The key is the position of the mirror. Since contrast measurement is
done with the sensor the mirror must be up and any mechanical shutter
must be open... live view. Phase detection is done with a beam splitter
that passes light through the main mirror and then bounces it off a
small secondary underneath to a sensor in the floor of the mirror box.
Very similar to the way OM's with TTL-OTF measure exposure when the
mirror is down.
Mirror down, shutter closed = normal shooting and phase detect only.
Mirror up, shutter open = live view and contrast measurement only.
As EVF's and contrast measurement improve I think the SLR and its mirror
will go the way of the dodo bird. The future will be in designs like
micro 4/3. Whether micro 4/3 itself will be good enough I don't know.
But I believe it's coming eventually. Too many cost and functional
benefits for it not to.
Chuck Norcutt
Andrew Fildes wrote:
> Thanks for that - the reason i asked is that on the press releases an
> specs., previous models have stated explicitly that Contrast
> Detection AF is available in LiveView - the E-450 does not, just the
> Contrast Detection AF Mode is available. It struck me that it may be
> possible to enable it in regular shooting where appropriate, but I
> can't find confirmation.
> Andrew Fildes
> afildes@xxxxxxxxxxxxx
>
>
>
> On 01/04/2009, at 10:54 AM, Chuck Norcutt wrote:
>
>> 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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>
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