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Author Topic: AF questions  (Read 1397 times)

Normalnorm

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AF questions
« on: August 23, 2013, 03:50:56 PM »
I am calling upon the collective wisdom here to help me understand something that has bedeviled me for some time

I generally use Canon lenses though I do own the Sigma 50 1.4 and in the past had an assortment of Tamron and Sigma lenses.
I am puzzled as to why some lenses hunt and others do not. For instance, a Tamron 24-75 and a Canon 28-105 that I used to own would hunt mercilessly.
If the AF module is in the camera and it sends its instructions to the lens why would the lens hunt? It seems to me that if the AF module sees OOF images it sends a correcting command to the lens and the lens should stop. Many comments I have seen seem to imply that the lens is making some calculations of its own but I can hardly imagine that to be the case as the light just passes through the lens.

For CDAF I understand it is an iterative process whereby the lens overshoots perfect focus and then backtracks to eventually settle at correct focus but what is the case for PDAF?

TIA
« Last Edit: August 23, 2013, 03:52:28 PM by Normalnorm »

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AF questions
« on: August 23, 2013, 03:50:56 PM »

tcmatthews

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Re: AF questions
« Reply #1 on: August 23, 2013, 06:07:34 PM »
The auto focus works by detecting edges in an image.  So the better that a lens is at transmitting edges the easier time the camera will have focusing.  So in general the sharper the lens and more contrast it has the better the auto focus.  There one other item that can make a difference some lens transmit distance information while other (mostly older) do not.   

Note: auto focus normally occurs wide open so if a lens is soft wide open it can present a challenge for the focus system. 
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Meh

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Re: AF questions
« Reply #2 on: August 23, 2013, 06:14:19 PM »
The camera very likely attempts to confirm focus once the lens' focusing element is moved to the predicted focus position.  If, when it attempts to confirm focus, the image is not in focus it will try again.  You can imagine that if the lens was damaged or otherwise not working properly such that it simply can not focus within the acceptable tolerance of the PDAF system, it will hunt.  This is not the iterative process of contrast detect, it is a failure of the lens to achieve focus.

Normalnorm

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Re: AF questions
« Reply #3 on: August 23, 2013, 06:50:02 PM »
Thanks but both the lenses are decent performers to the extent that I doubt that an AF module would see such a poor projected image of edges that the AF would be impaired.

I think the transmission of distance info was for the benefit of auto flash exposure.

The other issue is that these were both new lenses and both were sent in and returned without improvement.

I also note that commenters in other areas will note that "this lens focuses well on my xx camera" or "This lens struggles to find focus".

Mt Spokane Photography

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Re: AF questions
« Reply #4 on: August 23, 2013, 10:35:38 PM »
There are lots of possible reasons.  There is a brush in the lens that conducts the electrical signal from the position sensor to the lens electronics and feeds back the lens position to the camera.  If the brush is dirty and makes poor or no contact, the lens will hunt. 
 
There are many more possibilities, I just listed this to point out that a lens is complex and has many failure points.   A lens that binds or sticks will have the same issue.

Meh

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Re: AF questions
« Reply #5 on: August 23, 2013, 10:37:21 PM »
Well, there is either 1) something wrong with the lens, 2) something wrong with the body, or 3) something wrong with the particular combination of body + lens.  Or you are just being punished by camera fairies for some offense you have caused them.

Normalnorm

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Re: AF questions
« Reply #6 on: August 24, 2013, 12:06:58 AM »
Well, there is either 1) something wrong with the lens, 2) something wrong with the body, or 3) something wrong with the particular combination of body + lens.  Or you are just being punished by camera fairies for some offense you have caused them.

I'm going with the camera fairies. ;)

Thanks for the ideas. I do feel that I can understand some of the issues better now.

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Re: AF questions
« Reply #6 on: August 24, 2013, 12:06:58 AM »