From: Chris Malcolm on
Wilba <usenet(a)cutthisimago.com.au> wrote:
> Chris Malcolm 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. :- )
>>
>> Depends what you take "effective aperture" to mean. It could be taken
>> as effective diameter of lens exploited, which gives the numbers like
>> f2.8 and f5.6 often mentioned. It could be taken to be effective light
>> capture area of the AF sensor as a fraction of the lens aperture. But
>> since it doesn't look at the whole image, that esttimate could in turn
>> be adjusted by effective change of focal length to account for the
>> reduction on field of view. And so on.
>>
>> It all depends on what you want "effective aperture" to mean. Once
>> you've defined what you mean by "effective aperture" it's just
>> arithmetic on well known numbers to calculate it.

> Use the precise meaning I gave - the effective size of an AF sensor's
> virtual aperture. If you can tell me the well known diameter of the AF
> sensor's virtual aperture, we can work it out. :- )

But you still haven't explained what you mean by "virtual aperture" of
an AF sensor. The only clue you've given us is that you probably don't
mean what camera makers usually mean by it, since I don't think anyone
claims a virtual AF aperture as small as f11, and the f2.8 many do
claim you tell us is definitely not what you mean.

So what do you mean?

--
Chris Malcolm

From: Wilba on
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. :- )


From: Wilba on
Chris Malcolm wrote:
> Wilba wrote:
>> Chris Malcolm 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. :- )
>>>
>>> Depends what you take "effective aperture" to mean. It could be taken
>>> as effective diameter of lens exploited, which gives the numbers like
>>> f2.8 and f5.6 often mentioned. It could be taken to be effective light
>>> capture area of the AF sensor as a fraction of the lens aperture. But
>>> since it doesn't look at the whole image, that esttimate could in turn
>>> be adjusted by effective change of focal length to account for the
>>> reduction on field of view. And so on.
>>>
>>> It all depends on what you want "effective aperture" to mean. Once
>>> you've defined what you mean by "effective aperture" it's just
>>> arithmetic on well known numbers to calculate it.
>>
>> Use the precise meaning I gave - the effective size of an AF sensor's
>> virtual aperture. If you can tell me the well known diameter of the AF
>> sensor's virtual aperture, we can work it out. :- )
>
> But you still haven't explained what you mean by "virtual aperture" of
> an AF sensor. The only clue you've given us is that you probably don't
> mean what camera makers usually mean by it, since I don't think anyone
> claims a virtual AF aperture as small as f11, and the f2.8 many do
> claim you tell us is definitely not what you mean.
>
> So what do you mean?

The same thing you were talking about ages ago. See figure 13 of
http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf.


From: Paul Furman on
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. Fig 13 shows an f/2.8 AF sensor
setup, so it's picking rays from near the edges of the lens and ignoring
the less accurate central part by blocking it out. There is still some
space outside the green virtual apertures though, and that's the f/1.8
region, so this system only sees the f/2.8 rays. Yeah, I know I'm
reading stuff into the diagram but that's the idea I've been trying to
convey about how if the AF system is designed for f/2.8, it won't be
helped by a faster f/1.8 lens. I don't even know if this is a problem,
maybe those virtual apertures do reach up to the edge of faster glass
and that's just simplification in the diagram but it could be an issue.

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.

--
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:
>>>>>>> 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. :- )

That makes sense but doesn't match your previous diagram:

(must use fixed width font or this jpeg:
http://edgehill.net/1/temp/Clipboard01.jpg)

sensor subject background
_________________________________________________________________
|< 430mm >+ sensor to subject distance for optimal focus
_________________________________________________________________
|< 430mm >+ starting from near-side focus
_________________________________________________________________
|< 433mm >+ far-side focus
_________________________________________________________________
| |< 3.0mm >| the "beep band"
_________________________________________________________________
| |< 1.5mm >| back-focus error from center of beep band


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

all google groups messages filtered due to spam