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Old 06-12-2013, 10:33 AM
ericbowles ericbowles is offline
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Default Understanding the Nikon AF System

The Nikon D4 and D800/800E use the Multi-Cam 3500 FX focus module. With that module comes AF at f/8. But there is a big difference in AF across sensors. If you have a lens or Lens+teleconverter combination that operates beyond f/5.6, it's important to understand how AF sensors change.

Keep in mind this discussion is based on lens or lens+teleconverter combinations wide open. Regardless of the aperture selected, the camera focuses with the aperture wide open.

First some context. AF sensors are typically line sensors that evaluate light and dark areas looking for sharp differences in one direction. The Cross sensors take two line sensors and arrange the second at a 90 degree angle to the first, increasing the likelihood of sharp focus. The cross sensors perform much better with difficult subjects or motion.

With a lens that is at f/5.6 or lower, the central 15 sensors are cross type sensors and all sensors will AF. The outer 18 sensors on each side operate as regular line sensors.

With a lens that is >f/5.6 but < f/8, the center sensor and the adjoining 8 sensors are cross sensors, and the central 3 sensors to the left and right of this group are line sensors. Others sensors will not reliably AF.

With a lens that is f/8, only the center sensor is a cross type sensor and the line sensors are the four sensors to the left and right, plus the one sensor above and below the center sensor.

http://imaging.nikon.com/lineup/dslr...features03.htm

This has implications on the choice of sensors when you are using a lens with a variable aperture or a teleconverter.

If you are expecting to track a subject with AFC, you have some increased likelihood of losing focus as the subject moves through the frame. But there is nothing you can do.

The center sensor is the one that performs best and is a cross type sensor while using other sensors means you are more likely to miss AF, but it will depend on your lens. In any event the sensors outside of the center sensor are less reliable.

All of the diagrams for 11 point AF on current cameras show there are no cross type sensors - and the diagrams are next to diagrams of the 51 point AF which have highlighted cross sensors. I have previously chosen 11 points for speed in moving the cursor across the frame, and this will cause me to rethink that decision and choose 51 points. At most 3 sensors of the 11 sensors lineup are cross sensors, and depending on aperture, you may have just one cross sensor available.

The D600 has fewer sensors and fewer cross type sensors, but the conclusions are the same.
http://imaging.nikon.com/lineup/dslr...features02.htm

The D4 has the same CAM 3500 FX system as the D800.
http://imaging.nikon.com/lineup/dslr/d4/features02.htm

And the D7100 has a different layout with only one center sensor performing above f/5.6 and it is a cross sensor.
http://imaging.nikon.com/lineup/dslr...features01.htm

All of these cameras are going to be less reliable with AF using regular line sensors than cross sensors. All may focus depending on the target and light levels but the distinctions are relevant.

The problems come up when you are using a lens with a teleconverter that has an aperture above f/5.6, or a zoom with a variable aperture that goes above f/5.6. The Nikon 80-400 with a teleconverter, or popular long zooms like the Sigma 50-500, 150-500 or the Tamron 200-500 all run into this issue at the long end. The Sigma's for example are at f/6 at 300mm.
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