The 61-point high-density reticular AF sensor requires a wide incoming beam of light in order to increase the number of AF , but the size of the beam depends on the lens. AF systems of earlier EOS cameras were designed for a beam size that matched the greatest number of EF lenses to en-sure that all EF lenses could be used with similar ease. However, development and incorporation of intelligent viewfinders have made it possible to display different AF points depending on the lens, and so AF design ideas evolved toward fully exploiting the incoming beam available with each lens. AF points and focusing patterns are optimized for each type of lens.

The 61-point high-density reticular AF sensor provides a maximum of 41-point high precision cross-type AF sensor for lens with f/4 or larger aperture. The cross-type AF points are widely distributed throughout the whole AF area, with 21 point at the center and 10 points each on left and right. For lens with f/5.6 aperture, the AF sensor still provides 21-point cross-type AF sensor at center. The camera only shows AF points and pattern that can be used with the lens currently attached thanks to the introduction of intelligent viewfinder. In addition, cross-type points are illuminated while all other points flash on and off during AF point selection. Users don’t need to think twice about using a lens.

AF Sensor type distribution These focusing sensors are geared to achieve higher precision focusing using with f/2.8 or larger maximum aperture lenses. A diagonal cross pattern makes it easier to focus subjects that may be difficult to focus. They cover the five vertical AF points at the center. These focusing sensors are geared to achieve high-precision focusing with f/4 or larger max-imum aperture lenses. Since they have a horizontal pattern, they can detect vertical lines. These focusing sensors are geared for f/5.6 or larger maximum aperture lenses. Since they have a horizontal pattern, they can detect vertical lines. They covered the 3 columns of AF points at the viewfinder’s center. These focusing sensors are geared for f/5.6 or larger maximum aperture lenses. They can detect horizontal lines and cover all 61 AF points in a vertical pattern.

Non cross-type AF points flash on and off in viewfinder during AF point selection

Lens maximum aperture and Cross-Type AF points

Maximum aperture of f/2.8 or faster For lens with a maximum aperture of f/2.8 or "faster", five extra high-precision diagonal and twenty outer high-precision cross-type AF points are available — a total of 41 cross-type points — including the remaining standard-precision cross-type points in the central area. f/2.8 Dual cross-type AF point f/4 Cross-type AF point (f/4 horizontal; f/5.6 vertical) f/5.6 Cross-type AF point f/5.6 Vertical AF point Maximum aperture of f/4 41 cross-type AF points can be used with lenses having an f/4 maximum aperture. All 21 central AF points are cross-type, as well as 20 points on the left and right. The high precision autofocus performance of f/4L series lenses commonly used by professional photographers is achieved. f/4 Cross-type AF point (f/4 horizontal; f/5.6 vertical) f/5.6 Cross-type AF point f/5.6 Vertical AF point Maximum aperture of f/5.6 Even at maximum aperture of f/5.6, such as super telephoto lens with extender attached, 21-point cross-type AF still available in the central area. f/5.6 Cross-type AF point f/5.6 Vertical AF point Effective maximum aperture of f/8 To address the feedback from professional photographers, AF is still possible at the center AF point with cross-type coverage with latest firmware when effective maximum aperture is at f/8. For many nature and wildlife photographers, this expands their shooting possibility f/8 Cross-Type AF Point f/8 Vertical AF Point (Cannot be manually selected) f/8 Horizontal AF Point (Cannot be manually selected)

The number of usable AF points on 61-point AF System will differ depending on the lens. The lenses are classified into 9 groups from A to I. Check below link for which group your lens belongs to. (When using a lens from Group F to H, fewer AF points will be usable)
http://www.canon.com.hk/en/product/catalog/getProductTag.do?tagID=4672