From: Paul Furman on
Wilba wrote:
> Paul Furman wrote:
>> Wilba wrote:
>>>> Paul Furman wrote:
>>>>> Wilba wrote:
>>>>>> Paul Furman wrote:
>>>>>>> In the focus shift web page, he auto-focuses on the front eye but the
>>>>>>> final pic is focused on the back eye. I'm calling that 'back-focus'.
>>>>>> Right. That's the opposite of what's happening for me, it's
>>>>>> front-focus.
>>>>> No, this describes your results.
>>>>> Draw the two eyes and camera.
>>>>> Now draw your setup.
>>>>> They are the same.
>>>>> Back Focus.
>>> What do you see in this arrangement, back-focus or front-focus?
>>>
>>> Camera Subject Plane of focus
>> Back focus.
>>
>>> And therefore this is...?
>>>
>>> Camera Plane of focus Subject
>> Front focus.
>
> Excellent. So here we go.
>
> 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?

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

all google groups messages filtered due to spam
From: Wilba on
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. :- )


From: Wilba on
Chris Malcolm wrote:
> Wilba wrote:
>> Chris Malcolm wrote:
>>>
>>> Since the lens aberrations which affect focus increase with the width
>>> of a lens (focal length being constant), an uncorrected autofocus will
>>> only perfectly focus an image taken at an aperture which has the same
>>> width as the distance between the two AF sampling "holes". In fact
>>> because exact focus of a lens is a best compromise between the various
>>> focus affecting imperfections, an uncorrected phase detection AF
>>> system will only accurately focus images taken at a slightly larger
>>> aperture than the aperture defined by the distance between the two AF
>>> sensor apertures, probably around one stop larger.
>>>
>>> That's why the "effective AF aperture" in the sense of the aperture
>>> defined by the width apart on the lens of the two AF samples is an
>>> important parameter in knowing how an AF system will focus different
>>> lenses.
>>
>> So that's your definition of "effective AF aperture"? That would have
>> been
>> handy two weeks ago when you first started talking about it. :- )
>
> I explained it in the same terms over a week ago. It would have been
> handy if you'd paid attention :-)
>
>>> Of course if you're only concerned with the behaviour of a DSLR which
>>> has been optimised for best autofocus with one specific lens you can
>>> ignore all this. Those of us with more than one lens with critical
>>> focus can't ignore it.
>>
>> What do you do about it?
>
> Assuming your camera can't be adjusted for each lens individually,
> since you can't get them all to autofocus accurately, you have to
> decide which are the most important to get right, and if there's more
> than one, how to best compromise between them. Then you get the camera
> adjusted to the specific lens or compromise you have chosen, and when
> necessary use manual adjustmants of one kind or another to cope with
> the others when in conditions you know will be wrongly focused.
>
> In my case I adjusted the camera for perfect autofocus with the 500mm
> f8, which gave me perfect focus with everything else except apertures
> wider than f2.8 on my 50mm. So when I need apertures wider than f2.8
> on the 50mm I either focus it manually or lock autofocus and move
> forward the appropriate centimetre or so.
>
> Doing that has improved the sharpness of focus on the 500mm and 50mm
> by enough to raise the maximum size of sharp print by at least 40%.

Lovely. No surprises there.

I explained two weeks ago how all of that is irrelevant to my results. It
would have been far less challenging if you'd paid attention. :- )


From: Wilba on
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. 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.


From: NameHere on
On Sat, 16 Jan 2010 08:09:21 +0800, "Wilba" <usenet(a)CUTTHISimago.com.au>
wrote:

>Chris Malcolm wrote:
>> Wilba wrote:
>>> Paul Furman 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
>>>>
>>>> That makes sense. The AF system only sees things at the designed
>>>> aperture.
>>>
>>> I can see we're not going to agree on this, but I'd say it like, the AF
>>> system only sees things through an effective aperture, whose size is a
>>> consequence of the design of elements such as the prisms.
>>
>> It's not a consequence of the prism design, because the prisms are
>> always designed to accommodate the optical design of the entire AF
>> system, such as the size of the sensors, the amount of light they need
>> to operate, the intended width apart of their two samples of the image
>> from the lens, and so on. That sets the design parameters for the
>> prisms, not vice versa.
>
>Exactly. You don't start with choosing a virtual AF aperture and put stops
>in the AF system to enforce that, you start with a nominal exit pupil and
>design the elements of the system to suit.
>
>> The important thing you're ignoring is that the AF systems sees things
>> not through one virtual hole, which of course has an effective
>> aperture, but through two holes, whose images are combined or
>> compared. Those two holes are on opposite edges of the lens being
>> focused.
>
>LOL, be careful when you try to make someone else look stupid, 'cos it often
>backfires.
>
>> Since the lens aberrations which affect focus increase with the width
>> of a lens (focal length being constant), an uncorrected autofocus will
>> only perfectly focus an image taken at an aperture which has the same
>> width as the distance between the two AF sampling "holes". In fact
>> because exact focus of a lens is a best compromise between the various
>> focus affecting imperfections, an uncorrected phase detection AF
>> system will only accurately focus images taken at a slightly larger
>> aperture than the aperture defined by the distance between the two AF
>> sensor apertures, probably around one stop larger.
>>
>> That's why the "effective AF aperture" in the sense of the aperture
>> defined by the width apart on the lens of the two AF samples is an
>> important parameter in knowing how an AF system will focus different
>> lenses.
>
>So that's your definition of "effective AF aperture"? That would have been
>handy two weeks ago when you first started talking about it. :- )
>
>> Of course if you're only concerned with the behaviour of a DSLR which
>> has been optimised for best autofocus with one specific lens you can
>> ignore all this. Those of us with more than one lens with critical
>> focus can't ignore it.
>
>What do you do about it?
>

Ditch any camera design that causes anyone this much time and energy wasted
in trying to overcome or understand its design limitations.