I purchased Darrell Young's book ...mastering the D800. I am confused about sensors at apertures on page 457 in his book. ( I also purchased Thom Hogans e-book hoping this would be clarified for me) I guess I have a mental block understanding this concept. Am I understanding that the choice of dynamic area AF has to do with my chosen aperture, OR does the choice do with my lens? Nikkor 24-85 2.8-4. I am thinking this is what they are saying, just not getting a grasp on this. Can someone explain the concept a little better?
Amazing Darrell did a great job but this one page confuses me. I do understand AF-S, AF-C etc etc Thanks in advance
The sensors have nothing to do with the aperture you choose, but rather the wide open aperture of the lens and teleconverter where AF works reliably.
AF occurs wide open. The lens is opened up when you simply use AF. The DOF button lets you see the actual depth of field stopped down.
When we refer to an AF sensor working at f/8 - such as the center sensor - it means that an f/4 lens with a 2.0 teleconverter will reliably work assuming there is a good focus target and adequate light. That does not mean perfection. You may get adjoining sensors to work at f/8. And you might stop down to f/11 and still have AF. But the limit of the lens and teleconverter for reliable focus is f/8 with the center sensor, f/5.6 with the adjoining sensors, and f/4 if you get out toward the edges.
<So am I understanding this correctly, it really only applies to a lens with a teleconverter ? Thanks for your reply>
The aperture for AF is based on the wide open aperture. Lenses are designed for AF to work properly. The consumer Nikon zoom lenses have a variable zoom that normally has a maximum of f/5.6. That means they will AF throughout the zoom range. There are some lenses with a narrowest aperture of f/6.3 or more when fully zoomed, and they may struggle a bit with AF.
A teleconverter is not the only situation where AF may not work - it's just the most common.
In practice, I find a big difference between the center AF sensor, other cross sensors, and other AF sensors that are not cross sensors. When I get tough AF targets or if I experience hunting, the center sensor works like magic. It's much better for any testing, calibration, or simply general use under challenging conditions. You can use all the sensors, but if AF is a challenge, the center sensor is much better. This is true for all Nikon cameras.