Wiki says :
Phase Detection
Phase detection is achieved by dividing the incoming light into pairs of images and comparing them.
SIR TTL passive phase detection (secondary image registration,
through the lens) is often used in film and digital
SLR cameras. The system uses a
beam splitter (implemented as a small semi-transparent area of the main reflex mirror, coupled with a small secondary mirror) to direct light to an AF sensor at the bottom of the camera. Two
optical prisms capture the light rays coming from the opposite sides of the lens and divert it to the AF sensor, creating a simple
rangefinder with a base identical to the lens's diameter. The two images are then analysed for similar light intensity patterns (peaks and valleys) and the phase difference is calculated in order to find if the object is in
front focus or
back focus position. This instantly gives the exact direction of focusing and amount of focus ring's movement.
Although AF sensors are typically one-dimensional photosensitive strips (only a few pixels high and a few dozen wide), some modern cameras (
Canon EOS-1V,
Canon EOS-1D,
Nikon D2X) feature
Area SIR sensors that are rectangular in shape and provide two-dimensional intensity patterns for a finer-grain analysis.
Cross-type (CT) focus points have a pair of sensors oriented at 90° to one another, although one sensor typically requires a larger aperture to operate than the other. Some cameras (Canon EOS-1V, Canon EOS-1D,
Canon EOS 30D/
40D) ) also have a few 'high precision' focus points with an additional set of prisms and sensors; they are only active with '
fast lenses' of certain
focal ratio. Extended precision comes from the increased diameter of such lenses, so the base of the 'range finder' can be wider.
Contrast measurement (Contrast Detection)
Contrast measurement is achieved by measuring contrast within a sensor field,
through the lens. The intensity difference between adjacent pixels of the sensor naturally increases with correct image focus. The optical system can thereby be adjusted until the maximum contrast is detected. In this method, AF does not involve actual distance measurement at all and is generally slower than phase detection systems, especially when operating under dim light. As it does not use a separate sensor, however, contrast-detect autofocus can be more flexible (as it is implemented in software) and potentially more accurate. This is a common method in
video cameras and consumer-level
digital cameras that lack
shutters and reflex mirrors.
Some
DSLRs (including
Olympus E-420,
Panasonic L10,
Nikon D90,
Nikon D5000,
Nikon D300 in Tripod Mode,
Canon EOS 5D Mark II,
Canon EOS 50D) use this method when focusing in their
live-view modes. A new interchangeable-lens system,
Micro Four Thirds, exclusively uses contrast measurement autofocus, and is said to offer performance comparable to phase detect systems.
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My conclusion is whatever works better. PDAF is fast, and it will take a while before CDAF can do anything close to PDAF. But in the coming months, Olympus will release an updated AF system which we can only wait and see how these things work. They achieved so much with their PDAF system is only second to Canon.