What are the aperture limits for autofocus on Canon DSLR cameras?

Question

This is a follow-up to Why does Canon limit or disable autofocus below certain f-numbers? I'm doing this to separate the AF aperture requirements from the linked question above.

What apertures are required for Canon EOS DSLR cameras to autofocus? What apertures are needed to enable cross-type and/or high-precision autofocus?

Answer

TL;DR answer: With the exception of Canon EOS-1 series cameras, Canon DSLRs require a maximum aperture of at least f/5.6 to autofocus. Depending on the camera model, a maximum aperture of at least f/2.8 (f/4 for EOS-1 series cameras) enables cross-type and/or high-precision focusing. EOS-1 series cameras can autofocus at f/8, but only with the center point.

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Some definitions are needed here:

• A horizontal-sensitive point or vertical-sensitive point is an autofocus point that can detect horizontal or vertical lines, respectively. These points cannot detect both at the same time.
• A high-precision sensor is an autofocus sensor that is capable of focusing within 1/3 of the depth of field of the lens, instead of simply within the depth of field. These sensors require faster maximum apertures - typically at least f/2.8 - in order for them to work.
• A cross-type point is an autofocus point that can detect both horizontal and vertical lines simultaneously. Depending on the camera model, the maximum aperture may need to be at least f/2.8 (f/4 for EOS-1 series cameras) for a point to be cross-type, because the vertical-sensitive part of the AF point is a high-precision sensor.
• A twin cross-type point is an autofocus point that can detect diagonal lines as well as horizontal and vertical lines. The diagonal sensors require at least f/2.8 in order to function because they are high-precision sensors.

On Rebels since the XTi (400D) as well as the 30D, the center point is high-precision cross-type only when the aperture is at least f/2.8. If below f/2.8 but at least f/5.6, all points function as horizontal sensitive only. Below f/5.6, autofocus does not function.

On the 40D, 50D, 60D, and 7D, the center point is high-precision twin cross-type only when the aperture is at least f/2.8. If below f/2.8 but at least f/5.6, all points function as cross type, but the center point is single cross-type and normal precision only. Below f/5.6, autofocus does not function.

On the Canon EOS 5D Mark II, the center point is high-precision cross-type only when the aperture is at least f/2.8; the other points are either horizontal or vertical sensitive (but not both). If below f/2.8 but at least f/5.6, the center point remains cross-type but is normal precision only; all other points continue to function normally. Two of the six invisible Assist AF points used in AI Servo AF are cross-type at f/2.8, with the rest horizontal sensitive; if below f/2.8 but at least f/5.6, they are all horizontal sensitive. Below f/5.6, autofocus does not function.

On the Canon EOS-1D Mark IV, the following applies:

• If at least f/2.8, all 45 autofocus points will function, with 39 points high-precision cross-type. (If the AF points are selected automatically, or on the Canon EOS-1D Mark III and EOS-1Ds Mark III, 19 of the 45 points are high-precision cross-type.)
• If at least f/4, all 45 autofocus points will function, but only the center point is high-precision cross-type.
• If at least f/5.6, all 45 autofocus points will function as horizontal sensitive only.
• If at least f/8, only the center point functions as horizontal sensitive only. The other points will not work.
• Below f/8, autofocus does not function.
• There are exceptions:
  • All of the AF points will be horizontal sensitive only if a variable-aperture zoom lens is mounted. This excludes the EF 28-80mm f/2.8-4L USM lens, which allows the center point to work as cross-type.
  • Certain lenses do not allow the three points on the far left and far right to work as cross-type points (only horizontal sensitive) even though they have a maximum aperture of f/2.8. These include:
    • EF 24mm f/2.8
    • EF 28mm f/2.8
  • Some lenses and lens/extender combinations allow cross-type functionality with 39 of the 45 points, even though they have a maximum aperture of f/4. This applies to the Canon EOS-1D Mark IV only and include the following:
    • EF 17-40mm f/4L USM
    • EF 24-105mm f/4L IS USM
    • EF 70-200mm f/2.8L IS USM with Extender EF 1.4X II
    • EF 200mm f/2L IS USM with Extender EF 2X II
    • EF 300mm f/2.8L IS USM with Extender EF 1.4X II
    • EF 400mm f/2.8L IS USM with Extender EF 1.4X II

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