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Next PZ lens test report: Tokina AF 50-135mm f/2.8 AT-X Pro DX
#7
Hi Serkan:



Thank you for the answer.



However, I am still confused. According the article http://mansurovs.com/how-phase-detection...ocus-works, the phase-detection AF focus system works in the following ways (cited from that article):






  1. The light that passes through the extreme sides of the lens is evaluated by two image sensors

  2. Based on how the light reaches the image sensors, the AF system can determine if an object is front or back focused and by how much

  3. The AF system then instructs the lens to adjust its focus

  4. The above is repeated as many times as needed until perfect focus is achieved. If focus cannot be achieved, the lens resets and starts reacquiring focus, resulting in focus “hunting”

  5. Once perfect focus is achieved, the AF system sends a confirmation that the object is in focus (a green dot inside the viewfinder, a beep, etc)




"Once perfect focus is achieved, the AF system sends a confirmation that the object is in focus (a green dot inside the viewfinder, a beep, etc)". So I assume that when the AF sensors "sees" a state in the Figure 2 in that article the camera "thinks" that the subject is in foucus, then the camera tells the motor stop moving and let the lens elements stay in their positions, then images are taken. So, according to your answer, it can happen that for some problematic lenses, although the motor puts the lens elements in wrong positions, the AF sensors in the camera can still see a state as in the Figure 2 and then a out-of-focus image is taken. Is it right?



Best regards,

Frank



[quote name='PuxaVida' timestamp='1342770407' post='19480']

I don't think it's silly...



"So, what kind of problems with the lens can make the precise AF system of the camera think that the image is in focus while the image on the sensor plane is not in focus?"



You exclude the optical problems like focus shift and field curvature and ask about other potential problems arising from the lens itself. If we eliminate the optical elements, what remains is the built-in AF motor of the lens. AFAIK, the camera body does the math for PDAF / CDAF and -incase of a lens with AF motor- tells the lens motor to move the focusing elements based on these calculations. So in general I believe the body itself is mostly to blame (either the calculations or data stored for lenses in the internal tables or even maybe the parts like misaligned AF sensor / back mirror etc...). But maybe there's the possibility that the built-in AF motor in the lens does not listen to the data provided by the body.



Serkan

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Next PZ lens test report: Tokina AF 50-135mm f/2.8 AT-X Pro DX - by frank - 07-20-2012, 09:12 AM

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