From: Chris Malcolm on
Wilba <usenet(a)cutthisimago.com.au> wrote:
> David J Taylor wrote:
>> Wilba wrote:
>> []
>>> The remaining challenge is to explain why the AF system confirms focus
>>> over such a wide range. The idea of asymmetrical bokeh makes sense, so
>>> the obvious thing to do now is compare the near side and far side bokeh
>>> of the 50/1.8.
>>
>> Already explained, I think. The AF sensor has an f/number of 2.8 (or
>> whatever, depending on the camera), not 1.8, so it may report a focussed
>> condition over a wider range.

> OMG! You don't think that the "f/2.8" in "f/2.8 high-precision AF sensor" is
> some measure of an effective aperture of the AF sensor?! It would certainly
> explain a lot of bizarre thinking if you did.

Bizarre thinking? As bizarre as the diagram of how phase contrast AF
works in this wiki entry? :-)

http://en.wikipedia.org/wiki/Autofocus

--
Chris Malcolm
From: Chris Malcolm on
David J Taylor <david-taylor(a)blueyonder.delete-this-bit.and-this-part.co.uk.invalid> wrote:
> "Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
> news:00cd4aac$0$15575$c3e8da3(a)news.astraweb.com...
> []
>> OMG! You don't think that the "f/2.8" in "f/2.8 high-precision AF
>> sensor" is some measure of an effective aperture of the AF sensor?! It
>> would certainly explain a lot of bizarre thinking if you did.

> I was not talking of any specific implementation, simply that a particular
> AF sensor will have a range of lens apertures with which it will work. If
> the lens aperture is too small (too large an f/number) the AF sensor
> doesn't work. If the lens aperture is too large (too small an f/number)
> the focus point indicated may not be correct if the lens has spherical
> aberration.

>> Let's be perfectly clear - the "f/2.8" in "f/2.8 high-precision AF
>> sensor" refers to when the camera switches over to using that sensor.
>> When a lens is mounted that has an f/2.8 or better maximum aperture, the
>> high-precision sensor is used rather than the less accurate one that
>> requires a maximum aperture of f/5.6 or better.

> OK, but that still tells you nothing about the actual cone of light used
> by the "large aperture" AF sensor.

It's not quite as simple as "cone of light".

http://en.wikipedia.org/wiki/Autofocus

As the diagram of phase contrast AF there shows, the middle of the
cone is missing from the AF. That shifts the effective average best
focus from what the whole cone would see. Careful matching of the size
of that missing middle cone with the size of the hole in the middle of
the aperture of a catadioptric lens is how Minolta/Sony manage to
autofocus their 500mm catadioptric lens despite it being an f/8.

--
Chris Malcolm
From: David J Taylor on
"Chris Malcolm" <cam(a)holyrood.ed.ac.uk> wrote in message
news:7qjskoF4fcU4(a)mid.individual.net...
[]
> It's not quite as simple as "cone of light".
>
> http://en.wikipedia.org/wiki/Autofocus
>
> As the diagram of phase contrast AF there shows, the middle of the
> cone is missing from the AF. That shifts the effective average best
> focus from what the whole cone would see. Careful matching of the size
> of that missing middle cone with the size of the hole in the middle of
> the aperture of a catadioptric lens is how Minolta/Sony manage to
> autofocus their 500mm catadioptric lens despite it being an f/8.
>
> --
> Chris Malcolm

Thanks for that, Chris. I was trying to keep things relatively simple to
aid discussion. Perhaps that wasn't wise.

Cheers,
David

From: Wilba on
David J Taylor wrote:
> Wilba wrote
> []
>> OMG! You don't think that the "f/2.8" in "f/2.8 high-precision AF sensor"
>> is some measure of an effective aperture of the AF sensor?! It would
>> certainly explain a lot of bizarre thinking if you did.
>
> I was not talking of any specific implementation, simply that a particular
> AF sensor will have a range of lens apertures with which it will work. If
> the lens aperture is too small (too large an f/number) the AF sensor
> doesn't work. If the lens aperture is too large (too small an f/number)
> the focus point indicated may not be correct if the lens has spherical
> aberration.

