Focusing at f1.4: It is not "Sigma", it is phase detection limit..!

Started Jan 1, 2014 | Discussions thread
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Nanospeed1 Forum Member • Posts: 67
Focusing at f1.4: It is not "Sigma", it is phase detection limit..!

Hi guys, reading repeatedly about "AF inconsistencies" and "Sigma/Tamron bad AF alghoritms" etc., I would like to share with you a very interering debate I have had recently.

After discussing the AF issues with an optical engineer (a family friend) working for a well-known big photo brand on the AF systems development in Japan (for more than 20 years, retiring now), I learned that basically the phase detection AF system (PDAF) is far from ideal as far as the critical accuracy is concerned. Of course the marketing machine has never been mentioning this because generally the accuracy was sacrificed for the sake of speed and other benefits. Also, the PDAF system has relatively wide tolerances which simply make it an inappropriate system for any really critically accurate focusing by definition.

The big camera manufacturer he has been working for has also performed the accuracy tests for 2.8 lenses as the standard, they have never been allegedly critically fine-tuning the accuracy for 1.4 lenses.

When Contax (Kyocera) and Leica were testig the PDAF, they were unsatisfied with the accuracy results back in the 90s. Later Contax introduced Contax AX AF camera which autofocused by moving the whole block of film plane - this was much more accurate then the PDAF system. Of course it had some drawbacks (esp. the speed), too, but lets leave this behind. Before Leica could come up with their own and better alternative, it was (almost) a dead company fighting to survive....

He also said that basically the PDAF has been mainly designed for 2.8 lenses for focusing on moving objects in a mid and long distances. The closer you get or the faster lens you have, the more accuracy inconsistencies you are going to meet by the definition.

The companies also welcomed the increasing photoconsumerism in the last decade and by the digital photography because the criticism for the limits of PDAF system more or less disappeared, everybody was mostly concerned about sensor noise, dynamic range, megapixel, image stabilizers ets. But with the high-megapixel sensors, the basical limits of PDAF system are showing up again...

I was also told that much as the PDAF system has improved in many aread during the last 20 years (low-light sensitivity, the overall accuracy, cross-type points etc.), the basic (in)accuracy limitation is still there.

Yes, the assembly tollerances and manufacturing quality control (such as the infamous D800 left AF issue) is another aspect of PDAF which is also relevant but which I don´t want to mention here again.

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P.S. if you have a relevant info about the AF accuracy, please share it here.

 Nanospeed1's gear list:Nanospeed1's gear list
Sigma 35mm F1.4 DG HSM Art
Nikon D800
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