>>This is only true in the multi-point AF modes. In >>single-servo, single-point AF the only data that is >important >>is that which is under the selected focus point. > >If you're basing your comment on factual technical knowledge >about the way in which the AF system works, then you've just >added some really valuable information. The thing is, I don't >have precise technical knowledge about the AF system and I've >been making an assumption that even using a single point AF >mode it is still necessary for the AF system to use proximal >data in order to get the best possible focus lock. > >>If all of the focus points did work well in single-servo, >>single-point, I would bet a lot of money that the >performance >>in continous-servo and dynamic-area modes would improve >>greatly. > >I agree, and it seems self-evident. My own point has been that >the proposed test method doesn't allow the AF system's single >point mode to work to all of its strengths. > >>IMO it's a ignoratio elenchi to say "yes AF-S >>single-point doesn't work well in test scenarios, use >AF-C >>dynamic-area in the real-world." > >I suggest that it is not a good idea to critique grammar, >sentence construction and so on in a public forum. Irrelevant >conclusion (ignoratio elenchi) or not, I'd ask for a >clarification. For example, although throughout this part of >the thread the posters have been careful about noting that >they are expressing opinion and supposition and the results of >experiment, you state unqualifyingly that "In >single-servo, single-point AF the only data that is important >is that which is under the selected focus point" but >don't offer any technical information in support. So your >statement might be judged merely reductio ad absurdum (proof >by contradiction). Help us by explaining your understanding >that Nikon's AF system does not use or does not necessarily >need proximal data when it is set to use a single AF point. >Believe me, I need the education. >
Please don't misunderstand; I wasn't critiquing your grammar at all.
That said; assume for a second that in the single-servo, single-point mode the camera uses information from the nearby focus points. The designer of the algorithm could not assume anything about the shape of the object being focused on, and would therefore be hard pressed to use this data in any useful way. For instance, am I focusing on a brick wall or someone's nose? It could very well be that given the aperture, focal length, subject size, and distance that the selected focus point falls on someone’s nose, while the neighboring ones fall on their cheeks/eyes and are out of focus.
Therefore, based on logical reasoning like this and Nikon's description of the single-point mode (while in single-servo) I find it very unlikely that it uses nearby focus points in this mode. Simply put, this mode is trying to minimize the phase error measured between the two microlenses and sensors that make up the selected focus point.