Editing Autofocus (section)

==Active==
{{unreferenced section|date=March 2014}}

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 [[Ultrasound|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. [[Instant camera|Polaroid]] cameras including the Spectra and [[Polaroid SX-70|SX-70]] were known for successfully applying this system. In the latter case, infrared light is usually used to [[triangulation|triangulate]] the distance to the subject. Compact cameras including the [[Nikon]] [[Nikon Ti cameras|35TiQD and 28TiQD]], the [[Canon AF35M]], and the [[Contax T|Contax T2 and T3]], as well as early video cameras, used this system. A newer approach included in some consumer electronic devices, like mobile phones, is based on the [[Time of flight|time-of-flight]] principle, which involves shining a laser or LED light to the subject and calculating the distance based on the time it takes for the light to travel to the subject and back. This technique is sometimes called ''laser autofocus'', and is present in many mobile phone models from several vendors. It is also present in industrial and medical<ref>{{cite journal |last1=Fricke |first1=Dierk |last2=Denker |first2=Evgeniia |last3=Heratizadeh |first3=Annice |last4=Werfel |first4=Thomas |last5=Wollweber |first5=Merve |last6=Roth |first6=Bernhard |title=Non-Contact Dermatoscope with Ultra-Bright Light Source and Liquid Lens-Based Autofocus Function |journal=Applied Sciences |date=28 May 2019 |volume=9 |issue=11 |pages=2177 |doi=10.3390/app9112177|doi-access=free }}</ref> devices.

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