Autofocus Systems

There are basically 2 type of autofocus system that are being use in Interchangeable Lens camera systems (a third type is the hybrid system of both the 2 AF system and I am not going to touch on that just yet).

The two types of AF systems are known as Phase Detection Autofocus system and Contrast measurement AF system.

   

Phase Detection AF system: This AF system is mainly used in DSLR and DSLT. It is by far the faster AF systems among the two. This AF system works in the following way.

Light entered the camera through the lens and hit the mirror which was not really transparent, thus allowing some of the light to pass through and was reflected again by a secondary mirror behind the main mirror, while most of the light got reflected up to the massive prism in the DSLR/ DSLT and transfer to the metering system and also the viewfinder. The light that had passed through the light and reflected down by a secondary mirror would hit the AF sensor (do not confuse an AF sensor with the imaging sensor).

  

On top of the AF sensor is a set of precision optics (micro lenses) that splits the incoming light into 2 beams. Base on where this beams strike it, the sensor can determine how far out of focus the image is and whether the focus is infront or behind the focal plane. With these information, the AF sensor is able to calculate and produce a very accurate focus.

Contrast Detection (Contrast measuring) AF system: This AF system was normally being use in mirrorless interchangeable camera and small compact (point and shoot) cameras. This system make use of only 1 sensor (unlike phase detection AF system which differentiate both the imaging sensor and AF sensor) the contrast detection system combine both imaging and AF sensor into a single package.

The contrast detection AF system normally started off with the lens at infinity (focal length) and move back step by step to gauge at the contrast of the scene to see if it increases or decreases. If it increases, the system would know that it was getting toward an accurate focus point... and once the system get past that point and found that contrast decreases, the system would identify the peak of the contrast point and lock on to that point as focused point.

  

(note: from personal experience, sometime the system told me that it had achieved focus point, but everything are still blur... and this would easily occur especially with complicated scene... unlike my DSLR which utilized the phase detection AF system which is either was spot on... or simply cannot focus.)