From: Paul Furman on
Wilba wrote:
> Paul Furman wrote::
>> Wilba wrote:
>
>>> |< 430mm >| sensor to subject distance for optimal
>>> focus
>> 430mm from sensor to subject when focused.
>
> No, the POF is always 430mm from the sensor in the beep test. If the subject
> is there too, it will be in focus.
>
>>> 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.
>
> When the subject is 433mm from the sensor, and the POF is 430mm from the
> sensor, that's front-focus.
>
>> The diagram immediately following below shows front focus with the subject
>> 3mm further away from the plane of focus than it was before.
>
> Exactly! That's what I'm depicting. The POF is always 430mm from the sensor
> in this scenario.
>
>> It looks like you turned the focus ring.
>
> No, the sensor to subject distance increased while the POF remained 430mm
> from the sensor.
>
>> Didn't you say the subject was 430mm away from the sensor
>> when focus is perfect?
>
> No, I said if the subject is coincident with the POF (430mm from the
> sensor), it will be in focus.
>
>> This scenario still has the plane of focus at 430mm but the subject
>> isn't in focus any more.
>
> You got it!
>
>> Does not compute. Illogical.
>
> The whole scenario is - sensor to POF distance is constant, sensor to
> subject distance changes. Computes perfectly.
>
>> 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.
>
> Correct.
>
>>> 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.
>
> This is the bit I screwed up. See previous message.
>
>>> 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
>
> Incorrect. C to POF is always 430mm, so back-focus would look like this -
>
> C S POF
> | | |
> 0mm 427mm 430mm
>
> which never happens.
>
>>> Can you identify the point at which the diagram is "opposite" to the
>>> results, and describe in your words what you're seeing?
>> Done.
>
> From the above, I believe you have been reading these diagrams depicting the
> beep test as if they were depicting AF tests (in which the sensor to subject
> distance remains the same and the sensor to POF distances changes).
>
> If you can stand it, we should just work on the AF case, which looks quite
> different but shows the crossover phenomenon. Let me know if you have the
> will to carry on. :- )

The background is overwhelming, let me try to summarize my current
understanding.

The beep test uses a focus rail and if you stop to re-check rather than
holding a half-press from the initial position, then you see a
consistent focus band between a 3mm front focus and perfectly in-focus.

The AF test, letting the camera turn it's own focus ring shows the same
beep band, front focus or perfect focus but there is a crossover
phenomenon depending where initial focus starts, which is rather
counterintuitive. but still shows an overall front-focus.

Perhaps the crossover can be explained by the different character of the
bokeh from front & back, possibly it has to do with magnification
changes? Turning the focus ring changes the enlargement or focal length
if you want to say it that way... I don't know that this would produce
such a result but it's another idea, it is something that changes.
Anyways I'm going to say this doesn't matter, the beep band is more
relevant for analysis although yes I know the AF motor results are
what's real in the end, we don't have an explanation and the results are
the same.

The front-focus could be explained by mis-calibration but I recall you
said it works perfect at f/2.8? and by f/4 or 5.6 it's probably hard to
even see any discrepancy. My current explanation is aperture related
focus shift where the AF sensor sees it at f/2.8 but the final shot is
taken at f/1.8. This explanation should show a little back focus when
stopped down to f/4 or more but perhaps that's been calibrated out with
a pinch of front focus added to the correction tables??? Hell, I don't
know :-)

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

all google groups messages filtered due to spam
From: Wilba on
David J Taylor wrote:
> Wilba wrote:
>> David J Taylor wrote:
>>>
>>> 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.
>
> Exactly what you would expect when the focus system has a dead-zone
> or hysteresis, and when you are within that dead-zone the focus system
> reports "in-focus".

Yeah but, "likely stop at a different position, always within that
dead-band", is not true in this case. My focus does not stop _within_ that
dead-band, it always stops at one or other of the ends of it. It's quite
precise, not fuzzy as you'd expect from a hysteresis band.


