Passive Auto Focus (AF)

Passive AF systems determine correct focus by performing passive analysis of the image that is entering the optical system.

They generally do not direct any energy, such as ultrasonic sound or infrared light waves, toward the subject. (However, an autofocus assist beam of usually infrared light is required when there is not enough light to take passive measurements.)

Passive autofocusing can be achieved by phase detection or contrast measurement.

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 range finder with a base identical to the lens' 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 sensor is typically a one-dimensional photosensitive strip (only a few pixels high and a few dozen wide), some modern cameras (Canon EOS-1D, Nikon D2X) feature Area SIR sensors that are rectangular so as to provide two-dimensional intensity patterns.

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-1D, Canon EOS 30D/40D) ) also have a few 'high precision' focus points with 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.

Phase detection system

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. This is a common method in video cameras and consumer-level digital cameras that lack shutters and reflex mirrors. Some DSLRs (Olympus E-420, Panasonic L10, Nikon D300 in Tripod Mode) use this method when focusing in their live-view modes.