Autofocus

Autofocus (or AF) is a feature of some optical systems that allow them to obtain and maintain correct focus on a target.

General

Simple AF systems rely on a single focus sensor. Advanced systems consist of an array of focus sensors. The Canon EOS 3 has as many as 45 sensors, each accessible with their unique eye-control, to focus the camera on the photographer's subject. Most modern SLR cameras use data from the AF system to assist in the light measuring.

The speed and accuracy of autofocusing is normally better than what could be achieved manually. Modern cameras measure several areas of the picture and decide where the subject is. Some cameras are able to decide if the subject is moving towards or away from the camera, what speed it has, at what rate it accelerates, and can follow the subject as it crosses the image.

Different AF systems

There are two different approaches to systems for automatic focusing:

active systems
passive systems

Active systems

These systems usually work in two steps:

distance measuring (separated from the optical system)
focus adjustment to the optical system

There are several ways to measure distance: ultrasonic sound waves (some Polaroid cameras like Spectra, SX-70), infrared light (some compact cameras, early video camcorders).

Ultrasonic sound waves are emitted from the camera, and by measuring the delay in the reflection of the sound-waves, distance can be calculated.

The active AF system using infrared light is usually based on the triangulation. The camera scans the scene with infrared light then detects the reflection from the subject. Nikon 35TiQD, 28TiQD and Canon AF35M uses this system.

Focus adjustment is mainly controlled electronically. The data collected from the distance measuring are calculated in an electronic unit which activates a mechanical or electromechanical system to adjust the focus accordingly.

An exception to the two-step approach is the mechanical autofocus provided in some enlargers, which adjust the lens directly.

Passive systems

Passive systems don't rely on any energy (in form of electromagnetic rays or soundwaves) to be sent from the camera to the subject. Instead, these systems utilize the light that is naturally reflected by the subject. In dark scenes, this can be helped by an autofocus assist beam, which illuminates the scene. Autofocusing can be achieved in two ways:

phase detection
contrast measurement

Phase detection is achieved by dividing the incoming light into pairs of images and comparing them. This system is often utilized in film/digital SLR cameras. Contrast measurement is achieved by determining when maximum focus is adjusted to achieve the maximum contrast within the sensor field. This is a common method in video-cameras and digital still cameras, and is measured through the actual optics in use. This way, passive autofocusing may not involve actual distance measuring at all.

Pros and cons of the systems

Active systems will typically not focus through windows, since sound waves and IR light are reflected by the glass. With passive systems this will generally not be a problem, unless the window is stained.

Passive systems may not find focus when the contrast is low, notably on large single-coloured surfaces (walls, blue sky, etc) or in low-light conditions. Passive systems are dependent on a certain degree of natural illumination to the subject, while active systems may focus correctly even in total darkness.

References

Norman Goldberg. Camera Technology : The Dark Side of the Lens
Sidney Ray. Applied Photographic Optics
Ralph Jacobson, Sidney Ray, Geoffrey G Attridge, Norman Axford. Manual of Photography: Photographic and Digital Imaging

External links

photographic terms

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