What are the main differences in field use between phase detect AF and contrast based AF?
I understand the technical differences, but I am not sure what the implications from a photographer standpoint are. Is there a noticeable speed difference? Does one drain the battery faster?
To put it into manual focus terms, contrast-detect autofocus is like trying to focus an image on a plain ground-glass screen, while phase-detect is like using a split prism focus aid or a rangefinder. In the one scheme, you are looking for a local maximum on a gentle curve, while in the other, you're just looking for things to line up. It's a lot easier to decide when things are lined up than when things are maximally contrasty.
Now, electronics can make the absolute maximum contrast determination faster than we can, since they can be sensitive enough to go back the moment the contrast curve begins to fall, but that's still not quite as easy as comparing two images to see if they line up. And, since a phase-detect system knows which image is which, it should always know in which direction it needs to focus to make the correction. It's always a guess with contrast detection -- you focus in one direction, and if it gets worse instead of better, you reverse direction.
That said, some of the cameras they're making these days have incredible electronics, so there might not be an appreciable difference to the average photographer. Either way, driving the lens (and the backlight for the monitor, if you're using one) is going to be the main source of power drain. Yes, reading the entire imaging sensor is going to "cost" more than reading a few (or a single) specialized autofocus sensors, but is it a difference you're going to notice? Probably not. The real drain with contrast-detect AF is usually that you can't use an optical viewfinder, so the monitor (or EVF) is active the whole time, not that the focus system itself is running.
The fact that Phase-Detection stops at the focus-distance rather than exceeding and then coming back has a big impact (as Stan mentioned in his answer) on video since this motion is visible in the recording.
Another impact is that Phase-Detection requires dedicated sensors in fixed positions which is why the number of AF points is relatively small. Contrast-detection can be done anywhere and there are commonly cameras which can focus at 99 positions or more in the frame.
The most important advantage of Contrast-Detection is that it cannot suffer from front or back focus issues. When the sensor detects focus, things are in focus. Actually, on my blog I suggest how cameras which do both Phase-Detect and Contrast-Detect could even self-calibrate. We only have to wait and see who will implement it first.
Just be be clear. Any focus system can miss and will occasionally focus at the wrong distance. Conditions vary by camera and subject. Front or back-focus is different in that the camera always focuses too close or too far. When that happens, the camera must be calibrated to focus correctly with lenses in question. This happens with Phase-Detect AF the confirmation of focus is computed by the Phase-Detect sensor and not the camera's imaging sensor.
Contrast detect has much more information to work with which is why they do Face-Detection and other sophisticated tricks with it. Phase-Detection is pretty much all about distance.
Contrast-Detect uses more power and drains the battery faster because you have to keep the sensor powered and constantly read it. Modern cameras do that at 240Hz to get fast AF-speeds which is very power intensive.
The speed gap is getting smaller but Phase-Detect is considered faster still, although I suspect this is not the case under all circumstances.
The question insists on a practical, not a technical answer.
Phase detection is better at focusing on moving subjects, or subjects where contrast is not available because of low-light, for example, when shooting sports. Contrast-detection can be faster than phase detection where subjects are not moving or where contrast is plentiful, for example, in a studio lit for a portrait. This becomes a practical issue nowadays because the phase detection cameras and lenses are much larger.
Only the larger DSLR (Digital Single-lens Reflex) cameras with a pentaprism and mirror or DSLTs (T is for the translucent mirror) cameras have phase detection. One might object that the smaller SONY NEX5R and NEX6 are really tiny DSLTs and have phase detection, but there are so few lenses that can take advantage of this nascent technology, that one can still safely generalize that size matters. Awkward threatening huge lenses of DSLRS will continue to dominate the sidelines even if the form-fitting less-threatening tiny lenses of mirrorless cameras are favored by the old range-finder street-shooters, world travelers and outdoorsy types, clubbers who have realized the limitation of their smart phones, semi-professional soccer mom and b-camera camera assistants at weddings (heck, practically everyone else.)
Most people are NOT shooting sports or wild game professionally, so in 95% of all shooting scenarios the new smaller mirrorless contrast-detection cameras are enough. For about a year now these smaller camera have even had weather-sealed camera bodies, extra battery grips for long days of shooting, competitive image stabilization, and reasonably high frame-rates per second. The micro four-thirds ecosystem even includes fast bright lenses, with beautiful bokeh, to rival their dinosaur ancestors. And because, when shooting video, DSLRS also focus with contrast-detection, these smaller cameras are often better, simply because the interface is easier to use, for example, because of touch-screen focusing or because the screen swings out so you can hold the camera wherever you want.
Would you prefer a cheaper, lighter, smaller and subtle camera that can shoot most everything, or an expensive, heavy, large and obvious one that can shoot anything that moves? This is the practical choice nowadays between these two focus systems.
Phase detection AF is better suited to:
Contrast detection AF is better suited to:
Lenses with heavy lens groups don't work well with contrast AF, because they can't be moved fast enough with the many tiny steps required for contrast AF. Thus, such lenses are usually not offered for systems with contrast AF. They can be adapted (e.g. FT lenses for mFT), but then they do not work well.
So the practical limitation is that such lenses will not be available for contrast AF systems unless there is an advance in AF systems (e.g. hybrid AF). There will no be bright tele (especially zoom) lenses for those systems. So if you intent to do animal or sport photography, currently the contrast AF systems are not the best choice.
Another practical difference: because of the size/weight limits the contrast AF system imposes on the lens groups which can be still moved by quick micro steps, optical correction is often replaced by electronic correction.
This is a limitation if you prefer to shoot RAW and your RAW converter does not support electronic correction methods of the camera.
