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timujin said:
Because of the mirror. I want to guess that the AF unit in the Canon and Nikon require the image from the mirror (hence the blackout on LCD during focus) and moving the mirror back in and out to focus and show you the image on the LCD takes time. The NEX AF is designed just to use the information from the sensor and since there isn't a mirror in place you don't have to wait.
This is of course glossing over the specific implementation of contrast AF which might contribute as well.
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using the 'quick mode' focussing when on live view flips the mirror down, uses the faster normal af, then flips the mirror back up.
using the 'live mode' focussing is slow, because it reads all its information from the sensor and uses some sort of contrasty-algorithm.
but yeah, every other camera, P&S and EVIL like the NEX, they all read the contrast information from the sensor too, and they're faster than the 'live mode' in the dslr.
so just based on that, i'd say it was the algorithm. but canon and nikon also produce P&S cameras, which can only read information from the sensor. Does anyone have a Powershot handy? Has anyone compared the speed of focussing a powershot/ixus/whatever compared to a dslr on live-mode?
and there's also other factors, the size of the sensor, pixel density, and don't forget the size of the lenses, the travel distance from full macro to infinity focus.
(there's a thing that bugs me, if i use live mode, most of the time when it tries to focus it pulls the focus all the way from one extreme to the other to try get focus, the algorithm obviously doesn't realise it's going 'the wrong way' until it hits the end. obviously the algorithm could be improved in this respect).
using the 'live mode' focussing is slow, because it reads all its information from the sensor and uses some sort of contrasty-algorithm.
but yeah, every other camera, P&S and EVIL like the NEX, they all read the contrast information from the sensor too, and they're faster than the 'live mode' in the dslr.
so just based on that, i'd say it was the algorithm. but canon and nikon also produce P&S cameras, which can only read information from the sensor. Does anyone have a Powershot handy? Has anyone compared the speed of focussing a powershot/ixus/whatever compared to a dslr on live-mode?
and there's also other factors, the size of the sensor, pixel density, and don't forget the size of the lenses, the travel distance from full macro to infinity focus.
(there's a thing that bugs me, if i use live mode, most of the time when it tries to focus it pulls the focus all the way from one extreme to the other to try get focus, the algorithm obviously doesn't realise it's going 'the wrong way' until it hits the end. obviously the algorithm could be improved in this respect).
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timujin said:
It depends on the Camera, the NEX 3 uses contrast detect focus like point and shoot cameras. Its very slow compared to phase detect AF, but still usably fast.
If you use contrast detect in the live view mode of your Canon DSLR, its very slow, its old technology but getting faster in the newer models. Some DSLR's will use the much faster phase detect while in live view by flipping up the mirror focusing and dropping the mirror again. Unfortunately its only good for one shot AF and not for video or continuous AF.
To get continuous AF with a DSLR, domething like the Sony A55 with a pellicle screen which lets the camera do continuous phase detect AF.
We are all wondering if Canon is going to come up with new focus technology for DSLR's that will give AF while capturing video.
Meanwhile, if you want fast and continuous AF for your DSLR, there isn't any. The closest thing is the Sony A55 and its limititation of just a vew minutes before it overheats.
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Mt Spokane Photography said:
but the reason it overheats is because it's got sensor shift IS, no? ie, the sensor moves around to correct shake, so it's not bolted to anything, so there's no heat transfer to any other part of the body. if canon or nikon (who do IS/VR in the lens) bring in a pellicle mirror, then you get the best of both worlds, the sensor would be bolted to the body and wouldn't overheat as quick, and you'd get fast AF.
(and that's my best prediction of new features if any new 'video' bodies come out)
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dr croubie said:
That IS unit generates even more heat adding to that generated by the sensor, Sony's recommendation is to turn the IS off for video and double the time to overheat. But right now, that is all there is.
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