But do you know why a particular AF sensor will have a range of lens
apertures with which it will work? It seems to me that you're assuming that
it's fundamentally about matching an aperture through which the sensor
receives light. It absolutely isn't, it's about the base of the
rangefinder - essentially the distance between the two elements of the AF
sensor.

Here's a photo of the AF chip fitted to my camera -
http://a.img-dpreview.com/reviews/canoneos450d/images/whatsnew/canon_450d_afchip.jpg

See the four bars in a row through the centre? In discussions with others,
we believe that the inner two bars form the "f/5.6" standard-precision
sensor, and the outer two form the "f/2.8" high-precision sensor.

When talking about AF sensors, "f/5.6" and "f/2.8" refer ONLY to the
apertures of the lenses appropriate for each sensor. I assume that an f/5.6
lens can illuminate both elements of the "f/5.6" sensor, but can't
illuminate both elements of the "f/2.8" sensor in the way it needs to work.
AFAIK, that's all there is to it.

>> Let's be perfectly clear - the "f/2.8" in "f/2.8 high-precision AF
>> sensor" refers to when the camera switches over to using that sensor.
>> When a lens is mounted that has an f/2.8 or better maximum aperture, the
>> high-precision sensor is used rather than the less accurate one that
>> requires a maximum aperture of f/5.6 or better.
>
> OK, but that still tells you nothing about the actual cone of light used
> by the "large aperture" AF sensor.

Exactly. AFAIK it's irrelevant in the way you mean.

>> Getting back the idea you've just introduced, for me to make any sense of
>> it you'd have to give me a clue to _how_ the AF sensor might confirm a
>> focus over a wider range because of that difference.
>
> You already answered your own question - the high-precision sensor (in
> general terms) will have more light to work with, ...

So will the standard-precision AF sensor.

> ... and a larger cone of light coming from the lens.

I don't accept that assumption, since it seems to be based on a fanciful
notion.

> Thus if the other system parameters including the hysteresis are the same,
> the range of "in-focus" indication can be narrower.

But you said focus confirmation would be wider if the lens aperture is wider
than f/2.8. Now you're saying it's narrower?

> You would need to check the exact details of Canon's system for a more
> detailed answer.

I have. AFAICT you're lost in a bizarre fantasy based on a terrible
misunderstanding. :- )


From: Wilba on
Chris Malcolm wrote:
> Wilba wrote:
>> David J Taylor wrote:
>>> Wilba wrote:
>>> []
>>>> The remaining challenge is to explain why the AF system confirms focus
>>>> over such a wide range. The idea of asymmetrical bokeh makes sense, so
>>>> the obvious thing to do now is compare the near side and far side bokeh
>>>> of the 50/1.8.
>>>
>>> Already explained, I think. The AF sensor has an f/number of 2.8 (or
>>> whatever, depending on the camera), not 1.8, so it may report a focussed
>>> condition over a wider range.
>
>> OMG! You don't think that the "f/2.8" in "f/2.8 high-precision AF sensor"
>> is
>> some measure of an effective aperture of the AF sensor?! It would
>> certainly
>> explain a lot of bizarre thinking if you did.
>
> Bizarre thinking? As bizarre as the diagram of how phase contrast AF
> works in this wiki entry? :-)
>
> http://en.wikipedia.org/wiki/Autofocus

LOL - you guys are seriously ****-up. :- )

This document - http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf -
describes your idea as an "artifice" ("Getting real", p. 7), since it
doesn't actually exist.

In "Aperture dependency" (p. 17) the author describes the relationship
between the position of the two "virtual AF apertures" (note: virtual = not
in actual fact, form, or name; existing in the mind, especially as a product
of the imagination), and the lens aperture and exit pupil. That is the basis
of the "f/2.8" and "f/5.6" designations for the AF different AF sensors -
denoting only which lenses are compatible with which sensor, because of the
distance between the elements comprising each AF sensor.