Phase detection (PD) is achieved by dividing the incoming light into pairs of images and comparing them. Through-the-lens secondary image registration (TTL SIR) passive phase detection 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 micro-lenses 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 within the lens's diameter. The two images are then analysed for similar light intensity patterns (peaks and valleys) and the separation error is calculated in order to find whether the object is in front focus or back focus position. This gives the direction and an estimate of the required amount of focus-ring movement.[2]

PD AF in a continuously focusing mode (e.g. "AI Servo" for Canon, "AF-C" for Nikon, Pentax and Sony) is a closed-loop control process. PD AF in a focus-locking mode (e.g. "One-Shot" for Canon, "AF-S" for Nikon and Sony) is widely believed to be a "one measurement, one movement" open-loop control process, but focus is confirmed only when the AF sensor sees an in-focus subject. The only apparent differences between the two modes are that a focus-locking mode halts on focus confirmation, and a continuously focusing mode has predictive elements to work with moving targets, which suggests they are the same closed-loop process.[3]

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 TTL area SIR[citation needed] sensors that are rectangular in shape and provide two-dimensional intensity patterns for a finer-grain analysis. Cross-type 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 (Minolta 7, Canon EOS-1V, 1D, 30D/40D, Sony DSLR-A700, DSLR-A850, DSLR-A900) also have a few "high-precision" focus points with an additional set of prisms and sensors; they are only active with "fast lenses" with certain geometrical apertures (typically f-number 2.8 and faster). Extended precision comes from the wider effective measurement base of the "range finder".


Contrast detection
Contrast-detection autofocus 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 maximal contrast is detected. In this method, AF does not involve actual distance measurement at all. This creates significant challenges when tracking moving subjects, since a loss of contrast gives no indication of the direction of motion towards or away from the camera.

Contrast-detect autofocus is a common method in digital cameras that lack shutters and reflex mirrors. Most DSLRs use this method (or a hybrid of both contrast and phase-detection autofocus) when focusing in their live-view modes. A notable exception is Canon digital cameras with Dual Pixel CMOS AF. Mirrorless interchangeable-lens cameras typically used contrast-measurement autofocus, although phase detection has become the norm on most mirrorless cameras giving them significantly better AF tracking performance compared to contrast detection.

Contrast detection places different constraints on lens design when compared with phase detection. While phase detection requires the lens to move its focus point quickly and directly to a new position, contrast-detection autofocus instead employs lenses that can quickly sweep through the focal range, stopping precisely at the point where maximal contrast is detected. This means that lenses designed for phase detection often perform poorly on camera bodies that use contrast detection.

Focus motors
Modern autofocus is done through one of two mechanisms; either a motor in the camera body and gears in the lens ("screw drive") or through electronic transmission of the drive instruction through contacts in the mount plate to a motor in the lens. Lens-based motors can be of a number of different types, but are often ultrasonic motors or stepper motors.

Some camera bodies, including all Canon EOS bodies and the more budget-oriented among Nikon's DX models, do not include an autofocus motor and therefore cannot autofocus with lenses that lack an inbuilt motor. Some lenses, such as Pentax' DA* designated models, although normally using an inbuilt motor, can fall back to screwdrive operation when the camera body does not support the necessary contact pins.

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