09-24-2010, 12:31 AM
[quote name='photonius' timestamp='1285287166' post='3186']
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.
[/quote]
No idea why you want to be in the camp of that it would matter, if a lens has an USM motor or not, because that just does not make sense at all. USM is nothing special, it is just a motor just like the other motors are. They get talked to by the camera in a similar fashion too. It is like saying a diesel engine and a wankel motor somehow need a different acceleration pedal. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/blink.gif' class='bbc_emoticon' alt='
' />
About CCD or CMOS: Anyone who ever used a compact camera with CCD knows there was LIVE VIEW. So no need to speculate about reading subsets, the whole image got read and converted for live view, and the area used for AF evaluation... got evaluated for contrast.
Does not matter whether your compact digital has CMOS or CCD.
About "finding focus range"... that is "phase detect AF talk".
Contrast detect AF does never have a "focus range" or an idea about focus, all it does is see if contrast increases, until it decreases again (then it knows focus must have been reached).
If you think the 550D focusses as fast as a compact digital... you must have not used newer compact digitals <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt='' />.
Of course it is true that Canon has improved the speed of the algorithm a bit, compared to the earlier 450D for instance. But it still is... slow. Not comparable to what for instance Panasonic achieves with micro 4/3rds and micro 4/3rds lenses. And the 550D is not only a bit faster with an EF-S 60mm f2.8 macro, but also with a 35mm f2 for instance, without USM.
But that is it... just "a bit" faster. Nothing like contrast detect AF with dedicated lenses, or PD-AF.
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.
[/quote]
No idea why you want to be in the camp of that it would matter, if a lens has an USM motor or not, because that just does not make sense at all. USM is nothing special, it is just a motor just like the other motors are. They get talked to by the camera in a similar fashion too. It is like saying a diesel engine and a wankel motor somehow need a different acceleration pedal. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/blink.gif' class='bbc_emoticon' alt='
' /> About CCD or CMOS: Anyone who ever used a compact camera with CCD knows there was LIVE VIEW. So no need to speculate about reading subsets, the whole image got read and converted for live view, and the area used for AF evaluation... got evaluated for contrast.
Does not matter whether your compact digital has CMOS or CCD.
About "finding focus range"... that is "phase detect AF talk".
Contrast detect AF does never have a "focus range" or an idea about focus, all it does is see if contrast increases, until it decreases again (then it knows focus must have been reached).
If you think the 550D focusses as fast as a compact digital... you must have not used newer compact digitals <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt='
' />.Of course it is true that Canon has improved the speed of the algorithm a bit, compared to the earlier 450D for instance. But it still is... slow. Not comparable to what for instance Panasonic achieves with micro 4/3rds and micro 4/3rds lenses. And the 550D is not only a bit faster with an EF-S 60mm f2.8 macro, but also with a 35mm f2 for instance, without USM.
But that is it... just "a bit" faster. Nothing like contrast detect AF with dedicated lenses, or PD-AF.