Contrast Detection autofocus vs. Phase Detection autofocus

You’ve seen the specs – here for the Sony a6500:

“169 contrast detection points and 425 phase detection points!”  Well that’s cool, the numbers seem high, but what the hell is contrast detection and phase detection autofocus?  Why should I be impressed?

Contrast Detection autofocus explained

Contrast detection is the simplest, most accurate method of autofocus technology.  Thus it’s the cheapest autofocus technology.  The name should say it all: the camera looks at the contrast between edges and moves the focus motor until the contrast is the sharpest.

This is actually how our eyes work when we use unaided manual focus – we look at edges and move the focus ring until the edge contrast is the hardest.  Or what we call “sharp” in simple terms.  We go back and forth with large adjustments first, then smaller refinements until we get there.

Let’s look at these simple black cubes as an example of the steps involved in contrast detection as it zeroes in on the proper focus:

Contrast detection

Contrast detection autofocus – despite being the easiest, cheapest, and most accurate method of focusing – is also the slowest.  If you’ve ever seen the autofocus motor “hunt” back and forth, kinda like you would do when manually focusing, this is contrast detection at work.  It’s comparing focus distances to find the maximum contrast point.

Phase Detection autofocus explained

Okay ready to get really techie?  No?  We’ll try to keep this simple.

Imagine an image hitting a prism.  The prism then splits that image into two.  If the image is in focus, the split images will line up.  If not in focus, the images won’t line up.  This is actually the “split prism” manual focus aide you’d find in the middle of the viewfinder on old SLR film cameras.  You’d move the focus ring until the image lines up and voila, in focus.  Very fast.  Much faster than trying to detect where the contrast edge is the hardest.

When a digital camera says it has “425 phase detection points”, that means there are 425 places on the sensor where it can compare that split image.  Because the sensor knows which split image is which, it knows precisely in which direction and how much to move the focus motor to bring the split image together.

Let’s go back to our black cube to see what’s involved in phase detection:

Phase detection

DSLRs use an actual prism to split the image onto the focus sensor while mirrorless cameras do this directly on the sensor.  The technology is more expensive and in a DSLR adds a little more weight.

Which is better: Contrast or Phase Detection autofocus?

The answer is, like everything else in photography, “it depends.”

Still subjects and high contrast

Contrast detection will give you the most accurate focus when shooting single-shot autofocus with a still subject.  There are fewer chances of the camera focusing in front of or behind the subject as sometimes happens with phase detection.  This is called front or back focus, and you should still be aware of it with contrast detection.

But remember that the lens motor is moving more with contrast detection.  This means that it’ll use more juice.  The motor will also be moving a little slower with larger lenses with multiple glass elements.

Moving subjects

If your subject is moving phase detection will give you the fastest and most accurate autofocus.  You still have a risk of back or front focus, but with image tracking technology and multiple phase detection points, this is less of a factor.  You’ll be shooting in continuous or servo autofocus modes.

This is what happens with front focus for example.  The phase detect points picked up an object closer to where I wanted to focus and focused on that instead.  The green squares are what you would see on a Sony alpha.  Be aware of these; if you see them in the wrong spot, you should change to a different focus area.

DetectPoints

Contrast detection is not good with moving objects because of the time it takes to find the maximum contrast.  By the time that maximum contrast point is detected, the subject has already moved to a different distance, and the camera must find that position again.  Phase detection will immediately snap the motor to the proper focus point for continuous shooting.

Low light & low contrast

Just remember that both of these methods require light to focus.  If the image has no contrast or little light, the camera may not have enough data to use either focus method.  There are ways around this.

  • Some cameras, like my Sony alpha mirrorless cameras, will bump up the ISO when you hit the focus button.  This allows enough light in to focus, then the ISO will drop to what you have set.
  • Lights & autofocus assist beams.  Cameras & flash units will emit beams of light in an attempt to illuminate your (near) subject.  You can also use a high powered flashlight if your subject is further away.
  • Use back-button focus so that your camera won’t try to autofocus every time you hit the shutter button, especially with still scenes.  Point your focus point to an area of high contrast or higher luminance, hit the focus button, then recompose.  Now your focus is set & locked for that distance.

Hybrid autofocus explained

You’ll see some cameras advertise Hybrid Autofocus.  The Sony a6500 is boasting the fastest autofocus in the world right now at 0.05 seconds using hybrid autofocus.

Hybrid autofocus generally starts with the quick phase-detection method.  It then uses contrast detection to refine the edge, and because phase detection brought it to near that point, contrast detection doesn’t take nearly as long as it usually would.  So yeah, phase detection returns it to what our eyes would perceive as perfect focus, then takes it beyond that with contrast detection.

So now that you understand the differences between phase detection and contrast detection autofocus, I hope the understanding will improve your photography as well!  Any questions or comments?  Please leave them below!

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