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Cross-type and Extra-sensitive AF Sensors

DigitalDarrell

Knoxville, US
5987 posts

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DigitalDarrell Team Member Founding Member of the Nikonians writer Guild. Author of most of the NikoniansPress books. Donor Ribbon awarded for his generous support to the Fundraising Campaign 2014 Donor Ribbon awarded for his very generous support to the Fundraising Campaign 2015 Charter Member
Fri 08-Jun-12 05:56 PM | edited Fri 08-Jun-12 07:39 PM by DigitalDarrell

Just to whet your appetite for the upcoming Nikonians Press book, Mastering the Nikon D800, I thought you might enjoy a tiny section of the chapter Autofocus, AF-Area, and Release Modes. I have included a subject that is misunderstood and not well documented in the manual or online—how the cross-type sensors work in conjunction with lenses or lens/teleconvertor combinations having maximum apertures smaller than f/5.6. Enjoy this short unedited preview:

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Cross-Type AF Sensors at Various Apertures

Cross-type AF sensors will initiate autofocus in either a horizontal or a vertical direction, unlike standard AF sensors, which work only in a horizontal direction. The ability of the AF system to function properly is dependent on the maximum aperture of the lens in use, or lens and teleconverter combination. Lenses normally autofocus at maximum aperture and only stop down to the aperture you have selected when you take a picture. Most cameras are designed to autofocus with lenses having a maximum aperture of f/5.6 or larger (e.g., f/1.4, f/2.8, f/4).

The Nikon D800 is in a class of its own when it comes to the fact that its autofocus can work with lenses having a maximum aperture of smaller than f/5.6. The camera can autofocus using lenses or teleconverter/lens combos having maximum apertures from f/5.6 to f/8.

Figure 10.3 shows the various arrangements of cross-type and extra-sensitive AF sensors the camera can use when you are working with small maximum apertures. You must be sure to select one of the AF sensors shown in figure 10.3 if you are using a lens or teleconverter lens combo that has a smaller maximum aperture than f/5.6. The camera will not prevent or warn you if you try to use an AF sensor inappropriate to the small maximum aperture.

You cannot in some way select one of the overall patterns shown. You simply move your active AF sensor into one of the locations in the patterns, according to how small the maximum aperture happens to be. Study this carefully if you regularly use teleconverters on telephoto lenses with autofocus.


Figure 10.3 – (1) 15 Cross-type AF sensors at f/5.6 or larger aperture, in red. (2) 9 Cross-type AF sensors in red and 6 AF sensors sensitive to apertures between f/5.6 and f/8, in blue. (3) 1 Cross-type AF sensor in red and 10 AF sensors that work at f/8, in blue.

Figure 10.3, image 1, shows the 15 AF points (in red) that are cross-type AF points. The center three columns of AF points are cross-type and work with lenses having a maximum aperture of f/5.6 or larger.

Figure 10.3, image 2, shows the arrangement of nine cross-type AF sensors (in red), along with six extra-sensitive sensors (in blue) that are not cross-type, all of which work with lenses having a maximum apertures between f/5.6 and f/8.

Figure 10.3, image 3, shows the arrangement of one cross-type sensor (in red), along with 10 extra-sensitive sensors (in blue) that are not cross-type, all of which work with lenses having a maximum aperture of f/8.

All the rest of the 51 AF sensors not marked in red or blue are standard AF sensors that work only at apertures between wide open and f/5.6. You may be able to get the standard AF sensors to respond at smaller apertures, but you shouldn’t depend on consistency of autofocus when a standard-sensitivity AF sensor is in use at maximum apertures smaller than f/5.6.

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This book is going to be full of deeper camera information of this type. You can pre-order Mastering the Nikon D800, with its Nikonians Gold Membership 50% discount coupon, at the top of this page: http://www.pictureandpen.com/NikonBooks.asp

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Darrell Young (DigitalDarrell) www.pictureandpen.com
"Better too many words than not enough understanding."
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