When I started my photography journey 6 years ago, a simple question came to my mind one day while fiddling with a manual focus lens. How does a camera Autofocus work? Is it the lens doing all the work or the camera does something too? Being totally new to digital photography at that time, a simple question turned out to be a great learning experience for a few weeks. Using my T2i, I learned the basics of autofocusing and the difference between live view focus and viewfinder focus. The two very basic techniques of focusing in digital photography include: 

-Manual

-Autofocus

Manual, as the name suggests, is a technique where the user rotates the lens focus ring to achieve focus. In this mode, there is no help or assistance offered by the electronic circuit inside the camera to achieve focus. Night sky photography enthusiasts love the manual focus, as explained in my post about achieving infinity focus.  Coming to autofocus, half-pressing the shutter button activates the focus search mechanism to achieve focus. The camera and lens work together at the same time to achieve this (provided both support autofocus feature and are electronically connected). There are 3 different working principles to achieve autofocus:

How does contrast detect autofocus work? Basically, it detects the difference in the value of contrast in a scene. Imagine a black square and a white square side by side. If the line connecting them is sharp, it gets in focus easily. If it forms a gradual gradient, then it would be hard to focus.
Pros:

Very precise.
Cons:
Not very fast. Hunts more for focus.
Uses:
Smartphones, point and shoot cameras, micro-four-third cameras and mirrorless cameras. When a DSLR is in live view mode, CDAF is used. 

How does phase detect autofocus work? Light passes through the lens and hits the center of the camera. At this moment, it is split into two parts and bounced from a wall. If they reflect back and coincide at the center of the sensor, then it means in focus.
Pros:

Very fast.
Cons:
Not very precise.
Too much reliance on lens/path to be extremely well calibrated with longer lenses. 

Uses:
Most DSLR cameras (Viewfinder only). When it comes to viewfinder vs live view focusing, viewfinder focusing wins for most of the DSLR cameras.

With a lot of advancements in the autofocus field, there are two different ways to achieve phase detection autofocus: On-sensor module or off sensor module (behind the mirror). Comparing these two methods, I found on-sensor PDAF generally to be slower than a dedicated PDAF module. This is because:
-Dedicated PDAF module is bigger in size and thus more sensitive to light.
-Dedicated PDAF filters are monochromatic, whereas most of the on-sensor PDAF modules have a Bayer filter covering the light wells. This means less light is able to reach light wells.

It is a type of active autofocus system where some type of light radiation is used to achieve autofocus. Few examples include sonar, laser, etc. In this technique, a laser beam is emitted by the focusing gadget which hits the subject and returns to the sensor. By measuring the time taken by laser, the camera calculates the distance between subject and sensor. This distance is used as a parameter to achieve focus.

Uses:
Some smartphone cameras

The working principles discussed above form the basis of autofocus working in every camera. In recent years, new autofocusing technology names have been adopted by different manufacturers. Even though they sound different, the underlying basic principle will still be based on techniques already discussed. Some of the new autofocus technologies:

Used by Sony mirrorless cameras. It combines both phase detection autofocus and contrast detection autofocus. Cameras with hybrid AF have an on-sensor phase detect module + usual contrast detect. That's why it is claimed to be the best.
Cons:
Sony fast hybrid AF only works with compatible lenses. If the lens does not have this feature, default contrast phase detection autofocus is used.
Modern age mirrorless cameras especially Sony.

Dual pixel autofocus
From Canon, it improves focusing ability of a DSLR greatly in live mode shooting. It won't be wrong to refer to this as a special case of PDAF. One latest Canon camera with dual pixel CMOS autofocus is Canon 80D. List of other canon DPAF cameras:
Canon 70D, Canon 77D, Canon M5, Canon M6, Canon M50, Canon T7i,  Canon 7D Mark II, Canon 6D Mark II, Canon 5D Mark IV, Canon 1DX Mark II. 

Phase detection autofocus vs Optical image stabilization:
One user on my Facebook page asked me which one is better? Hard to comment, as both are totally unrelated technologies for focusing. Phase detection autofocus is a technique used by the camera body, whereas optical image stabilization is a lens stabilization feature with all the circuit built inside the lens itself. 

Tip: Professional moviemakers do not use autofocus cameras. They employ a person called as focus puller whose main job is to stand right next to the camera and move the focus ring with a precise pre-measured amount during the shot.

Hope you learned something new about autofocusing in this post. I know I skimmed quickly through the physics part of the autofocus mechanism in this article, will make a separate in-depth post. Share the photography love by sharing this post. :)