Tuesday, September 9, 2014

What are Apple iPhone 6 "Focus Pixels"?

One of the new features of the iPhone 6 and 6 Plus is that it has "Focus Pixels" which supposedly help the iPhone "focus faster and better."  Apple has not yet been clear on what exactly they mean by "Focus Pixels."  However, in their keynote, they mentioned that it's a technology that's found in DSLRs, and they also showed a demonstration of the iPhone's continuous autofocus.  Both of those point to Focus Pixels being either phase detection or a hybrid autofocus that uses both phase detection and contrast detection.  What does this mean and will it really do what Apple claims, or is it just marketing exaggeration?

Traditionally, point and shoot cameras as well as cameras in phones use contrast detection to autofocus.  That means the camera will analyze the contrast in the photo and adjust the lens.  At the point when contrast is maximized, the shot is deemed to be in focus.  The benefit of contrast detection is that it is accurate, and it is immune from focusing errors (front or back focus).  The disadvantage is that it is generally much slower than phase detection, because the camera doesn't know whether an out-of-focus subject is near or far, therefore it uses trial and error by going back and forth.
DSLRs use phase detection which means that the camera uses two sensors to evaluate the target from two points of view.  By comparing the two images, the camera can determine whether the subject is in front or behind the current focus point, and the degree to which the image is out of focus, and the camera can adjust accordingly.  There is no trial and error as in the case of contrast detection.  The advantage of phase detection is speed, especially for continuous autofocus (as when shooting a continuous burst, or when shooting a video).
Hybrid autofocus is a relatively recent innovation that is used in cameras such as Sony's a6000 and a5100.  With hybrid autofocus, the camera uses phase detection sensors to inform the camera about the degree to which the subject is in front or behind the current focus point.  That information reduces the amount of trial and error needed for contrast detection to achieve focus.  Indeed, the a6000 is among the fastest-focusing cameras.  Here is an explanation of Sony's implementation of its hybrid AF: http://petapixel.com/2014/03/08/works-sonys-super-fast-hybrid-af-explained/

Apple hasn't clearly explained what Focus Pixels are, but based on their description it seems that the iPhone 6 and 6+ use either phase detection, or more likely, hybrid autofocus (with both phase detection and contrast detection).  If so, here are the benefits we should expect from the iPhone 6 and 6+:
1. Faster focusing, especially for moving subjects.  Great for taking candid shots of kids and babies.
2. The ability to keep a moving subject in focus when shooting a burst of several frames.
3. The ability to keep a moving subject in focus while shooting video.
4. Might have better autofocus ability in low light, compared to the 5S and older iPhones.
Again, Apple hasn't confirmed that this is in fact what "Focus Pixels" are, but it seems from the information they provided that this is what it is about.  (And if Focus Pixels aren't phase detection sensors, then I'm curious what technology Apple will be using to improve autofocus speed.)

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