How do you handle the inconsistent D800/D800E PDAF with different color temperature?

Started Feb 19, 2013 | Discussions thread
Marianne Oelund
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Clarification re: AF point selection
In reply to rayman 2, Mar 3, 2013

rayman 2 wrote:

Yup thats how it works and to some extent vice versa.....

It uses rgb data to get the right focus......!

The RGB sensor helps in the process of selecting an AF point, in modes where the camera is allowed to do so.  Face detection comes into play only when auto-area AF is used.  In single-point, 9-point dynamic, 21-point dynamic, 51-point dynamic or 3D mode, the camera will start with the user's chosen AF point (or an adjacent one if that point is not on usable detail).  If the subject moves within the AF array, it is tracked by color and AF is automatically handed off to other points as required.

In area focus and tracking mode extensively but in all the other modes it might do something to your focus... enough so that lens cal systems are NOT reliable enough !

The mere fact that the block diagram shows the RGB-sensor dataflow going to the AF block, does not infer that such data is always used - in spite of what any technician may insist.  The conditions for use will only be revealed in a lower-level diagram showing the logic within the blocks, or perhaps only at the source-code level.

It is true that there is only one combination of AF settings where the user can force the camera to use their selected AF point:  Single-point AF, with AF-S mode.  If you use AF-C, or any multi-point mode, then the camera may use a point adjacent to your selected point - but will only do so if your point does not fall on usable detail. This selection is not based on information from the RGB sensor, but rather the AF sensors themselves.  The owner's manual does not discuss this operational detail.

However, this auto-select function does not invalidate calibration systems, nor does it make them unreliable, provided the target is aligned normal to the optical axis. At worst, it can only cause confusion between results for adjacent points.  It is trivial to avoid this problem by using AF-S and single-point AF when calibrating AF fine-tune, or by simply ensuring that the selected AF point does have sufficient detail for focusing.

In all cases, there is no "biasing" or averaging of AF errors calculated from various AF points.  The camera always evaluates focus error from a single point, once it has been selected, either manually or automatically.

To Summarize:

The RGB sensor is used for face-detection to select an AF point for initial focus in auto-area AF.

The RGB sensor is used to assist tracking of subjects moving across the AF array, in dynamic-area or 3D AF, by passing color information to the AF system so that it can select a suitable AF point.

The RGB sensor is not involved in the AF system's function of defaulting to an adjacent AF point when the user's selected point does not encompass usable detail.

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Qualities possessed by God in infinite proportion: Love, Grace, Power, Righteousness, Wisdom, . . .
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- Marianne

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