   #4
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I don't have the Sigma 18-250 but I do have a Sigma 100-300 and a 50-500mm and they do quite well even in low light settings. Slow? Not really as most of my shots are of small birds they need to be semi-fast anyway.

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Old Apr 23, 2009, 7:51 AM   #5
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I have the Pentax 18-250 which I carry ifI only have room for one general purposelens. I have used itindoors as well as outdoors, and the lens focuses quick enough to catch kids sports and wildlife. The onlytimes that I really find the lensslow to focus is when you are shooting ator near 250mm andyou focus on asubject that is close to you and then switch to onethat isoff in the distance (or vice versa). The lens has to move through the entire focus range and will occaisionally miss the focus at the other end, back up a bit and try again. If on the other hand you are shooting several subjects that aren'tat complete opposite ends of thefocus range it does just fine.

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Old Apr 24, 2009, 9:18 AM   #6
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Oh, yeah . . . I didn't mean to sound all negative about the lens.

I love my 18-200 . . .

Even though it's slow . . . when you only want to carry around one lens (or your subject matter bounces around a lot to require you to zoom-in / zoom-out), having the super zoom is nice & fun!

I really only take it off the camera when I have enough time (or a specific want) to use one of my other lenses.

Take care,

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