Digital Photography Tutorial - Camera Auto Focus System
By Joe Y Chin

There's a feature in cameras these days that made photography a much easier process. Before this feature was invented, only professional photographers were the ones who can actually take wonderful photos. This feature is really technical and through innovative technology and research, the feature eventually ended up in your common digital camera. What feature am I talking about? It is the magnificent but sophisticated auto-focus system. However quite a number of people are confused about how it actually works. There are actually three kinds of auto-focus system for a camera.

The first one is relatively simple and it is categorised as active detection. The system measures the distance of the camera from the subject and focus the lens according to the distance. This can be achieved by releasing an infrared beam and record the time taken for the beam to return to the camera from the subject. Although this system is easy to develop, it actually has a limit. This system cannot work if the subject is too far away from the camera, because the infrared diffuses as the beam travels at a larger distance. Therefore the reflected infrared might not even reach back to the camera. This system only works well for static macro photography.

The second and third auto-focussing technology are categorised as passive detection

The second auto focus system is achieved by your camera's sophisticated sensor. Your sensor is actually a light sensor that
can detect the brightness and the darkness of the subject (this is how they function in Auto mode!). Utilizing this sensor's ability, the auto-focussing system actually progressively detects the contrast of the intended subject. The camera then focuses the lens until the sensor detects a maximum contrast of the subject before claiming that the subject is in focus. (Imagine a blur subject and a subject in-focus, the contrast of an unfocussed subject is lower because of the scattered light from the object).

The third one is one of the most advance technologies in auto focussing system and currently is only implemented in DSLRs. This is called phase difference detection system. This system uses the Secondary Image Registration Through The Lens (SIR TTL) technology where the light from the lens will be split by the reflex mirror creating two separate light waves: one to the sensor and one to the auto-focussing prism. The two light detected will be analysed by an Auto Focus processor and the phase difference of the light beams detected will cause the lens to be adjusted making both waves to be in phase. (This may seem really confusing. I was confused the first time as well).

That is the summary for all three auto focus systems. There are much more features in digital camera nowadays that one needs to learn in order to take professional photos. If you want to learn more about photography techniques, tips and tricks; subscribe at for free! You will even get a free photography report upon subscribing.