Autofocus
Autofocus accuracy within 1/3 of the depth of field (DOF) at the widest aperture of the lens is common in professional AF SLR cameras.
A variation of autofocus is an electronic rangefinder, 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 dependent on the widest aperture offered by the lens at the current focal length.
Faster lenses than this (e.g.: f/1.4 or f/1.8) typically have very low depth of field, meaning that it takes longer to achieve correct focus, despite the increased amount of light.
The Pentax ME-F, which used focus sensors in the camera body coupled with a motorized lens, became the first autofocus 35 mm SLR in 1981.
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.
Through-the-lens secondary image registration (TTL SIR) passive phase detection is often used in film and digital SLR cameras.
[5] 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.
[6] 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.
Some cameras (Minolta 7, Canon EOS-1V, 1D, 30D/40D, Pentax K-1, 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" Some modern sensors (for example one in Librem 5) include about 2% phase detection pixels on the chip.
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.
The assist light (also known as AF illuminator) "activates" passive autofocus systems in low-light and low-contrast situations in some cameras.
The lamp projects visible or IR light onto the subject, which the camera's autofocus system uses to achieve focus.
This aids the autofocus system in the same fashion as a dedicated assist light, but has the disadvantage of startling or annoying people.
Similar stroboscopic flashing is sometimes used to reduce the red-eye effect, but this is only intended to constrict the subject's eye pupils before the shot.
Another way to assist contrast based AF systems in low light is to beam a laser pattern on to the subject.
An IR or ultrasonic system based on reflection will work regardless of the light conditions, but can be easily fooled by obstacles like window glasses, and the accuracy is typically restricted to a rather limited number of steps.
Phase-detection autofocus "sees" through window glasses without problems and is much more accurate, but it does not work in low-light conditions or on surfaces without contrasts or with repeating patterns.
In July, 2010, Fujifilm announced a compact camera, the F300EXR, which included a hybrid autofocus system consisting of both phase-detection and contrast-based elements.
Active systems will typically not focus through windows, since sound waves and infrared light are reflected by the glass.
Passive systems may not find focus when the contrast is low, notably on large single-colored surfaces (walls, blue sky, etc.)
AI Servo is an autofocus mode found on Canon SLR cameras, and in other brands such as Nikon, Sony, and Pentax, under the name "continuous focus" (AF-C).
AI refers to artificial intelligence: algorithms that constantly predict where a subject is about to be based on its speed and acceleration data from the autofocus sensor.
[14] The magnetic field interacts with coils to produce motion for adjusting the lens position quickly and accurately based on focus requirements.
[15] Magnets are ideal for this purpose because they enable smooth and rapid adjustments without direct physical contact, enhancing durability and response time.
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.
