Focus Modes: Quick Mode vs Live Mode

Since Live Mode is very slow and, I've read, not particularly accurate, why is it an option within the camera? What is it's benefit? Why isn't the method of focusing that is used in Quick Mode used rather than the "contrast" method (if I've got that right)?
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Mark.

Is it because the image sensor (as opposed to the dedicated AF sensor) isn't capable of the more advanced Quick Mode method?

So the only benefit to using Live Mode is that the mirror doesn't flip down as much because it doesn't flip down to obtain focus (making it quieter I guess)?
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Mark.

what if someone needed to shoot above a crowd? or around a corner? or at ground level? or from at hip/chest level without stooping over? how would that photographer frame their shot?

an lcd screen live view would help to frame the photo instead of guessing before taking the picture when a photographer can't use the viewfinder for any reason. a move-able lcd screen (like on the t3i/600d) could help even more.

the contrast method can be more accurate than the phase detect method. the phase detect method (mirror/viewfinder) is subject to front- rear-focusing errors. the contrast method will never have front- rear-focus problems, but it is very slow/impossible for moving subjects or low light photographs where contrast is low. olympus/sony/panasonic are making contrast detection faster and faster with each new camera, but it is still not as fast as phase detection.

quick mode (as i understand it) opens/closes the shutter more than just using the viewfinder for focusing or default live view focusing. no camera shutter will work forever (though they can be used for 1000s or 10s of 1000s of shutter activations before they break). i would not consider that a significant reason to stop using live view and/or quick mode

markannab said:

Since Live Mode is very slow and, I've read, not particularly accurate, why is it an option within the camera? What is it's benefit? Why isn't the method of focusing that is used in Quick Mode used rather than the "contrast" method (if I've got that right)?

Click to expand...

Thanks for your reply. Yes, I understand the benefits of the LCD (it's the 600D I own). I was just the Live MODE over Quick Mode that was puzzling.
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Mark.

Quick Mode requires the mirror to be in place to direct light to the sensor. When in Live View mode the mirror is flipped out of te way. LiveView really only of value, to me difficult angle macro and video, except for video I would tend to use my LX5 for that . Stuart

This link may be of help...

http://en.wikipedia.org/wiki/Autofocus

Quick Mode is Canon's name for Phase Detection. Live View is Cannon's name for Contrast Measurement.

As the article mentions; Contrast Measurement is a somewhat more accurate, but slower, auto-focus method. Phase Detection is a far faster method but, by and large, not as accurate. Phase Detection is what we use when looking through the viewfinder. Contrast Measurement is what we use when looking at the LCD screen.

When in Live View (Contrast Measurement) the mirror is locked in the up position and auto-focus is achieved using the camera's main sensor. When using Quick Mode auto-focus is achieved using a special AF sensor and the mirror is in the down position.

Best that I can do.

Contrast detect (live mode) actually has potential to be more accurate, as data from main sensor itself, not a separate device, is used for focusing. When you have your camera on a tripod, and your subject is not going anywhere, live mode can often yield better results. It is not all that slow out in good light. In fact, if you are already focused sufficiently close, it is a minor adjustment which is usually very fast. Especially if you got a USM lens.

Then, you can focus anywhere on the sensor, where in quick mode, only the 9 focus points of the phase AF are available.

markannab said:

Is it because the image sensor (as opposed to the dedicated AF sensor) isn't capable of the more advanced Quick Mode method?

Click to expand...

That is correct.

markannab said:

So the only benefit to using Live Mode is that the mirror doesn't flip down as much because it doesn't flip down to obtain focus (making it quieter I guess)?

Click to expand...

That and Live Mode is generally more accurate, at least for stationary subjects. For moving subjects, Live Mode is just about useless.

markannab said:

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Mark.

Click to expand...

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scorrpio said:

Contrast detect (live mode) actually has potential to be more accurate, as data from main sensor itself, not a separate device, is used for focusing. When you have your camera on a tripod, and your subject is not going anywhere, live mode can often yield better results. It is not all that slow out in good light. In fact, if you are already focused sufficiently close, it is a minor adjustment which is usually very fast. Especially if you got a USM lens.

Then, you can focus anywhere on the sensor, where in quick mode, only the 9 focus points of the phase AF are available.

Click to expand...

And you can zoom in x10 for extra accuracy (tripod recommended)

DavePlugh wrote:

Phase Detection is what we use when looking through the viewfinder. Contrast Measurement is what we use when looking at the LCD screen.

Very good explanation (and thanks to all others for your replies). Just one clarification to the point above. While generally true, it seems possible to use the LCD with phase detection. That is, having the camera set to Quick Mode, then clicking to use LCD. In that scenario, the mirror flips down for a split second to obtain focus (I assume, from the dedicated AF sensor). Hence, quick focus. It certainly doesn't search around for focus like it does when using contrast measurement. Is that correct?

markannab said:

Very good explanation (and thanks to all others for your replies). Just one clarification to the point above. While generally true, it seems possible to use the LCD with phase detection. That is, having the camera set to Quick Mode, then clicking to use LCD. In that scenario, the mirror flips down for a split second to obtain focus (I assume, from the dedicated AF sensor). Hence, quick focus. It certainly doesn't search around for focus like it does when using contrast measurement. Is that correct?

Click to expand...

During the very short period of time that the mirror is flipped down the camera is using Phase Detection for auto-focusing. But, at the same time the camera is auto-focusing the LCD goes blank. So, technically, when using Quick mode auto-focusing you aren't 'using' the LCD at all. But, you are correct that in Quick mode it doesn't search around for focus.

That's why they offer you the choice of focusing methods while Live shooting.

markannab said:

Since Live Mode is very slow and, I've read, not particularly accurate, why is it an option within the camera? What is it's benefit?

Click to expand...

As many have said it's usually more accurate. It's also slightly more consistent, and you can do it anywhere you want over most of the frame. In some cases (e.g. when using extension tubes), it can achieve AF where Quick mode will fail.

If Canon wanted to improve it, they could let you change the size of the AF window so that you can do a kind of "spot focus" with it.

markannab said:

Why isn't the method of focusing that is used in Quick Mode used rather than the "contrast" method (if I've got that right)?

Click to expand...

The trick is to use the two methods appropriately. If you're on a tripod with a static subject, the only thing getting in the way of you having the advantages of contrast detection is your own impatience.

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