From: Wilba on
Paul Furman wrote:
> Wilba wrote:
>> Paul Furman wrote:
>>> Wilba wrote:
>>>> Paul Furman wrote:
>>>>>
>>>>> Looking at that again prompted me to try another paper cutout test. I
>>>>> cut two holes in a black paper on each side like that, a little bit
>>>>> larger than the scale of that drawing and it works as a phase detect
>>>>> AF system. The whole scene through the viewfinder turns into
>>>>> double-vision ghosts that split apart and overlap when out of focus
>>>>> but it all snaps together when in focus. Pretty cool.
>>>>
>>>> I couldn't make it work. How big did you make the holes and how far
>>>> apart?
>>>
>>> Here's the results & specs:
>>> http://www.flickr.com/photos/edgehill/4271241493/
>>> -and the three previous shots of the guitar
>>
>> Hmm, I just get a masking effect.
>
> It didn't work on my 50mm. Maybe I got lucky.

I tried again with a zoom at 100mm and didn't get anywhere. I've seen that
two hole thing before, so I'm sure it does work somehow.


From: Wilba on
Paul Furman wrote:
> Wilba wrote:
>> Paul Furman wrote:
>>> Wilba wrote:
>>>> Paul Furman 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.
>>>>>
>>>>> OK, I see what you are saying now. Those apertures exist for the
>>>>> purpose of selecting rays from opposite sides of the lens so that you
>>>>> have two images to compare for the phase detect.
>>>>
>>>> I'm uncomfortable with the way that sounds. Those virtual apertures
>>>> don't exist as real holes in anything anywhere,
>>>
>>> Fig 14 shows an AF aperture plate with real holes at the prisms. Those
>>> project out to the 'virtual' entrance pupil where it's easier to see how
>>> it relates to the lens...
>>
>> Yep, but I don't believe that it's as simple as tracing back from the
>> holes the prism are mounted in. Around figures 9 to 12 he explains how
>> the prism diverts the light falling on it, but only a subset gets to the
>> AF sensor. I believe that's the fundamental thing that the virtual
>> aperture is derived from, not the prism holes.
>
> Fig 13 has 'AF field stops' that select light from only small parts of the
> subject, so that's happening in the mix too.

Yeah, I know, but I still don't know of any reason why the fundamental
operation of the AF system would require a stop in the path. Several may be
there for other reasons, such as preventing light falling on other things
where it isn't wanted and maximising the mirror area, but I find no
justification for it in the AF system itself.

I'm sure the "AF field stops" would be designed to accomodate rather than
control the rays that reach the AF sensor. IOW, I fundamentally disagree
with, "those apertures exist for the purpose of selecting rays from opposite
sides of the lens so that you have two images to compare for the phase
detect." I'm convinced that the prisms effectively do that, and I don't see
how a careful reading of Kerr could lead one to any other conclusion. :- )

Anyway, let's say for argument's sake the "AF field stops" were controlling
the rays that reach the AF sensor, what would be the relevance, to what, and
in what way?


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

--
Chris Malcolm
From: Paul Furman on
Wilba wrote:
> Paul Furman wrote:
>> Wilba wrote:
>>> Paul Furman wrote:
>>>> Wilba wrote:
>>>>> Paul Furman 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.
>>>>>> OK, I see what you are saying now. Those apertures exist for the
>>>>>> purpose of selecting rays from opposite sides of the lens so that you
>>>>>> have two images to compare for the phase detect.
>>>>> I'm uncomfortable with the way that sounds. Those virtual apertures
>>>>> don't exist as real holes in anything anywhere,
>>>> Fig 14 shows an AF aperture plate with real holes at the prisms. Those
>>>> project out to the 'virtual' entrance pupil where it's easier to see how
>>>> it relates to the lens...
>>> Yep, but I don't believe that it's as simple as tracing back from the
>>> holes the prism are mounted in. Around figures 9 to 12 he explains how
>>> the prism diverts the light falling on it, but only a subset gets to the
>>> AF sensor. I believe that's the fundamental thing that the virtual
>>> aperture is derived from, not the prism holes.
>> Fig 13 has 'AF field stops' that select light from only small parts of the
>> subject, so that's happening in the mix too.
>
> Yeah, I know, but I still don't know of any reason why the fundamental
> operation of the AF system would require a stop in the path. Several may be
> there for other reasons, such as preventing light falling on other things
> where it isn't wanted and maximising the mirror area, but I find no
> justification for it in the AF system itself.
>
> I'm sure the "AF field stops" would be designed to accomodate rather than
> control the rays that reach the AF sensor. IOW, I fundamentally disagree
> with, "those apertures exist for the purpose of selecting rays from opposite
> sides of the lens so that you have two images to compare for the phase
> detect." I'm convinced that the prisms effectively do that, and I don't see
> how a careful reading of Kerr could lead one to any other conclusion. :- )
>
> Anyway, let's say for argument's sake the "AF field stops" were controlling
> the rays that reach the AF sensor, what would be the relevance, to what, and
> in what way?

No particular relevance to the focus shift issue :-)

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

all google groups messages filtered due to spam
From: Paul Furman on
Chris Malcolm wrote:
> Wilba <usenet(a)cutthisimago.com.au> 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

That makes sense. The AF system only sees things at the designed aperture.

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

all google groups messages filtered due to spam