From: David J Taylor on
"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
news:03612b6c$0$1272$c3e8da3(a)news.astraweb.com...
[]
> Oh no, the results _are_entirely_ explicable. The question that remains
> is why the AF system confirms focus asymmetrically, depending on which
> side you start from. Asymmetrical DOF is a very plausible factor, but
> ARFD (either the as-we-know-it version or the effective/virtual AF
> aperture version) is not, AFAICT.

That sounds like simple hysteresis - final position depends on starting
position.

David

From: Paul Furman on
Wilba wrote:
> Paul Furman wrote:
>> Wilba wrote:
>>> C = Camera,
>>> POF = Plane of Focus,
>>> DOF = Depth of Field,
>>> S = Subject.
>>>
>>> When the plane of focus is a long way (i.e. much greater than the DOF,
>>> >>DOF) in front of the subject -
>>>
>>> C POF<------ >>DOF ------>S
>>>
>>> I call that "near focus". You can think of it as initial gross
>>> front-focus.
>>>
>>> If I start like that, autofocus and beep focus both put the plane of
>>> focus coincident with the subject. Lovely.
>>>
>>> When the plane of focus is a long way behind the subject -
>>>
>>> C S<------ >>DOF ------>POF
>>>
>>> I call that "far focus". You can think of it as initial gross back-focus.
>>>
>>> If I start like that, autofocus and beep focus both put the plane of
>>> focus just outside the DOF (>DOF/2) on the _front_ side of the subject -
>>>
>>> C POF<-- >DOF/2 -->S
>>>
>>> Summary - initial gross front-focus results in optimal focus, and initial
>>> gross back-focus results in _front-focus_.
>>>
>>> This outcome -
>>>
>>> C S POF
>>>
>>> never occurs with my gear in my tests using autofocus or beep-focus.
>>>
>>> Let me know if that doesn't make sense. :- )
>> OK I finally read that carefully. It does not make a whole heck of a lot
>> of sense.
>>
>> Do you agree that this is the opposite of what you reported initially or
>> do I need to find and read that again?
>
> You can trust me that the same story has been repeatedly told with great
> care and consistency, or you can go back and check. Either way you'll end up
> at the same place. :- )
>
> Here's what I said on the 27th of December -
>
> "I found that my phase detect AF sensor has sidedness. If I start with the
> lens focussed closer than the subject, the results are uniformly excellent,
> whether autofocussing or manually focussing using the AF confirmation
> (as Doug described above)."
>
>
> I.e. the beep test.
>
> "If I start with the lens focussed behind the subject, and I manually focus
> using the AF confirmation [the beep test] focus is always off by the
> same tiny amount (one click towards infinity in the EOS Utility will bring
> it into optimal focus)."
>
> If you have a front-focus, which way do you have to turn the focus ring to
> correct it? Towards infinity.
>
> "With initial focus behind the subject and PD autofocus, about seven shots
> out of ten are out by the same one click as the manual focus [beep test],
> and the rest are optimal, like when starting from the nearside. I assume
> that the good ones come about from the lens overshooting and then the
> system corrects towards infinity (so it ultimately approaches focus from
> the nearside)."
>
> The key is accepting that there is a cross-over. I expect you pre-supposed
> that an initial gross back-focus must result in a final back-focus, but it
> doesn't, it results in a final front-focus. It's counter-intuitive, but it's
> the Goddam truth. :- )


I was concentrating on this diagram you posted which was the opposite:

Try viewing this text with a fixed width font (e.g. copy into Notepad) -

|< 430mm >| sensor to subject distance for optimal
focus
| |< 2.2mm >|< 2.2mm >|
theoretical DOF
|< 430mm >| nearside
focus
|< 433mm >| farside
focus
| |< 3.0mm >| the
"beep band"
| |<0.8mm>| outside
the DOF

--
Paul Furman
www.edgehill.net
www.baynatives.com

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From: Paul Furman on
David J Taylor wrote:
> "Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
> news:03612b6c$0$1272$c3e8da3(a)news.astraweb.com...
> []
>> Oh no, the results _are_entirely_ explicable. The question that
>> remains is why the AF system confirms focus asymmetrically, depending
>> on which side you start from. Asymmetrical DOF is a very plausible
>> factor, but ARFD (either the as-we-know-it version or the
>> effective/virtual AF aperture version) is not, AFAICT.
>
> That sounds like simple hysteresis - final position depends on starting
> position.

Memory? Does hysteresis band simply mean fuzz factor?


--
Paul Furman
www.edgehill.net
www.baynatives.com

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From: David J Taylor on
"Paul Furman" <paul-@-edgehill.net> wrote in message
news:hisucr$cui$1(a)news.eternal-september.org...
> David J Taylor wrote:
[]
>> That sounds like simple hysteresis - final position depends on starting
>> position.
>
> Memory? Does hysteresis band simply mean fuzz factor?
>
>
> --
> Paul Furman

No, not memory, the fact that there is a dead-band where focus will be
deemed to be good enough if you are anywhere within that band. Approach
from different starting points, infinity or close up, and you will likely
stop at a different position, always within that dead-band, though (at
least in theory).

Cheers,
David

From: Paul Furman on
David J Taylor wrote:
> Paul Furman wrote
>> David J Taylor wrote:
>
>>> That sounds like simple hysteresis - final position depends on
>>> starting position.
>>
>> Memory? Does hysteresis band simply mean fuzz factor?
>
> No, not memory, the fact that there is a dead-band where focus will be
> deemed to be good enough if you are anywhere within that band. Approach
> from different starting points, infinity or close up, and you will
> likely stop at a different position, always within that dead-band,
> though (at least in theory).

It's not clear his camera gives a range, just one beep. And he got
consistent dead-on results from the front. And consistently front
focused coming from the rear so a fuzz factor band appears not to
explain it. My Nikons give little bracket arrows when approaching
acceptable focus, one side or the other or both, then a green dot in the
middle when it's optimal.

--
Paul Furman
www.edgehill.net
www.baynatives.com

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