From: Wilba on
Chris Malcolm wrote:
> Wilba wrote:
>> Chris Malcolm wrote:
>>> Wilba wrote:
>>>> Chris Malcolm wrote:
>>>>> Wilba wrote:
>>>>>> Chris Malcolm wrote:
>>>>>>>
>>>>>>> "Back and front focus errors are common with very fast lenses (and
>>>>>>> others). The AF sensors in all DSLRs operate at effective apertures
>>>>>>> between f/2.8 and f/7.1, and 'see' the focus point as if the lens
>>>>>>> was
>>>>>>> stopped down to the exact virtual f-stop the sensor in use
>>>>>>> imposes. When spherical aberration causes a shift in focus on
>>>>>>> stop-down, both wider and smaller apertures will no longer focus on
>>>>>>> the targeted plane. This is corrected in camera calibration, general
>>>>>>> or lens-specific."
>>>>>>>
>>>>>>> This quote comes from the section "Focus Accuracy" in this article
>>>>>>> in
>>>>>>> the British Journal of Photography.
>>>>>>>
>>>>>>> http://www.bjp-online.com/public/showPage.html?page=837772
>>>>>>
>>>>>> Excellent! So, now that you have some corroboration for that idea,
>>>>>> please remind me what you're claiming as a consequence of it.
>>>>>
>>>>> In the quote above it says that there are misfocusing problems at
>>>>> effective apertures different from the effective focus aperture of the
>>>>> AF sensors, defined by the width between them, and that these are
>>>>> corrected by calibration. But that calibration can never be
>>>>> perfect. When focus is not highly critical that doesn't matter because
>>>>> the residual uncorrected imperfections are swallowed in the DoF. But
>>>>> when focus is highly critical with very shallow DoF, such as wide
>>>>> apertures on high quality lenses, the inevitable imperfection of
>>>>> general lens and camera calibrations matters. That's why the very top
>>>>> end DSLRs, even though they have the very best AF systems,
>>>>> nevertheless allow the user to trim the calibration for each lens, and
>>>>> those users who have used it report that it gives useful improvements
>>>>> in focus accuracy.
>>>>>
>>>>> Of course if your camera has been calibrated for best focus with one
>>>>> specific lens, you're happy with the results, and you will never
>>>>> acquire another critical focus lens, then you won't ever need to know
>>>>> any of this. But since we're having this discussion in public, and
>>>>> some of our audience does have more than one lens of critical focus,
>>>>> it's worth mentioning these general issues.
>>>>
>>>> That's it?! Okay, since none of that has ever been contested, and it's
>>>> never
>>>> been shown to be relevant to my test results, unless you have something
>>>> that
>>>> is, you can retire from the discussion satisfied that you have made
>>>> your
>>>> point. Thank you. :- )
>>>
>>> Since your test results remain by your own admission completely
>>> inexplicable it can't be claimed that none of that will turn out to be
>>> relevant when the explanation is found :-)
>
>> Oh no, the results _are_entirely_ explicable.
>
> Ah. Obviously I misinterpreted what you wrote yesterday:
>
> "... I can't find anything whatsoever that can possibly explain my
> results."

You sure did. Here's the full quote with the context fully clarified for
those who can't hold more than a single sentence in mind -

"When I read all that [Paul's attribution of the beep band to ARFD] and
consider everything else we've talked about [forms of ARFD and their
possible relationships with the beep band], I can't find anything whatsoever
[in ARFD] that can possibly explain my results."

>> 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.
>
> Since the presence of ARFD due to chromatic aberration would limit
> lens sharpness and also phase detection AF accuracy it could be what
> decided the makers to calibrate the lens with the degree of reported
> in-focus width you have experimentally discovered.

Yep, it could be a design consideration, not it's the primary cause in
itself.


From: Chris Malcolm on
Wilba <usenet(a)cutthisimago.com.au> wrote:
> David J Taylor wrote:
>> Wilba wrote:
>>> David J Taylor wrote:
>>>>
>>>> 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.
>>
>> Exactly what you would expect when the focus system has a dead-zone
>> or hysteresis, and when you are within that dead-zone the focus system
>> reports "in-focus".

> Yeah but, "likely stop at a different position, always within that
> dead-band", is not true in this case. My focus does not stop _within_ that
> dead-band, it always stops at one or other of the ends of it. It's quite
> precise, not fuzzy as you'd expect from a hysteresis band.

Why would you expect fuzziness in a hysteresis band? I've seen plenty
of systems with hysteresis with very precise boundaries to the
hysteresis band.

Note by the way the the in-focus band defined by the acceptable focus
error band isn't a hysteresis band. But there's no reason why it
should have fuzzy edges either.

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

> The background is overwhelming, let me try to summarize my current
> understanding.
>
> The beep test uses a focus rail and if you stop to re-check rather than
> holding a half-press from the initial position, then you see a consistent
> focus band between a 3mm front focus and perfectly in-focus.

It doesn't matter whether you stop and recheck or hold a half-press all the
way through, you see the same thing.

> The AF test, letting the camera turn it's own focus ring shows the same
> beep band, front focus or perfect focus but there is a crossover
> phenomenon depending where initial focus starts, which is rather
> counterintuitive.

Yes.

> but still shows an overall front-focus.

I find that a very artificial concept, like saying that on average a man who
is one half of a heterosexual couple is half female.

> Perhaps the crossover can be explained by the different character of the
> bokeh from front & back, possibly it has to do with magnification changes?
> Turning the focus ring changes the enlargement or focal length if you want
> to say it that way... I don't know that this would produce such a result
> but it's another idea, it is something that changes.

New and interesting, but a long shot I think. :- )

> Anyways I'm going to say this doesn't matter, the beep band is more
> relevant for analysis although yes I know the AF motor results are what's
> real in the end, we don't have an explanation and the results are the
> same.

Right.

> The front-focus could be explained by mis-calibration but I recall you
> said it works perfect at f/2.8? and by f/4 or 5.6 it's probably hard to
> even see any discrepancy.

I haven't gone very far with stopping down (maybe up to f/5.6 or so), but I
don't see a drift. We know the lens applies a looked-up correction for that
in AF.

> My current explanation is aperture related focus shift where the AF
> sensor sees it at f/2.8 but the final shot is taken at f/1.8.

So we come back to where we were three weeks ago. If the remote Live View
image (at 100% on the PC monitor) is in optimal focus, either by AF or beep,
where's the shift? There is no shift that I can see. The misfocus happens
under identical conditions, except we started with the POF on the other side
of the subject. I still cain't see no shift.

> This explanation should show a little back focus when stopped down
> to f/4 or more but perhaps that's been calibrated out with a pinch of
> front focus added to the correction tables??? Hell, I don't know :-)

Well, we can eliminate the influence of the correction tables by doing the
beep test, and we get the same results (same two focus points) as we do with
AF, so where's the shift?