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5 Days Ago   #1
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pdaf explanation

Can a competent teacher give me a lesson for dummies about the nature of pdaf?
With a well adjusted SLR the focus confirmation, ( splitfield - microprism ) is not related to the lens, the user has to turn right or left.
The Af module has the same function, it tells the lens to turn right or left. A sharp line is a sharp line regardless of the lens, why is pdaf lens dependable?
Steve Brot has stated that pdaf is not good for curved motives. Why?
Are some lenses more difficult than others. I guess that my pentax D 50 mm 1.8 is rather imprecise, I only use single point af
Hope that my questins are understandable
thanks on beforehand

4 Days Ago - 1 Like   #2
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QuoteOriginally posted by niels hansen Quote
Can a competent teacher give me a lesson for dummies about the nature of pdaf?
Here is a link to an explanation (now quite dated but still relevant) that made PDAF understandable to me.

Doug Kerr, The Pumpkin | Principle of the Split Image Focusing Aid and the Phase Comparison Autofocus Detector in Single Lens Reflex Cameras (PDF download, 151Kb)

To understand the split-image focus aid is to understand PDAF.

QuoteOriginally posted by niels hansen Quote
why is pdaf lens dependable?
PDAF accuracy and precision varies according to the lens' ability to provide an unambiguous image of detail edges to the AF point's arrays of linear light sensors. Ideally, the light distribution will be symmetric with little spread. If there are multiple and/or asymmetric and/or flattened peaks, the ambiguity causes problems. When the ambiguity is influenced/caused by other than perfect alignment (both lateral and linear) of lens elements, that lens may require AF fine adjustment.

QuoteOriginally posted by niels hansen Quote
Steve Brot has stated that pdaf is not good for curved motives. Why?
Yes, I said that.

A curved surface is ambiguous even for manual focus using a split-image focus aid. The short answer is that we can't expect the camera to read our minds as to where on the receding cheekbone or skin of orange we want it to lock focus, if indeed it is able to.

I hope this helps.


Steve
4 Days Ago   #3
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QuoteOriginally posted by stevebrot Quote
Here is a link to an explanation (now quite dated but still relevant) that made PDAF understandable to me.

Doug Kerr, The Pumpkin | Principle of the Split Image Focusing Aid and the Phase Comparison Autofocus Detector in Single Lens Reflex Cameras (PDF download, 151Kb)

To understand the split-image focus aid is to understand PDAF.



PDAF accuracy and precision varies according to the lens' ability to provide an unambiguous image of detail edges to the AF point's arrays of linear light sensors. Ideally, the light distribution will be symmetric with little spread. If there are multiple and/or asymmetric and/or flattened peaks, the ambiguity causes problems. When the ambiguity is influenced/caused by other than perfect alignment (both lateral and linear) of lens elements, that lens may require AF fine adjustment.



Yes, I said that.

A curved surface is ambiguous even for manual focus using a split-image focus aid. The short answer is that we can't expect the camera to read our minds as to where on the receding cheekbone or skin of orange we want it to lock focus, if indeed it is able to.

I hope this helps.


Steve
quite a lot
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