From: Wilba on
Chris Malcolm wrote:
>
> 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.

In my experience (with a static camera and subject), contrast detect AF
can't be beat until you get close enough to start focussing by moving the
camera. Then Live View manual focus is the way to go.


From: Wilba on
Chris Malcolm wrote:
> Wilba 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?

Yes.

> 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.

Yes.

> The rays which pass through all parts of the lens contribute to the
> formation
> of all parts of the image.

Yes.

Waaay-back someone said that focussing with an f/1.8 lens using an "f/2.8"
AF sensor could explain the results I got in the tests that followed the OP.
I've yet to see a credible description of how that could be. I'm still here
because even if that never comes, I'll learn _something_ useful.


From: Wilba on
David J Taylor wrote:
> Wilba wrote:
> []
>> Um... I don't really care where the extreme rays focus as long as it's
>> sharp in the centre.
>
> Well you would, should the extreme rays be used (such as by the "f/2.8" AF
> sensor, and if those rays don't focus at the same point as the main image.

But the fact remains that I _do_ get images in which the the centre is well
focussed, so why should I care that they might, theoretically, not be?

> As Chris pointed out, the extreme rays are those from the edges of the
> exit pupil, not those at the edge of the image.

Correct. I was referring to the image not the exit pupil.

>> 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?
>
> I would hope that an f/1.8 lens from a reputable manufacturer would have
> very little spherical aberration, rendering the effects we have been
> discussing to be very small, so they will be present but all being well
> below the limits you can observe in normal circumstances.

So why should I care?! :- )

Does this effect explain my test results? (I haven't seen anything so far
that tells me so.) Therefore, do I need to modify my test procedure to
control for this effect? (That's the bottom line.)

> As you know, that lens is quite soft even in the centre of the image at
> full
> aperture:
>
> http://www.dpreview.com/lensreviews/canon_50_1p8_ii_c16/page3.asp
>
> and likely that softness would mask the effects I am discussing.

Both effects have the same cause - spherical aberation, right? So they're
inextricably linked...


From: Wilba on
David J Taylor wrote:
> Wilba wrote:
>> David J Taylor wrote:
>>>
>>> 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.
>
> The focus system uses /only/ the rays which are not coming from the
> centre of the lens exit pupil. These are more likely to be the rays
> showing spherical aberration.

Yes, but I'm thinking the "f/2.8" AF sensor is only going to see the rays
that it would see from an f/2.8 exit pupil, so it doesn't see the rays from
the extremes of the f/1.8 exit pupil. 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."
>
> To expand on what I wrote:
>
> 1 - "I would expect the "f/2.8" precision focus sensor to have a smaller
> range of "in-lock" indication [than the "f/5.6" standard focus sensor]."

Correct. Canon say their "f/5.6" AF sensors should focus "within the DOF",
and the "f/2.8" sensors should focus "within the inner 1/3 of the DOF".

> 2 - "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
> [than a focus sensor optimised for an f/1.8 lens]."

This bit, "[than a focus sensor optimised for an f/1.8 lens]", is crucial to
understanding what you meant. :- )


From: Chris Malcolm on
Wilba <usenet(a)cutthisimago.com.au> wrote:
> Chris Malcolm wrote:
>> Wilba wrote:
>>> 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.
>>
>> But it's a useful simplifying fiction for those learning about this
>> stuff for the first time, just like Newtonian physics or the earth
>> being a sphere. You started out this discussion not wanting to go into
>> more details than necessary, and not wanting to have to understand
>> anything not strictly necessary to answer your specific questions. You
>> specifically asked for explanations to be given in simple terms as
>> though to someone who was completely new to the topic.
>>
>> But you're now criticising the explanations you were given because
>> they used used various "in effect" "as it were" "virtually"
>> etc. didactic simplifications.
>>
>> You can't have it both ways. Either you must make the effort to
>> understand the full technical complexities, or you've got to accept
>> the simplfying fictions which give a good general idea of what is
>> going on.

> That's what happens when you won't answer straight-forward questions -
> people start performing backflips to try to get anything like an answer out
> of you.

You're making the invalid assumption that all direct questions have a
sensible answer.

> And still you don't answer direct questions. Makes it look like you
> don't know what you're talking about. :- )

An appearance that obviously doesn't deceive any of those who're
performing backflips to try and get an answer :-)

--
Chris Malcolm