Canon eos5d manual EOS 5D Mark II AF Systems

IV. PERFORMANCE AND RELIABILITY 32

EOS 5D Mark II AF Systems						The AF system on the EOS 5D Mark II
						includes 9 visible AF points and six
						invisible assist points, for a total 15
		f/5.6-sensitive			Intermediate	points concentrated toward the center.
		major defocus				The outer points on the extreme left and
					AF points
		detection sensors
						right are actually located at the same
						positions as the corresponding points in
	f/2.8 / f/5.6-sensitive sensors		: Sensitive to f/5.6	: Sensitive to f/2.8		the EOS-1Ds Mark III, giving the
	f/5.6 sensors		Assist AF points	Assist AF points		camera a wide field for detecting moving
						subjects crossing the scene or improved
	AF sensor configuration		AF points and invisible AF points
						ability to focus on off-center subjects.
						While only 9 AF points are visible
						through the viewfinder, all can be revealed
			using ZoomBrowser or ImageBrowser software. Among the six Assist AF
			points, the top and bottom points at the center are vertical-line sensitive
			with lenses featuring f/2.8 or brighter maximum apertures, while all other
			points are sensitive at f/5.6 or brighter. Focusing is started using the f/5.6,
			horizontal line-sensitive central sensors with individual pixels arranged in
			a zigzag pattern for improved extreme defocus detection. When focusing is
			nearly achieved (and when using an f/2.8 or brighter lens) focusing is handed
AF Sensor			off to the f/2.8 sensitive vertical line sensors.

The purpose of the supplemental points is to enhance the camera’s subject tracking performance by filling in the gaps between the highly sensitive central AF point and the 8 adjacent linear sensors.

As on the EOS 50D, the EOS 5D Mark II’s AF system is sensitive to light levels as low as EV -0.5, and capable of locking on to a moving subject and tracking it across the diamond- shaped AF area. Improved precision over previous AF systems stems from each camera’s ability to adjust for different light sources while calculating focusing distance. Here’s how it works: during Phase Detection AF, the AF engine rapidly measures the density of horizontal and vertical details in a scene and the camera processor uses this data to determine where the sharpest edges and details are located. When found, the AF is locked on target. But under fluorescent lighting and some other artificial lighting sources, a rapid, imperceptible flickering of the light occurs along with sudden color temperature shifts. These tend to throw off the density measurements and therefore the accuracy of the AF calculations. In both the EOS 50D and EOS 5D Mark II, the type of lighting is taken into effect, and readings are averaged and processed with the help of the super-fast DIGIC 4 Image Processor. The result is improved AF accuracy and speed when shooting under lighting conditions that could fool the AF systems found on earlier Canon and competitive models alike.