I was looking at the EXIF information from a D500 file (I use the Exiftool program to see this information) and saw a mention of “Primary AF point” followed by a “C9”. What’s that? I didn’t have a particular need to understand the entry, so I just moved on.
I was recently trying to understand how a D500 uses focus points in controlling lens focus, and found out that none of the image editors use very much (and sometimes none) of the focus point information. Exif data, however, keeps track of what’s going on with the focus points.
You can find Nikon-provided information that discusses which focus points will work with which lens; not all focus points work with every lens. This will greatly complicate figuring out the logic behind how the focus algorithms use the focus points.
A picture is in order:
Nikon D500 focus sensors
I found out that Nikon saves focus sensor information in the picture EXIF data much like a chess board. The middle focus sensor, for instance, is called “E9”. You can only select focus sensors that belong to the rows labelled “A, C, E, G, or I”. You can only select focus sensors in the columns labelled “1, 3, 5, 7, 9, 11, 13, 15, or 17”.
Note that “cross” sensors are in red, while the less-capable “line” sensors are in black (they all look black in your viewfinder, of course). You can only select the sensors with a little box around them, either red or black (as shown above). You can’t even see the non-selectable sensors in your viewfinder.
The auto-focus algorithms that get executed while trying to keep your subject in focus, however, can make use of ALL of the focus sensors (at least with large-aperture lenses). Using something like “Dynamic-Area 25” AF can make use of up to 25 total focus sensors, or two “concentric” boxes of sensors around your selected sensor, which is a mix of both selectable and non-selectable sensors. The D5 includes “Dynamic-Area 9”, but it’s missing on the D500.
The D500 in auto-area AF mode, visually appears to ignore any focus sensors except the one that the user selects (called the “Primary AF Point”). Even viewing the photos in an editor with “Show Focus Point” selected will only ever show you the originally-selected focus point, and not what it actually used to focus.
The selected focus point shown in Capture NX-D
EXIF data for the picture above.
As shown above, the EXIF data indicates that I selected the center E9 focus sensor, and it only used that sensor at the time of the exposure.
Focus algorithm used more sensors
In the above example, several focus points were active at the time of exposure.
At the time of this writing, the only way to reverse-engineer what the focus algorithms must be doing requires use of this EXIF information. Best of luck figuring out the focus algorithms. I suppose hammering out 10 fps while wiggling the camera around various targets could get you enough EXIF data to figure out “how they do it”, but you’d better have a lot of time on your hands.