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Autofocus (AF) could also be called power-focus, as it often uses a computer to
run a miniature motor that focuses the lens for you. Focusing is the moving of
the lens in and out until the sharpest possible image of the subject is
projected onto the film. Depending on the distance of the subject from the
camera, the lens has to be a certain distance from the film to form a clear
image.
DSLR Know-How Autofocus Episode 2: Andre's 1-minute tip about focus points and focus modes
General
Auto-focus systems can capture a subject in a variety of ways; here, the focus is on the person's image
in the mirror.
Autofocus systems rely on one or more sensors to determine correct focus. Some AF systems rely on a single
sensor, while others use an array of sensors. Most modern
SLR cameras use through-the-lens optical AF sensors,
which also perform as light meters.
The speed and accuracy of through-the-lens optical autofocusing is now often more precise than what can be
achieved manually with an ordinary viewfinder. (More precise manual focus can, of course, be achieved with
special accessories such as focusing magnifiers.) Autofocus accuracy within 1/3 of the
depth of field (DOF)
at the widest aperture of the lens is not uncommon in professional AF SLR cameras.
Most multi-sensor AF cameras allow manual selection of the active sensor, and many offer automatic selection
of the sensor using algorithms which attempt to discern the location of the subject. Some AF cameras are able
to detect if the subject is moving towards or away from the camera, including speed and acceleration data,
and keep focus on the subject -- a function used mainly in
sports and other action photography.
The data collected from AF sensors is used to control an electromechanical system that adjusts the focus of
the optical system. A variation of autofocus is called an electronic rangefinder, a system in which focus
data are provided to the operator, but adjustment of the optical system is still performed manually.
The speed of the AF system is highly dependant on the maximum aperture offered by the lens.
F-stops of
around f/2 to f/2.8 are generally considered optimal in terms of focusing speed and accuracy. Faster
lenses than this typically have very low depth of field, meaning that it takes longer to achieve correct
focus, despite the increased amount of light. Most consumer camera systems will only autofocus reliably
with lenses that have a maximum aperture of at least f/5.6, while professional models can often cope with
lenses that have a maximum aperture of f/8, which is particularly useful for lenses used in conjunction
with teleconverters.
History
The first mass-produced autofocus camera was the Konica C35 AF, a simple point and shoot model released in
1977. The Polaroid SX-70 was the first autofocus
single-lens reflex camera, released in 1978. The Pentax
ME-F, which used focus sensors in the camera body coupled with a motorized lens, became the first autofocus
35mm SLR in 1981. In 1983 Nikon released the F3AF, their first autofocus camera, which was based on a similar
concept to the ME-F. The Minolta Maxxum 7000, released in 1985, was the first SLR with an integrated
autofocus system, meaning both the AF sensors and the drive motor were housed in the camera body, as well
as an integrated film advance winder - which was to become the standard configuration for SLR cameras from
this manufacturer, as it would for Nikon.
Canon, however, elected to develop their EOS system with
motorized lenses instead.
Active autofocus
Active AF systems measure distance to the subject independently of the optical system, and subsequently
adjust the optical system for correct focus.
There are various ways to measure distance, including ultrasonic sound waves and infrared light. In the
first case, sound waves are emitted from the camera, and by measuring the delay in their reflection,
distance to the subject is calculated. Polaroid cameras including the Spectra and SX-70 were known for
successfully applying this system. In the latter case, infrared light is usually used to triangulate the
distance to the subject. Compact cameras including the Nikon 35TiQD and 28TiQD, the Canon AF35M, and the
Contax T2 and T3, as well as early video cameras, used this system.
An exception to the two-step approach is the mechanical autofocus provided in some enlargers, which adjust
the lens directly.
General
Passive autofocus
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
Phase Setection System
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 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-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 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 (Olympus E-420, Panasonic L10,
Nikon D90,
Nikon D300 in Tripod Mode) 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.
Comparison of active and passive systems
High contrast bands can be projected onto low contrast subjects, seen here for Canon EOS 100Active
systems will typically not focus through windows, since sound waves and infrared light are reflected by
the glass. With passive systems this will generally not be a problem, unless the window is stained.
Accuracy is often considerably less than passive systems.
Active systems may also fail to focus a subject that is very close to the camera (e.g., macro photography).
Passive systems may not find focus when the contrast is low, notably on large single-colored surfaces
(walls, blue sky, etc.) or in low-light conditions. Passive systems are dependent on a certain degree of
illumination to the subject (whether natural or otherwise), while active systems may focus correctly even
in total darkness when necessary. Some external flash units have a special low-level illumination mode
(usually orange/red light) which can be activated during auto-focus operation to allow the camera to focus.
See also
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