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PostPosted: Thu Feb 10, 2011 2:18 am 
hi there,
Few weeks ago i bought 16-85mm as a walk-around lens for my D7000.
I didn't have any problems with it, but tonight I was taking indoor pictures - not very bright, but not too dark either. What is bothering me is the autofocusing - it was unable to focus, unless I move the camera close to the light source.
I did some tests - it was autofocusing perfectly from 16-24mm - after that the entire range up to 85mm - just zooming back and forth without success.
The story was valid for ISOs from 200 all the way to 6400, where it started to autofocus normally in all focal lengths.
My question is - is that something normal. From my tests it seems that the autfocus is determined by the quantity of light coming through the lens.
Or I have a defective copy of 16-85mm, which has troubles to autofocus.
Please share your thoughts. Any help will be appreciated. I can still return the lens if the problem is in it.
Thanks in advance!


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PostPosted: Thu Feb 10, 2011 8:36 am 
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whitenebula wrote:
From my tests it seems that the autfocus is determined by the quantity of light coming through the lens.

Completely right, and also the middle 9 sensors of the AF field are more light sensitive (cross-type) than the ones around the middle. The higher aperture number (5.6 for instance) as maximum aperture, the more difficulty the AF sensor has to snap focus.

And, if you put the camera to AF-S (single servo) then you will have the built-in AF assist light helping you IF still turned on in the menu.

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PostPosted: Thu Feb 10, 2011 12:52 pm 
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Cross-type simply means there's two strips of sensors (one horizontal, one vertical), they're not more sensitive to light by definition afaik.

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PostPosted: Thu Feb 10, 2011 2:09 pm 
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I was told they work better in low light situations, so assumed they were more sensitive to light.

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PostPosted: Thu Feb 10, 2011 2:20 pm 
Thanks, WoutK89!

I continued with the tests:
I don't have this problem with either my 35mm (1.8) or 10-24mm.
I see some problems with 50mm (1.4)
and I do have problem to autofocus with my Tokina 100mm (2.8).
It seems wide angle lenses handle better low light conditions, speaking about autofocusing.
Is that because of the dept of field?
I know that 24mm lens has four times the depth-of-field as a 50mm lens, which causes more of the image to be in focus.
But I didn't know this affects the autofocus as well..


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PostPosted: Thu Feb 10, 2011 3:39 pm 
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Well, the center point IS often more sensitive to light, it just has nothing to do with being cross-type. :)

Wideangles being easier do make sense, there's less difference between fully out of focus and fully in focus.

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PostPosted: Thu Feb 10, 2011 11:03 pm 
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I use centre point focusing only (too old to take action now???) and I've never had a problem. I have found that I need the focus assist light only in the lowest lighting situations.

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PostPosted: Fri Feb 11, 2011 5:14 am 
Strange - in the opposite of what we discussed here, I tried my 70-300mm in the same room/ same light conditions and....
... it was focusing every single time without any problems at all focal lengths. I set it up to 4.5/5.6 and shot at ISO 200.
Although the images were totally underexposed, the lens didn't have any problem to focus.
This means it's not the light coming through the lens which determines the autofocus!
Any comments??


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PostPosted: Fri Feb 11, 2011 7:08 am 
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I would love to comment, but I cant get a clear picture :lol: of what you actually take a picture of and the difference between them in focus accuracy.

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