From: Wilba on
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?


From: Wilba on
Chris Malcolm wrote:
> 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 answer was) "The same thing you were talking about ages ago. See figure
13 of
http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf."

> That document is the only one I've seen to use "virtual AF aperture"
> with that meaning.

Give me some references to other documents that give "virtual AF aperture" a
different meaning and explain what that is.

How about you explain what you mean by "effective aperture", e.g. "The AF
sensors... their construction gives them an effective aperture of their
own", and how that's different to Kerr's virtual AF aperture. Please refer
to the elements of Kerr's figure 13
(http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf),
http://en.wikipedia.org/wiki/File:US_pat_5589909_fig_2.png, or similar if
that helps.

> But if that's what you want then the next question is what useful
> information you want to derive from that information. For example,
> if it's virtual aperture with respect to comparable light gathering
> power then it will need to be adjusted for the (very approximately)
> half silvered mirror loss.

The only reason I'm talking about it is to explain that the "f/2.8" in
"f/2.8 high-precision AF sensor" is not the size of Kerr's virtual AF
aperture (AFAIK). If you have any evidence to the contrary, please present
it.


From: Paul Furman on
Wilba wrote:
>
> The only reason I'm talking about it is to explain that the "f/2.8" in
> "f/2.8 high-precision AF sensor" is not the size of Kerr's virtual AF
> aperture (AFAIK).

Right.

--
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:
>>>> 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.

--
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:
>>>>>>> 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.


>> ... and in my home made experiment, I placed them out in front of the
>> lens. I'm doing it a little different but it works.
>> http://www.flickr.com/photos/edgehill/4271241371/in/photostream/
>>
>>> they just define the areas on an ideal lens through which passes the
>>> light that ends up reaching the AF sensor. As I understand it, what light
>>> through yonder window breaks is determined more by the AF prisms than
>>> anything else (see figure 9 and the two paragraphs above it). You have to
>>> start at the sensor and work back, not the other way around.
>>>
>>>> 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.
>>> Again, those rays aren't blocked by an aperture as such - if they reach
>>> the prisms they don't end up shining on the AF sensor.
>>>
>>>> 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.
>>> It makes more sense to me this way - the system only needs an f/2.8 exit
>>> pupil to illuminate the "f/2.8" AF sensor. An f/1.8 lens has a larger
>>> exit pupil than required, it has "excess capacity". :- )
>>>
>>>> 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.
>>> Sure, still for me the preferred connotation is that the "f/2.8" AF
>>> sensor _requires_ an f/2.8 (or better) lens for effective illumination,
>>> rather than there being a sense of optimisation to work well only with
>>> lenses of exactly f/2.8.
>>>
>>> Did you read all the way through Kerr's article? The very last section
>>> sums it up nicely. (And AFAIK, the 20D system is a direct ancestor of the
>>> 450D system, so I would assume that the principles hold for both.)
>>>
>>>> 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.
>>> No, I'm sure they snuggle up to the size of an f/2.8 exit pupil. An f/1.8
>>> exit pupil would simply be larger than that.
>
>


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

all google groups messages filtered due to spam