Contrast measurement is implemented entirely within the sensor and is
used on point and shoots and focusing in live view with the mirror up
(and shutter open, of course) Phase detection is used in SLRs and DSLRs
when not in live view. Phase detection uses a beam splitter to form and
compare two images. Phase detection can tell if an out-of-focus image
is either back or front focused and start driving the lens immediately
in the right direction to achieve focus. It's faster than conrast
measurement which may have to hunt a bit to get started. But, while
slower, contrast measurement is potentially more accurate.
"Shutter open" above assumes there is an actual mechanical shutter that
needs to be opened. That's not necessarily true. There may only be an
electronic shutter which, again, is implemented within the electronics
of the sensor. Or there may be both mechanical and electronic shutters
as in my Mynolta A1.
See the nitty-gritty in this Wiki article
<http://en.wikipedia.org/wiki/Autofocus>
Chuck Norcutt
Andrew Fildes wrote:
> Indeed. But they are sticking with 'small'.
> It appears to me that Contrast Detection AF has been used only in
> Live View so far but is now an available mode for normal shooting.
> Have I got that right? Anyone know the advantage?
> Andrew Fildes
> afildes@xxxxxxxxxxxxx
>
>
>
> On 01/04/2009, at 8:46 AM, Donald wrote:
>
>>> Is the 420 Contrast Detection AF - I'll have to go check. That's
>>> developed for the Micro 4/3rds system.
>>> I did expect them to get IS into it though - using the reduced size
>>> IS module and circuit board from the 620.
>>> Andrew Fildes
>>> afildes@xxxxxxxxxxxxx
>>>
>> Yes, that would have been nice. At the entry level it kinda makes
>> sense, no?
>
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