From: Wilba on
Chris Malcolm wrote:
> Wilba 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.

Right-oh, so there's nothing we can think of so far that directly attributes
the width of the beep band to any form of ARFD.


From: Paul Furman on
Peter wrote:
> Paul Furman wrote
>
>> 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.

OK that's just the D700. It's kind of confusing to pay attention while
watching the scene. The D200 just has the green dot.

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

all google groups messages filtered due to spam
From: Paul Furman on
Wilba wrote:
> 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 manually focus from infinity it won't confirm focus till
you've over shot it, even when moving very slowly on a focus rail?


> But if you put the subject in that band, the
> system will confirm focus.

So you must be holding a half shutter press while turning the focus
rail? If you release & press again, it won't overshoot? Is there a
continuous focus like for tracking moving subjects?


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

all google groups messages filtered due to spam
From: Peter on
"Paul Furman" <paul-@-edgehill.net> wrote in message
news:hitsph$d2e$1(a)news.eternal-september.org...
> Peter wrote:
>> Paul Furman wrote
>>> 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.
>
> OK that's just the D700. It's kind of confusing to pay attention while
> watching the scene. The D200 just has the green dot.
>


thanks, I have a D300, but never paid much attention on my D200.


--
Peter

From: Paul Furman on
Wilba wrote:
> 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

There definitely is confusion involved :-)


> 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


430mm from sensor to subject when focused.



> | |< 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 -


433mm is more than 430mm so the text on the line above describes back
focus. The diagram immediately following below shows front focus with
the subject 3mm further away from the plane of focus than it was before.
It looks like you turned the focus ring. Didn't you say the subject was
430mm away from the sensor when focus is perfect? This scenario still
has the plane of focus at 430mm but the subject isn't in focus any more.

Does not compute. Illogical.

The DOF symbols with various numbers of angle brackets are just
theoretical and make the diagrams confusing. I deleted them in my
version. You could discuss them later but they aren't part of the
measured data.



> 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.

It does if optimal focus is 430mm and the camera's AF system confirms
focus at 433mm. That's back focus.

C S POF
| | |
0mm 430mm 433mm


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

Done.

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

all google groups messages filtered due to spam