Autofocus with the EOS-3

Information supplied by Chuck Westfall of Canon

38 of the EOS-3's 45 focusing points are normal-precision single-axis AF sensors which remain functional with all EF lenses or EF lenses plus EF Extenders as long as the effective maximum aperture reported back to the camera through the electronic lens mount is at least f/5.6 or faster.

6 of the 7 focusing points in the central area function as high-precision cross-type sensors with all EF lenses or EF lenses plus EF Extenders as long as the effective maximum aperture reported back to the camera through the electronic lens mount is at least f/2.8 or faster. These same 6 focusing points function as normal-precision single axis AF sensors with all EF lenses or EF lenses plus EF Extenders as long as the effective maximum aperture reported back to the camera through the electronic lens mount is slower than f/2.8 but at least as fast as f/5.6 throughout the focusing range of the lens.

The central focusing point functions as a high-precision cross-type sensor with all EF lenses or EF lenses plus EF Extenders as long as the effective maximum aperture reported back to the camera through the electronic lens mount is at least f/4 or faster. The central focusing point also functions
as a normal-precision, single axis horizontal line-sensitive AF sensor with all EF lenses or EF lenses plus EF Extenders as long as the effective maximum aperture reported back to the camera through the electronic lens mount is slower than f/4 but at least as fast as f/8 throughout the focusing range of the lens.

What do we mean by "normal-precision" vs. "high-precision" AF? A "normal-precision" AF sensor produces focusing accuracy within the depth of focus for the lens' maximum aperture. A high-precision" AF sensor produces focusing accuracy within 1/3 the depth of focus for the lens' maximum aperture. Therefore, when activated, the high-precision AF sensors have up to 3 times the precision of the normal-precision sensors.

To put all this in perspective, consider the following:

Until the EOS-3, only the EOS-1 and 1N have offered high-precision AF, which has been limited to a single AF point and then only with lenses having a maximum aperture of f/2.8 or better, vs. 7 such points on the EOS-3. The EOS-1N delivers normal-precision, single axis AF to maximum
apertures as slow as f/5.6 with all 4 outer focusing points. The EOS-1 and 1N deliver normal-precision, single-axis AF with the central focusing point with EF lenses slower than f/2.8 and equal to or faster than f/5.6. All AF sensors on all other EOS cameras are considered normal-precision.

The purpose of the chart on the bottom of pp. 28-29 in the EOS-3 brochure is not only to identify which lens/extender combinations retain autofocus when used with that camera, but also to clarify whether autofocusing is possible at high-precision (HP), normal precision (Normal), or not possible at all (NP).

Unfortunately, there are a few typographical errors in the chart. Here is a revised version with corrected information: (In this chart, NR stands for 'not recommended')

                       with EF 1.4x              with EF 2x
Lens                   Center/Central*/Others    Center/Central*/Others
---------------------------------------------------------------------------

135/2L                 HP/HP/Normal              HP/Normal/Normal
180/3.5L Macro**       Normal/Normal/Normal      NP/NP/NP         
200/1.8L               HP/HP/Normal              HP/Normal/Normal 
200/2.8L               HP/Normal/Normal          Normal/Normal/Normal
200/2.8L II            HP/Normal/Normal          Normal/Normal/Normal
300/2.8L               HP/Normal/Normal          Normal/Normal/Normal
300/4L                 Normal/Normal/Normal      Normal/NP/NP
300/4L IS              Normal/Normal/Normal      Normal/NP/NP
400/2.8L               HP/Normal/Normal          Normal/Normal/Normal
400/2.8L II            HP/Normal/Normal          Normal/Normal/Normal
400/5.6L               Normal/NP/NP              NP/NP/NP
500/4.5L               Normal/NP/NP              NP/NP/NP
600/4L                 Normal/Normal/Normal      Normal/NP/NP
1200/5.6L              Normal/NP/NP              NP/NP/NP
70-200/2.8L***         HP/NR/NR                  Normal/NR/NR
100-400/4.5-5.6L IS*** Normal/NP/NP              NP/NP/NP

* 6 focusing points surrounding the central focusing point.
** Effective maximum aperture falls below f/8 at close focus, so AF is
disabled when used with EF 2x.
*** Only the central focusing point should be used when these zoom lenses
are used with EF Extenders.