Photography Techniques

Auto Focus – All about active and passive auto focus

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Written by Dave Peterson

Auto Focus –  All about active and passive auto focus

Auto Focus – All about active and passive auto focusThe feature of the digital camera that makes it possible to focus correctly on the subject without doing manual adjustments for focus is termed as Auto Focus (AF). In modern day cameras there is a miniature motor that moves lens to and fro as per the requirement to form sharpest image of the subject onto the sensor or film. As per the distance of the subject from the camera, the lens has to be at a certain distance from the sensor to form a clear image. One or more sensors are used to establish right focus.

Most of the digital SLR cameras have through the lens optical AF sensors. The advantage of such sensors is its precision and the speed. In most AF with multi-sensor cameras there is option for manual selection of active sensor.  Some AF cameras check and detect whether the subject is approaching or moving away from the camera, keeping focus on the subject, this feature is used remarkably in action photography. The speed of AF system heavily depends on maximum aperture that the lens can offer.

Auto Focus –  All about active and passive auto focus

There are two types of auto focus systems – active and passive. Active AF systems has a mechanism that measures distance between camera and subject not depending on optical system and accordingly corrects the focus by adjusting optical system. To measure the distance use of infrared light and ultrasonic sound waves is done. The infrared light from camera is allowed to fall on the subject and distance is calculated while in the second method ultrasonic sound waves are incident on the subject and the reflection of these waves are received to calculate the distance.

Auto Focus – All about active and passive auto focusThere are certain situations where infrared light method creates a problem. If you have a open flame in the subject then infrared light from such a source will confuse the infrared sensor in the camera. The black surface in the subject will absorb the incident infrared beam. Also the infrared light beam may reflect from the front of the subject rather than from the subject, making all the calculations wrong for knowing exact distance. The advantage of active auto focus system is it has no limitation of dark areas and thereby makes flash photography easier.

In passive AF systems no transmission of energy like infrared or ultrasonic sound is done towards the subject. These system use contrast measurement or phase detection to execute auto focus. Both the systems active as well as passive auto focus systems have their own advantages and disadvantages. You cannot focus through windows when using active systems as glass will reflect both sound waves and infrared rays. This is not a drawback of passive systems.

Auto Focus –  All about active and passive auto focus

The accuracy of active systems is less than the passive systems many times. Active system is unable to focus a subject very close to the camera while passive systems find it hard to focus in situations where contrast or light condition is low. Auto focus overall is a great time saver in modern cameras and helps you improve the quality of pictures.

In the next part of the tutorials more about auto focus will be discussed.

Tutorial Twenty Three – Auto Focus – Its type and various advantages and disadvantages

Tutorial Twenty One –Image stabilization – Necessary to overcome camera shake

About the author

Dave Peterson

Dave Peterson
Passion for adventure and sharing his life long journey with as many others as possible.

"What lies behind us and what lies before us are tiny matters compared to what lies within us." HENRY S. HASKINS

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