Phase Detection vs Contrast Detection Autofocus (AF)

[tmyuen]

I don't understand the technologies and merits behind these two autofocus (AF) modes, but I noticed a few camera manufacturers (Canon, Olympus, ..) have added Contrast Detection AF in their new models of DSLR. Does this imply that Contrast Detection is a newer technology and superior to Phase Detection AF?

[Raycer]

To my understanding Phase detection is faster and more accurate but it requires the mirror down which means it will block the sensor.
Contrast detection is useful for live view. At this time it is slower but face detection can be programmed in to the computer.

[McQ]

Phase Detection Autofocus
This AF method is most prevalent in digital SLR cameras. It requires dedicated hardware (line and cross-type autofocus sensors) which can increase the overall cost of a camera. Phase detection AF still uses contrast to assess where to focus, but contrast just isn't used as iteratively or throughout the image...only at the autofocus sensors. A disadvantage with the phase detection method is that it requires pre-calibration, and can therefore always misfocus if incorrectly calibrated.

Examples of phase detection AF sensor arrays
left: higher-end digital SLR (Canon 1-series) , right: midrange digital SLR (Canon 30D/40D/etc)

Contrast Detection Autofocus
This AF method is most prevalent in compact cameras and newer SLR cameras with Live View. This is because these cameras can be made less expensive by using the sensor itself to perform autofocus, with the (relatively) cheaper processing power used to determine maximal contrast. A disadvantage to contrast AF is that it cannot gauge whether the camera lens is front or back focused-- just that it is out of focus. This is primarily why it has to be used iteratively; contrast measurements at at least two lens positions are needed to assess which direction the camera should be focusing (ie, in front or in back of where it was previously focusing). The contrast detection is generally more fail proof (if given lots of light and enough time to focus) since it not reliant on pre-calibration.

-------->
increasing contrast (better focus)

Also take a look at the tutorial on Camera Autofocus (AF) for more on this topic...

[tmyuen]

Thanks for the explanation. After reading the tutorial on Camera AF, I have a question in mind.

At the end of that article, a technique is described:

In the example to the left, the stairs are comprised primarily of horizontal lines. If one were to focus near the back of the foreground stairs (so as to maximize apparent depth of field using the hyperfocal distance), one might be able to avoid a failed autofocus by orienting their camera first in landscape mode during autofocus. Afterwards one could rotate the camera back to portrait orientation during the exposure, if so desired.

My question is: when I do AF in landscape mode (horizontal) first, then back to portrait mode (vertical), do I have to keep half-pressing the shutter between the horizontal-to-vertical camera rotation?

[McQ]

If you were taking the photo in one-shot AF mode (with autofocus remaining activated the entire time), then yes, you would need to keep the button halfway pressed. However, it would be much easier to switch the lens into manual focus once you achieve focus lock. This way you could compose as you wish without having to hold the AF button, and you could use the central AF point for exposure metering.

That being said, the "90 degree focus then recompose technique" is really a very very specific example of how certain types of subject matter can play tricks on your camera's AF. In practice, this would only be used in low light with a camera/lens combination that does not have a cross-type AF sensor...and even then, only a small fraction of subject matter would benefit from it. Frankly, even in those situations I would rather bring a tiny flashlight to place where I wanted to focus, autofocus on it, and then remove the flashlight and take the shot. Alternatively, one could bring a flash and use it for AF assist.

[eimclark]

Hi,
I'm struggling with the comment on the autofocus page under the AF sensor arrays images.

Cameras used for left and right examples are the Canon 1D MkIII and Canon 400D, respectively.
For these cameras autofocus is not possible for apertures smaller than f/8.0 and f/5.6.

I have an EOS 450D and autofocus works at any f-stop. Or am I right? The dot lights up red and the camera beeps telling me that its found the focus.

Its a great article, but I wonder if someone can just clear up my confusion with the highlighted comment.

Thanks,
Martin

[Raycer]

I believe the smallest aperture refers to the largest aperture available on your lens. for example, for 18-55mm 1:3.5-5.6, the largest aperature at 18mm is f/3.5.