Phase Detection Vs Contrast Detection Autofocus

Introduction

If you’re looking to understand the difference between phase detection vs contrast detection, then this guide is exactly what you need. 

You’ll leave knowing precisely what phase detection autofocus and contrast detection autofocus actually are–and how to decide on the best option for your needs.

Let’s get started.

Phase Detection vs Contrast Detection Autofocus: What’s the Difference?

Phase detection autofocus and contrast detection autofocus are two popular types of AF systems. 

Most of the cameras on the market today use one or the other–but what’s actually the difference? And how do these difference systems affect your photography?

Let’s take a closer look at each option in turn:

Phase Detection Autofocus

All digital cameras have a sensor of some sort, which takes in light and converts that light to a usable image.

However, in most cameras, a phase detection autofocus uses a second sensor–a dedicated AF sensor–which also captures light.

But instead of turning the light into an image, phase detection systems analyze the light to determine whether certain areas are in focus or out of focus.

The technical details are a bit complex, but phase detection systems essentially compare light coming in from two different locations on the AF sensor. If the light rays converge completely, the image is in focus; if the light rays fail to converge, the lens needs to adjust focus in order to produce a sharp shot.

Based on the difference between the light rays, the AF sensor can quickly predict how the lens must be adjusted to obtain focus, and it communicates this information; the lens then shifts, giving you an in-focus image.

Now, in order for light to be sent to a second sensor (the AF sensor), phase detection AF systems often uses a mirror that reflects light away from the imaging sensor.

For this reason, DSLRs all use phase detection autofocus when the mirror is down. 

When the mirror is up–that is, when a DSLR is using Live View–the camera generally switches to contrast detection autofocus.

However, it’s important to recognize that there are alternative methods of creating phase detection autofocus systems. 

While DSLRs use at least one dedicated AF sensor, some camera models–chiefly mirrorless, but also a few DSLRs in Live View–use a second type of phase detection system:

On-sensor phase detection.

Here, the camera ensures that the phase detection process can be done on the imagining sensor, either by adding phase detection sensors around the imaging pixels, or by using pixels engineered to work as both imaging pixels and autofocus sensors. 

Contrast Detection Autofocus

Contrast detection autofocus, unlike the standard form of phase detection autofocus, coopts the image sensor.

And it simply analyzes the data that’s read by the sensor.

Then it moves the lens’s point of focus back and forth until it achieves maximal contrast.

(Hence the name: Contrast detection AF.)

So, with contrast detection autofocus, there are no separate images, there is nothing to compare or predict. 

Instead, data is continuously read by the sensor while the lens changes the point of focus. Once things look contrasty (that is, in focus), the camera stops focusing.

For a long time, contrast detection autofocus was the only option for DSLRs using Live View; after all, the mirror would be flipped up, which meant that phase detection AF was useless. 

The early mirrorless cameras also used contrast detection pretty consistently.

But, due to the serious drawbacks of contrast detection AF, engineers worked hard to develop the alternative discussed above:

On-sensor phase detection AF.

Because the latter system could work without a mirror, it was possible to apply it to DSLRs in Live View, as well as mirrorless systems.

Now I’ll cover the pros and cons of phase detection vs contrast detection, and why phase detection has been rapidly replacing contrast detection in every area of the market.

Phase Detection Autofocus vs Contrast Detection Autofocus

Now that you understand the basics of phase detection and contrast detection autofocus, let’s take a look at the strengths and weaknesses of these two AF systems.

Phase Detection Autofocus: Benefits

As you may recall, phase detection autofocus involves prediction. 

When the light rays don’t converge, phase detection systems are able to determine the precise changes that need to be made so that the rays do converge. 

This is done in fractions of a second, which means that phase detection autofocus is extremely fast. 

This is the primary reason why phase detection autofocus is so respected by camera manufacturers; it acquires focus quickly.

And when you’re photographing moving subjects, such as birds in flight, fast autofocus is a necessity.

Phase Detection Autofocus: Drawbacks

Dedicated sensor phase detection systems had one big problem:

When the dedicated AF sensor was out of alignment with the dedicated imaging sensor, you could end up with consistent misfocusing. 

Obviously, repeatedly focusing in front of the subject and behind the subject is not desirable. But on-sensor phase detection systems have done away with this problem completely; now that there is no separate AF sensor, there is nothing to be misaligned, which means that focusing is both fast and accurate.

Phase detection systems also tended to be pretty limited in terms of the number of focus points they could use (where more focus points meant better tracking and greater precision).

But with the advent of on-sensor phase detection systems, the number of AF points has skyrocketed.

Contrast Detection Autofocus: Benefits

The primary benefit of contrast detection autofocus is its accuracy. 

Because contrast detection involves analyzing data from the imaging sensor, it’s very rarely inaccurate (so there are no focusing errors caused by misalignments).

Contrast detection autofocus also allows for plenty of AF points. This was a benefit over the old phase detection systems, but recent innovations have resulted in the expansion of phase detection point counts (which can now go head-to-head with contrast detection point counts).

Contrast Detection Autofocus: Drawbacks

Contrast detection autofocus is slow.

This has always been the case, which meant that mirrorless cameras, for a long time, couldn’t keep up with their DSLR counterparts. 

(Once on-sensor phase detection autofocus was included in mirrorless cameras, however, this focusing discrepancy disappeared.)

Contrast detection AF just isn’t good for tracking subjects around the frame, nor is it good at quickly acquiring focus with moving subjects. 

Should You Use Phase Detection Autofocus or Contrast Detection Autofocus?

On-sensor phase detection autofocus offers the best of both worlds:

Fast focusing that’s also accurate.

That said, certain photographers aren’t especially bothered by contrast detection autofocus. For instance, if you’re a still life photographer, fast autofocusing isn’t important, which means that contrast detection AF will do just fine; the same is true for landscape photographers (who often focus manually, anyway).

Also note that, when comparing on-sensor phase detection AF versus dedicated sensor phase detection AF, on-sensor tends to perform better, thanks to a larger number of AF points.

However, if you’re purchasing a DSLR and you rarely use Live View, making sure that your camera can perform on-sensor phase detection autofocus should not be a priority, because you’ll very rarely need it.

Phase Detection Vs Contrast Detection: Conclusion

Phase detection autofocus and contrast detection autofocus can seem like difficult topics to master. 

But, as you now know, they’re both easy to understand.

So the next time you see a camera manufacturer talking about phase detection autofocus or contrast detection autofocus, you’ll be well equipped to understand what’s at stake.