From: Peter on
"Paul Furman" <paul-@-edgehill.net> wrote in message
news:hitafs$t0p$1(a)news.eternal-september.org...

> 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.
>

Which models are they?
I have not noticed this feature on any Nikon I own, or have owned.


--
Peter

From: Wilba on
David J Taylor wrote:
> Wilba wrote:
> []
>> 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.

The beep band is 3mm deep (around the same size as the DOF), yet the system
seems capable of achieving optimum focus at 0.1mm increments of
camera-to-subject distance. Does that fit?


From: Chris Malcolm on
Wilba <usenet(a)cutthisimago.com.au> wrote:

> Okay, let's assume you're right, that there is a miscalibration which
> results in a consistent offset in the focus (I've accommodated that
> possibility in my other reply today). How does what we've been talking about
> account for the difference between the two focuses? One is consistently
> _here_, and other is consistently _there_. That's the significant thing. How
> do you account for the width of the beep band?

In many modern cameras and lenses which communicate electronically the
width of the "beep band", i.e., the change in focus over which the
subject is deemed to be in focus by the AF system, is set by an
acceptable focus error parameter for that lens. In other words focus
is considered to be ok not when the focus error is at its smallest,
but when it's less than this acceptable error parameter. Very sharp
lenses will have a smaller value than softer lenses. The lens chip
holds the value, supplied to it at lens calibration time, and it
supplies this value to the camera along with its other parameters such
as max aperture, focal length, etc..

When you send a lens off to be recalibrated one of the things they may
do is to rewrite the lens focussing parameter table with values which
more accurately reflect the performance of that particular copy of the
lens, and which are more appropriate to your specific camera and
purposes.

--
Chris Malcolm
From: Wilba on
Paul Furman wrote:
> 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:

I think you're confused about what that text diagram shows. It's only
purpose is to give a graphical depiction of the distances involved. The
results are described in words in messages that preceded it. If you didn't
understand those descriptions you won't understand the text diagram.

You're possibly stumbling on the labels. The labels "nearside focus" and
"farside focus" refer to the initial sensor to subject distances (initial
gross front-focus, and initial gross back-focus), and the distances shown
("|< Xmm >|"), depict the sensor to subject distances achieved from them
(optimal focus, and front-focus).

Here's the diagram by itself. View 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" case
|< 430mm >|< 3.0mm >| "farside
focus" case
| |< 3.0mm >| the "beep
band"
|< 432.2mm >|<0.8mm>| outside the
DOF

And here it is with a full commentary. View with a fixed width font (e.g.
copy into Notepad).

C = sensor
POF = plane of focus
S = subject
DOF = depth of field

The lens is focussed at 430mm and doesn't refocus at any time. If the
subject is also at that distance, it will be in optimal focus -
C POF
S
|< 430mm >| sensor to subject distance for optimal
focus

A conventional DOF calculation gives 4.4mm -
C DOF min. POF DOF max.
| |< 2.2mm >|< 2.2mm >| theoretical DOF

If I start with the POF on the nearside of the subject (subject more than
430mm from the sensor, initial gross front-focus) -
C POF<------ >>DOF ------>S
and reduce the sensor to subject distance (so the POF approaches the subject
from the nearside), I get a beep (focus confirmation) at 430mm = optimal
focus -
C POF
S
|< 430mm >| "nearside
focus" case

If I start with the POF on the farside of the subject (subject less than
430mm from the sensor) -
C S<------ >>DOF ------>POF
and increase the sensor to subject distance (so the POF approaches the
subject from the farside), I get a beep (focus confirmation) at 433mm, i.e.
front-focus -
C POF<-------- >DOF/2 -------->S
|< 430mm >|< 3.0mm >| "farside
focus" case
This is the counter-intuitive "cross-over" bit - you'd expect this case to
put the subject 427mm from the sensor for a 3mm beep band.

The system will confirm focus with a beep if 430mm <= sensor-to-subject
distance <= 433mm -
C S
| |< 3.0mm >| the "beep
band"

The focus achieved in the farside case puts the subject 0.8mm outside the
calculated DOF -
|< 432.2mm >|<0.8mm>| outside the
DOF

This outcome -
C S POF
| | |
never occurs.

Can you identify the point at which the diagram is "opposite" to the
results, and describe in your words what you're seeing?


From: Wilba 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?
>
> 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).

Like Paul said, my AF system focuses precisely and repeatedly at the ends of
a band, never in the middle. But if you put the subject in that band, the
system will confirm focus.