09-24-2010, 12:12 AM
[quote name='Brightcolours' timestamp='1285259412' post='3178']
All the sensors now can deliver 30fps... so they should be up to the task. It is not the USM that makes the fast contrast AF possible, but the implementation of motor electronics commands. The lens motor type is not important, really. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/blink.gif' class='bbc_emoticon' alt='
' />
So... three things possible:
New mount with contrast detect AF only lenses, old lenses (with mount adapter) offer slower AF.
New lens "models" which offer both the PD-AF command set and the contrast AF command set, old lenses offer slower AF with contrast AF.
PD-AF done on sensor level, no need for contrast AF and old lenses (with mount adapter) offer fast PD-AF still.
[/quote]
I'm in the camp that at least with USM lenses, it shouldn't be the limitation of the lens.
Two things to remember: Compact cameras have smaller sensors, larger DOF, they do not need to focus as precisely as APS-C or FF, which need to be resolved much finer.
Second possibility: CCD versus CMOS, and their implementations.
With compact cameras they may actually only read subsets of the whole sensor for analysis (selected regions, or binned images - since DOF is sufficient)
With CMOS sensors, it seems until recently one had to read each whole image into the processor and analyse of course a whole series - processor and memory demanding. Now, there is the movie mode, reading only samples of the chip - 30fps. Alas, it's resolution limited, so probably not 100% suitable for fast lenses with tiny DOF.
Another issue is finding the initial right focus range. With compacts, due to high DOF, contrast detect has something to work with from the start usually. With many dSLR lenses (e.g. tele), a contrast detect may just see mush at the beginning and going through the whole focus range is much longer, due to the finer resolution needed because of the smaller DOF the lenses have to handle- if the AF knew that it could jump in larger steps, it would go faster of course.
In any case, it seems the difference is not that much nowadays. THe EF-S 60mm USM macro seems to focus quite fast on a 550D - not really that much worse than P&S, once it's in a range where the contrast detect has something to work with.
I think the trick will be to have a little lever with a phase detect system pop-in from the side to give initial fast focus.
All the sensors now can deliver 30fps... so they should be up to the task. It is not the USM that makes the fast contrast AF possible, but the implementation of motor electronics commands. The lens motor type is not important, really. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/blink.gif' class='bbc_emoticon' alt='
' /> So... three things possible:
New mount with contrast detect AF only lenses, old lenses (with mount adapter) offer slower AF.
New lens "models" which offer both the PD-AF command set and the contrast AF command set, old lenses offer slower AF with contrast AF.
PD-AF done on sensor level, no need for contrast AF and old lenses (with mount adapter) offer fast PD-AF still.
[/quote]
I'm in the camp that at least with USM lenses, it shouldn't be the limitation of the lens.
Two things to remember: Compact cameras have smaller sensors, larger DOF, they do not need to focus as precisely as APS-C or FF, which need to be resolved much finer.
Second possibility: CCD versus CMOS, and their implementations.
With compact cameras they may actually only read subsets of the whole sensor for analysis (selected regions, or binned images - since DOF is sufficient)
With CMOS sensors, it seems until recently one had to read each whole image into the processor and analyse of course a whole series - processor and memory demanding. Now, there is the movie mode, reading only samples of the chip - 30fps. Alas, it's resolution limited, so probably not 100% suitable for fast lenses with tiny DOF.
Another issue is finding the initial right focus range. With compacts, due to high DOF, contrast detect has something to work with from the start usually. With many dSLR lenses (e.g. tele), a contrast detect may just see mush at the beginning and going through the whole focus range is much longer, due to the finer resolution needed because of the smaller DOF the lenses have to handle- if the AF knew that it could jump in larger steps, it would go faster of course.
In any case, it seems the difference is not that much nowadays. THe EF-S 60mm USM macro seems to focus quite fast on a 550D - not really that much worse than P&S, once it's in a range where the contrast detect has something to work with.
I think the trick will be to have a little lever with a phase detect system pop-in from the side to give initial fast focus.