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

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.

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.

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.

--
Chris Malcolm
From: Chris Malcolm on
Wilba <usenet(a)cutthisimago.com.au> wrote:
> Chris Malcolm wrote:
>> Wilba wrote:
>>> Chris Malcolm wrote:
>>>> Wilba wrote:
>>>>> Paul Furman wrote:
>>>>>> Wilba wrote:
>>>>>>> Paul Furman wrote:
>>>>>>>>
>>>>>>>> The wider rays from f/1.8 won't be seen by the AF sensor. It's view
>>>>>>>> comes through a system that only gathers narrower f/2.8 angled rays.
>>>>>>>>
>>>>>>>> The wider rays will get clipped, bouncing around side surfaces,
>>>>>>>> never
>>>>>>>> reaching the AF sensor.
>>>>>>>
>>>>>>> I guess you're saying that the effective size of an AF sensor's
>>>>>>> virtual
>>>>>>> aperture doesn't change with the lens aperture, which is fair enough,
>>>>>>> because that's part of the explanation of why there is no ARFD
>>>>>>> occurring in the AF system. Still, my system's performance meets that
>>>>>>> standard (from an initial near side focus), so I'm not concerned
>>>>>>> about
>>>>>>> that aspect.
>>>>>>>
>>>>>>> It would be interesting to get a better idea of the effective size of
>>>>>>> an AF sensor's virtual aperture (I think I read somewhere that it
>>>>>>> might
>>>>>>> be something like f/11, but I can't recall where). As long as we're
>>>>>>> clear
>>>>>>> that "f/2.8" doesn't refer to that. :- )
>>>>>>
>>>>>> Heh, now I don't know what you're talking about :-)
>>>>>
>>>>> Isn't this fun!? :- )
>>>>>
>>>>> See figure 13 of http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf
>>>>> (but read your way down to it from the beginning so that it makes
>>>>> sense).
>>>>>
>>>>> The light that reaches the AF sensor effectively comes through a
>>>>> virtual
>>>>> aperture in the lens ("virtual AF apertures" in figure 13), as we have
>>>>> been discussing for quite some time. If we knew the diameter of one of
>>>>> those virtual apertures, we could work out the f-number for it. IIRC,
>>>>> I read somewhere that it is typically something like f/11. As long as
>>>>> we
>>>>> are clear that the "f/2.8" in "f/2.8 high-precision AF sensor" does not
>>>>> refer
>>>>> to the size of the virtual AF aperture.
>>>>
>>>> "As we have all been discussing for some time"?
>>>>
>>>> Since that document is the only one I've ever seen which uses "virtual
>>>> AF aperture" in that sense, and all the other uses I've seen, by
>>>> camera makers, reviewers, etc., have used it in the other sense of the
>>>> width of lens the pair stretch across, I'm pretty sure everyone (except
>>>> you) in this discussion has been using it in that sense too.
>>>
>>> Why won't you provided references to any such documents?
>>
>> "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.

--
Chris Malcolm
From: Paul Furman on
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.

OK. The important thing is the difference between an f/5.6 and f/2.8 AF
setup and that's what he calls the 'baseine'. It's just a matter of
geometry being set in the angle of the prisms so that the rays actually
make it to the AF sensor. In my home made example I suppose that would
be the separation between the holes? Apparently there are focusing
screens for various focal lengths and speeds with different prism angles.

page 17
http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf
"Autofocus precision
There will always be some uncertainty in the determination of the
relative positions of the two images, leading ultimately to a residual
focus error after the AF system has made the correction indicated by the
detector system. Because of geometric considerations, the greater the
separation of the two virtual AF apertures in the exit pupil, the
greater will be the displacement of the two images for a given focusing
error; conversely, the greater that separation, the smaller will be the
residual error for any given uncertainty in determination of the
relative positions of the two images. The distance of this separation
between the two virtual AF apertures is often spoken of as the
�baseline� of the detector system, by analogy with the
somewhat-corresponding parameter of a rangefinder.

....the focus detectors are usually designed to have virtual AF apertures
near the edges of the exit pupil for a modest camera aperture�an
aperture likely to be available on most lenses to be used on the camera.
Of course, this compromise (a �reduced baseline�) results in a reduction
in the precision of the focus detection system."

Few lenses are slower than f/5.6... quite a few are that slow at the
long end. So they have to accommodate that. F/3.5 lenses will be a
little off wide open and the camera won't make full use of the wider
aperture in focusing.

