We all know what does we mean by saying a focused picture. Well, if you don’t know, look at the below image. Clearly the picture on the right looks more pleasant and clear to our eyes while the other picture seems blurred.
There are two ways we can adjust the focus. 1. The manual focus adjustment. 2. Autofocusing systems.
In manual mode, focal length of the lens is adjusted manually by hand so as to get a clear picture. In autofocusing systems, we rely upon electronic-mechanical systems to adjust the lens focal length. If you have ever used a magnifying glass to get the focused image, you will get this concept easily. To get a clear image, you will have to adjust the lens up and down. Indirectly you are doing the autofocus. It is your brain (eyes are the sensors and controller the brain) telling you to adjust the glass.
Most common autofocusing systems are:
- Phase detection auto focus (PDAF)
- Contrast detection auto focus (CDAF)
- Laser auto focus (LAF)
- Hybrid auto focus (HAF)
Among these, PDAF and CDAF are the most popular autofocusing methods.
Contrast detection autofocus
This system relies on the contrast of the image which it is received. Sensor detects the contrast of the image in first lens position, and moves the lens forward or backward to get the contrast of the image in the new position. Then it compares the first result and second result and hunts for next possible best position of the lens. You might notice some sounds coming from the camera while it performs these, even from smartphone cameras which employs this.
Pros of CDAF
- Simple technology
- Good enough for static pictures
Cons of CDAF
- It takes some time for the system to find the best position for the lens, probably even few seconds
- It fails on a moving object, results in poor focusing
- Not an ideal solution for videos
Due to the disadvantages of contrast detection AF, PDAF is introduced
Phase detection autofocus
This type of autofocus is usual in today’s smartphones. PDAF relies on image processing more than mechanical adjustments to get the correct focus. It splits the image into 2. Then the system internally tries to move the image and align the lens accordingly. When it is aligned we get a focused image.
Pros of PDAF
- Mechanical elements are not involved in focusing, making it faster than CDAF.
- It is good at focusing moving images and videos
Cons of PDAF
- Certain percentage of the pixels in sensor will be allocated to focusing.
Today’s smartphones and and DSLR’s are using dual-pixel auto focusing systems. Example : Samsung Galaxy S7
Dual Pixel autofocus
With PDAF, only 5 % of the available pixels were part of focusing. But in Dual pixel focusing, a phase detection photo diode is embedded into each and every pixel on the camera sensor.
Dual pixel focusing systems are superior than PDAF and hybrid systems.
This system employs a small laser transmitter and a receiver. It sends a laser beam to the subject and it bounces back to reach the receiver. Using some mathematical algorithms, It calculates the time taken for the beam to hit the subject and come back.
Pros of Laser autofocus
- It is the fastest autofocusing
- Works in very low lighting
Cons of Laser autofocus
- Fails to focus subjects on the long distance as the laser transmitter is not a powerful one.
Some cameras employ multiple focusing systems to minimize the disadvantages of both the systems. For example, many of the SONY sensors are using both PD & CD together. Another example would be Google Pixel, which uses PDAF and Laser autofocus.
Picture Courtesy : giffgaff community