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Autofocus discussion, for the Canon EOS-1 series cameras, as it particularly relates to C.Fn-13 (Number of AF points) and C.Fn-17 (AF point activation area) All material here, without exception, is from the Canon 1D Users Manual or from written comment from Chuck Westfall.
Compiled by Bill Neukranz – March 3, 2004

AF Sensitivity and the Len’s Maximum Aperture

 The EOS-1D’s AF points are all horizontal-line sensitive.  However, bright lenses with a large maximum aperture enable certain AF points to work as cross-type sensors for higher AF precision.

With lenses whose maximum aperture is f/2.8 or faster the 7 AF points aligned vertically with the center AF point, including the center AF point, will be cross-type sensors sensitive to both vertical and horizontal lines.  The remaining 38 AF points will only be horizontal-line sensitive.  The cross-type sensor’s vertical line sensitivity is 3 times higher than the horizontal-line sensitivity.

For lenses, except the 70-200 f/2.8L, but including the 70-200 f/2.8L IS, whose maximum aperture is f/2.8 or faster combined with an Extender EF 1.4x, or the 135 f/2.0L or 200 f/1.8L with Extender EF 2x, the center AF point will be a cross-type sensor for high precision AF.  The remaining 44 focusing point will only be horizontal-line sensitive.

The 70-200 f/2.8L used together with Extender EF 1.4x will enable the center AF point to work as a cross-type sensor.  The other AF points cannot be used for AF and will cause an error.

For the 100-400 f/4.5-5.6L IS with a combined Extender EF 1.4x, the center AF point will be horizontal-line sensitive only.  The other AF points cannot be used for AF.

A ‘normal precision’ AF point produces focusing accuracy within the depth of focus for the lens’ maximum aperture.  A ‘high precision’ AF point produces focusing accuracy with one-third the depth of focus for the lens’ maximum aperture.

One-Shot AF for Still Subjects

The AF point which achieves focus flashes briefly. The focus confirmation light in the viewfinder lights at the same time.

AI Servo AF for Moving Subjects

When focus is achieved the viewfinder’s focus confirmation light does not light and the beeper does not sound. If the viewfinder’s focus confirmation light blinks, focus has not been achieved. The camera will focus track a subject which steadily approaches or retreats from the camera:

Automatic AF point selection

The camera first uses the center AF point to focus.  If the subject later moves away from the center AF point, focus tracking continues as long as the subject is covered by another AF point.  The active AF point does not light. Manually selected AF point: the AF point will flash in red and focus track the subject.

Use C.Fn-20 to change the AI Servo AF’s subject tracking sensitivity.  This Custom Function does not affect the AI Servo AF tracking speed.

C.Fn-13 Number of AF points / Spot metering

The camera can be set to automatic AF point selection.  C.Fn-13’s settings are meaningless when the camera is set to automatic AF point selection.

When in automatic AF point selection:

All 45 points will be subject to automatic selection. From among the 45 AF points, the camera will select the AF point automatically to suit the subject.

Tracking from point to point with all 45 AF points is only possible when the camera is set for the combination of AF point selection with AI Servo AF.

When in manual AF point selection:

The metering mode must be set to spot metering before it can be linked to the AF point.

0: 45/Center AF point

You can set one of the 45 AF points manually. If double AF points are selected, both AF points will be used for shooting.

Spot metering is limited to center AF point.

FEL is possible with center AF point only.

1: 11/Active AF point

The manually selectable AF points are limited to 11.

Spot metering is limited to the active AF point.

FEL is possible with any AF point you select.

2: 11/Center AF point

The selectable AF points are limited to 11.

Spot metering is limited to the center AF point.

3: 9/Active AF point

The selectable AF points are limited to 9.

Spot metering is limited to the active AF point.

FEL is possible with any AF point you select.

C.Fn-17 AF point activation area

The main reason for the existence of C.Fn-17 is to allow the camera to select an alternate AF point when a manually selected AF point is unusable for some reason. C.Fn-17 is only effective with manual AF point selection.

Enabling C.Fn-17 does not affect FEL in any way.

C.Fn-17 does not work any differently in Single Shot versus AI Servo AF mode.

C.Fn-17 is not affected by C.Fn-13’s (Number of AF Points) settings.

0: Single AF point

If there is insufficient contrast at the manually selected AF point, the camera will fail to AF and the viewfinder’s in-focus indicator blinks rapidly.

1: Expand (Total of 7 AF points)

The AF point activation area expands by one adjacent point all around the manually selected AF point.  A total of 7 AF points become active. This setting is effective for when one manually selected AF point is unable to focus track a subject moving erratically.

The AF point activation area centers on the selected AF point.  Therefore, if a peripheral AF point is manually selected, the AF point activation area may be smaller.

If focusing is initially achieved by an AF point other than the manually selected one, the camera will always favor the manually selected focusing point whenever possible.

If the number of manually selectable AF points has been reduced to 11 or 9 via C.Fn-13, then the camera may end up using an AF point that is not manually selectable under the C.Fn-13 condition.

The expanded AF point activation area is not displayed in the viewfinder.

When focus is achieved in One-Shot AF mode, the selected AF point and the AF point(s) achieving focus will light.

2: Automatic Expand (Max. 13)

The camera automatically sets the AF point activation area to 7 or 13 points to suit lens focal length, AF mode, and the subject’s speed during predictive AF.  This is effective when the subject is moving erratically.

The AF point activation area centers on the selected AF point.  Therefore, if a peripheral AF point is selected, the AF point activation area will be smaller.

If the AF Mode is One-Shot:

and the lens focal length is shorter than 300mm, the activation area does not expand automatically.

and the lens focal length is 300mm or longer, the activation area expands by one point.

When focus is achieved, the selected AF point and the AF point(s) achieving focus will light.

If the AF Mode is AI Servo AF:

and the lens focal length is shorter than 300mm, for a slow moving subject the activation area does not expand automatically;  for a fast moving subject the activation area expands by one point.

and the lens focal length is 300mm or longer, for a slow moving subject the activation area expands by one point; for a fast moving subject the activation area expands to 13 points.

If focusing is initially achieved by an AF point other than the manually selected one, the camera will always favor the manually selected focusing point whenever possible.

If the number of manually selectable AF points has been reduced to 11 or 9 via C.Fn-13, then the camera may end up using an AF point that is not manually selectable under the C.Fn-13 condition.  Again, C.Fn-13 setting does not affect C.Fn-17.

 

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