Auto Focus – All About Active and Passive Auto Focus

Auto Focus –  All About Active and Passive Auto Focus

The 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 the lens to and fro as per the requirement to form the 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 the right focus.

Most of the digital SLR cameras have through the lens optical AF sensors. The advantage of such sensors is its precision and speed. In most AF with multi-sensor cameras, there is an 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 the focus on the subject, this feature is used remarkably in action photography. The speed of the AF system heavily depends on the maximum aperture that the lens can offer.

Auto Focus –  All About Active and Passive Auto focus

There are two types of autofocus systems – active and passive. Active AF systems have a mechanism that measures the distance between camera and subject not depending on the optical system and accordingly corrects the focus by adjusting the optical system. To measure the distance use of infrared light and ultrasonic sound waves is done. The infrared light from the 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.

There are certain situations where infrared light method creates a problem. If you have an 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 the exact distance. The advantage of active autofocus system is it has no limitation of dark areas and thereby makes flash photography easier.

In passive AF systems, no transmission of energy like the infrared or ultrasonic sound is done towards the subject. These systems use contrast measurement or phase-detection to execute autofocus. Both the systems active as well as passive autofocus 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 passive systems many times. An 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. Autofocus 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 autofocus 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

Main Image Source : Pixabay

Also See : Picture Perfect – 100 Comments to make about Photographs

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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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