From: Chris Malcolm on
David J Taylor <david-taylor(a)blueyonder.delete-this-bit.and-this-part.co.uk.invalid> wrote:

> "Chris Malcolm" <cam(a)holyrood.ed.ac.uk> wrote in message
> news:7qmc4fF4csU1(a)mid.individual.net...

>> You're right that the "f2.8" AF sensor will be *capable* of higher
>> precision focus because of what in effect is the wider baseline of its
>> rangefinding phase contrasting. But can it safely *use* that higher
>> precision? One of the things that calibrated lenses with on chip
>> information can tell the camera is just how sharply they should be
>> focussed, i.e., what maximum size of focus error still counts as being
>> in focus. That's what sets the range of manual focus that gives the AF
>> confirmation "green light". A very sharp lens can use a small value,
>> which if used with a softer lens would result in it never being able
>> to focus.

> Accepted.

>> But there's a nastier problem than that. The higher precision AF
>> sensors selectively use rays refracted from the edges of a wider
>> lens. And some lenses are sharp in the middle, but soft at the edges
>> (I'm referring to the middle and edges of the lens, not the image). So
>> the AF sensor is limited in the precision of focus it can exercise by
>> how good the edges of the lens are, which are often not as good
>> as the middle.
>>
>> This problem is at its worst in the ultra wide angle rectilinear
>> perspective projection zoom lenses. They're very difficult optical
>> designs, and the edges of the lens are often a lot more optically
>> aberrant than the centre. So phase contrast AF sensors often have
>> unusual difficulty accurately focusing the shortest focal lengths of
>> these lenses.
>>
>> --
>> Chris Malcolm

> Thanks for that further information, Chris. It would seem that, for best
> results, you should have the option of excluding the wide-aperture focus
> sensors under certain conditions. Do any cameras provide this option?

I've never heard of such an option. My guess would be that they'd be
more likely to include it as a feature automatically invoked by the AF
algorithm under appropriate circumstances, since knowing all the lens
details provided by modern lenses to the camera should have all the
required information. And our modern DSLR makers don't seem to be too
concerned if their cameras rather snobbishly refuse to co-operate
intelligently with older less communicative lenses, or third party
lenses.

> Mine doesn't have dual AF sensors so the issue doesn't arise.

Nor mine. And since I have a 50mm f1.4 lens I missed the high
precision wide aperture sensor. But now that I have a camera with very
easy extremely precise max magnification live view manual focus (Sony
A550) I'm now not sure that's not even better to have than higher
precision wide aperture AF.

--
Chris Malcolm
From: David J Taylor on
"Chris Malcolm" <cam(a)holyrood.ed.ac.uk> wrote in message
news:7qmp91FgicU1(a)mid.individual.net...
[]
> I've never heard of such an option. My guess would be that they'd be
> more likely to include it as a feature automatically invoked by the AF
> algorithm under appropriate circumstances, since knowing all the lens
> details provided by modern lenses to the camera should have all the
> required information. And our modern DSLR makers don't seem to be too
> concerned if their cameras rather snobbishly refuse to co-operate
> intelligently with older less communicative lenses, or third party
> lenses.

Agreed, but what with Canon being forced to include the facility to
calibrate lenses, I thought that might have needed to include even further
kludges....

>> Mine doesn't have dual AF sensors so the issue doesn't arise.
>
> Nor mine. And since I have a 50mm f1.4 lens I missed the high
> precision wide aperture sensor. But now that I have a camera with very
> easy extremely precise max magnification live view manual focus (Sony
> A550) I'm now not sure that's not even better to have than higher
> precision wide aperture AF.
>
> --
> Chris Malcolm

I now have an f/1.8 lens, but the circumstances where I'm likely to use
that will be more for its lower-light capability, when the ability to
capture /anything/ may be more important than precise focus. Perhaps
after a few months' use I will be able to confirm or refute that.

Cheers,
David

From: Wilba on
David J Taylor wrote:
> Wilba wrote:
> []
>> 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.
>
> What he is saying is that the "f/2.8" AF sensor will not respond to rays
> form a greater exit pupil than "a typical f/2.8 lens". Hence it is quite
> possible that the extreme rays from an f/1.8 lens will not be used by the
> AF sensor, and hence if those extreme rays focus at a different point than
> the central rays, the focus point indicated by the AF sensor from the more
> central rays will be incorrect.

Um... I don't really care where the extreme rays focus as long as it's sharp
in the centre.

You can try to explain why my lens should not focus well in as many ways as
you like, but the fact is it does. Can you explain why your theoretical
predictions fail to come true?


From: Wilba on
David J Taylor wrote:
> Wilba wrote:
> []
>> 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.
>
> According to your reference:
>
> http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf
>
> it's about the exit pupil of the imaging lens.

That's another way to say it, yes, the two are directly related as I
understand it.

>> 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.
>
> If your assumptions are correct, the point is that there is light from the
> f/1.8 lens which is not used for the focus measurement, and the fact that
> this unused light come from the extremes of the lens ray bundle is
> critical should the lens have significant spherical aberration.

I don't get why is it critical. You're saying that the focus system won't
use the aberant rays? That sounds like a good thing to me.

Anyway, we all accept that spherical aberation creates a challenge for AF
systems. If that is corrected for (as Canon do with that kind of lens), and
a particular unit is calibrated to perfection to remove any error of that
kind (as mine is), what's the problem?

>> 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. :- )
>
> It sounds as if you may be confusing narrower and wider with f/numbers
> being bigger or smaller.

No.

> I would expect the "f/2.8" precision focus sensor to have a smaller range
> of "in-lock" indication. I have never said otherwise.

You said -

"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."


From: Chris Malcolm on
Wilba <usenet(a)cutthisimago.com.au> wrote:
> David J Taylor wrote:
>> Wilba wrote:
>> []
>>> 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.
>>
>> What he is saying is that the "f/2.8" AF sensor will not respond to rays
>> form a greater exit pupil than "a typical f/2.8 lens". Hence it is quite
>> possible that the extreme rays from an f/1.8 lens will not be used by the
>> AF sensor, and hence if those extreme rays focus at a different point than
>> the central rays, the focus point indicated by the AF sensor from the more
>> central rays will be incorrect.

> Um... I don't really care where the extreme rays focus as long as it's sharp
> in the centre.

Are you talking about the centre and edges of the image? That's quite
a different thing from the rays which pass through the centre and
edges of the lens, which is what this AF discussion is about. The rays
which pass through all parts of the lens contribute to the formation
of all parts of the image.

--
Chris Malcolm