Then you add a precision f/2.8 AF system on top of that and it
absolutely must accommodate f/2.8 lenses because there's a bunch of pro
zooms with that exact spec. Faster lenses don't add to the accuracy of
that system, in fact they will tend to front-focus wide open, and
back-focus shut down from this designed aperture baseline of f/2.8.

I think that explains what you've experienced and the focus shift dude's
free intro - using the logic in the split-prism article. What do you
think? I'm pretty sure I've nailed it here. The angle of the prism sets
the working aperture of the AF system.

A more sophisticated system might have an adjustable baseline. That
would require changing the prism angles with mirrors or sliding holes or
something.

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

all google groups messages filtered due to spam
From: Paul Furman on
Wilba wrote:
> Paul Furman wrote:
>> Wilba wrote:
>>> Paul Furman wrote:
>>>> Wilba wrote:
>>>>> Paul Furman wrote:
>>>>>> Wilba wrote:
>>>>>>> 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. :- )
>>>> That makes sense but doesn't match your previous diagram:
>>> Yeah, but that's _your_ diagram, which is wrong. :- )
>> Well hell.
>> Maybe it is focus shift then.
>
> No! No! No! No! :- )
>
> It's all about the beep band (see below). From one direction focus is
> confirmed in the right place, and from the other it's confirmed in the wrong
> place. Where is the focus shift in that?

Could be miscalibrated.
That's where my averaging idea has relevance.


>>> Here's mine again and we'll go through it line by line. Constant width
>>> font required for scale. Disregard any previous misunderstandings - these
>>> are the results. :- )
>>>
>>> --------------------------------------
>>>
>>> Beep Test (focus by changing the sensor to subject distance until the
>>> system beeps to confirm focus)
>>>
>>> Sensor to plane of focus distance -
>>> |< 430mm >|
>>> (The lens remains focussed at 430mm through the beep test.)
>>>
>>> Theoretical DOF -
>>> | |< 2.2mm >|< 2.2mm >|
>>> (The calculated DOF goes from 427.8 to 432.2mm.)
>>>
>>> Sensor to subject distance you get with an initial near-side focus -
>>> |< 430mm >|
>>> (Start with the subject 440mm from the sensor and decrease that distance.
>>> When the system beeps the subject is perfectly in focus.)
>>>
>>> Sensor to subject distance you get with an initial far-side focus -
>>> |< 433mm >|
>>> (Start with the subject 420mm from the sensor and increase that distance.
>>> When the system beeps the subject is 3mm further away than the plane of
>>> focus, i.e. front-focus.)
>>>
>>> The "beep band" -
>>> | |< 3.0mm >|
>>> (The system will confirm focus over this range of sensor to subject
>>> distances, for a plane of focus 430mm from the sensor.)
>>>
>>> Outside the DOF -
>>> | |<0.8mm>|
>>> (In the initial far-side focus case, the subject ends up 0.8mm further
>>> from the sensor than the far limit of the theoretical DOF.)
>>>
>>> --------------------------------------
>>>
>>> AF Test
>>>
>>> Sensor to subject distance -
>>> |< 430mm >|
>>> (The sensor to subject distance remains the same throughout the AF test.)
>>>
>>> Sensor to plane of focus distance you get with an initial near-side
>>> focus -
>>> |< 430mm >|
>>> (Start with the lens focussed at 420mm from the sensor and invoke AF.
>>> When the system beeps the subject is perfectly in focus.)
>>>
>>> Sensor to plane of focus distance you get with an initial far-side
>>> focus -
>>> |< 427mm >|
>>> (Start with the lens focussed at 440mm from the sensor and invoke AF.
>>> When the system beeps the plane of focus is 3mm short of the subject,
>>> i.e. front-focus.)
>>>
>>> Theoretical DOF for an initial far-side focus -
>>> | |< 2.2mm >|< 2.2mm >|
>>> (The calculated DOF goes from 424.8 to 429.2mm.)
>>>
>>> The "beep band" -
>>> | |< 3.0mm >|
>>> (The system will confirm focus over this range of sensor to plane of
>>> focus distances, for a subject 430mm from the sensor.)
>>>
>>> Outside the DOF -
>>> | |<0.8mm>|
>>> (In the initial far-side focus case, the subject ends up 0.8mm further
>>> from the sensor than the far limit of the theoretical DOF.)
>
>


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

all google groups messages filtered due to spam
From: Wilba on
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?