From: Wilba on
David J Taylor wrote:
> Wilba wrote:
>> []
>> Na, I don't buy it. I can't find any write up apart from speculation in
>> this thread (which seems to me to be based on a misunderstanding
>> of the original mention of the effective aperture of the AF sensor),
>> that describes what you're talking about in relation to aperture-related
>> focus shift. Can you give me some references?
>
> I already pointed you to the Wikipedia article showing how lens
> aberrations could cause a shift of focal point as the f/number was
> changed:
>
> http://en.wikipedia.org/wiki/Spherical_aberration
>
> Look at this diagram:
>
> http://upload.wikimedia.org/wikipedia/commons/9/92/Spherical_aberration_2.svg
> and see how the rays from the edge of the lens are brought to a
> focus nearer the lens than those from the centre part of the lens.
> Of course, a good lens design will try to minimise this effect.
> Depending on the exact lens design, the shift of minimum circle
> of confusion as the aperture changes may be either towards
> or away from that at smaller apertures.

So you're saying that changing the aperture for the exposure can cause the
focus to shift with a spherically-aberrant lens. Lovely. Wonderful. That's
aperture-related focus shift as we know it
(http://diglloyd.com/diglloyd/free/FocusShift/index.html).

So I ask the exact same question again - if there is no, N-O, none, nil,
nought, and zero aperture change, how then can you get a focus shift due to
this phenomenon?

If you don't understand the question, ask one. :- )

If you are talking about some other kind of aperture change or focus shift,
I need a name and references to _that_. Thanks.

> Remember that the auto-focus on some cameras can be open-loop
> rather than closed loop, and therefore subject to varying degrees of
> error. The sensor says the focus is off by so much, the firmware
> says move the lens by a certain amount, the lens is moved, and that's
> that. There's no second check to see how much error remains.
> In continuous-focus mode rather than single-shot-focus mode,
> performance may differ again.

This is a closed-loop system, so not relevant.


From: David J Taylor on
"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
news:00c9e335$0$15627$c3e8da3(a)news.astraweb.com...
[]
> So you're saying that changing the aperture for the exposure can cause
> the focus to shift with a spherically-aberrant lens. Lovely. Wonderful.
> That's aperture-related focus shift as we know it
> (http://diglloyd.com/diglloyd/free/FocusShift/index.html).

Correct.

> So I ask the exact same question again - if there is no, N-O, none, nil,
> nought, and zero aperture change, how then can you get a focus shift due
> to this phenomenon?

If the focus sensors have a different f/number than that of the lens, they
may focus at the incorrect position, i.e. where the convergence of the ray
bundle that the focus sensors can see is minimum.


> This is a closed-loop system, so not relevant.

It may not be closed loop if you rely on the auto-focus and don't use
continuous mode. If you are using manual focus there is still any
backlash in the system.

Cheers,
David

From: Wilba on
David J Taylor wrote:
> Wilba wrote:
> []
>> So you're saying that changing the aperture for the exposure can cause
>> the focus to shift with a spherically-aberrant lens. Lovely. Wonderful.
>> That's aperture-related focus shift as we know it
>> (http://diglloyd.com/diglloyd/free/FocusShift/index.html).
>
> Correct.
>
>> So I ask the exact same question again - if there is no, N-O, none, nil,
>> nought, and zero aperture change, how then can you get a focus shift due
>> to this phenomenon?
>
> If the focus sensors have a different f/number than that of the lens, they
> may focus at the incorrect position, i.e. where the convergence of the ray
> bundle that the focus sensors can see is minimum.

OK, again, there is no aperture _change_ in that idea, only a static
difference, and I can't see how, if that difference were significant, it
would not be dealt with by calibration.

Again, since that idea has nothing to do with aperture-related focus shift
as we know it (http://diglloyd.com/diglloyd/free/FocusShift/index.html), I
need a name and references for that phenomenon.

>> This is a closed-loop system, so not relevant.
>
> It may not be closed loop if you rely on the auto-focus and don't use
> continuous mode.

It is.

> If you are using manual focus there is still any backlash in the system.

Irrelevant in this case.


From: David J Taylor on
"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
news:00ca9eeb$0$15649$c3e8da3(a)news.astraweb.com...
[]
> OK, again, there is no aperture _change_ in that idea, only a static
> difference, and I can't see how, if that difference were significant, it
> would not be dealt with by calibration.

I don't know the Canon system, but I would imagine that you could
calibrate out such a difference.


>>> This is a closed-loop system, so not relevant.
>>
>> It may not be closed loop if you rely on the auto-focus and don't use
>> continuous mode.
>
> It is.
>
>> If you are using manual focus there is still any backlash in the
>> system.
>
> Irrelevant in this case.


Unless the focus point is infinitely small, there is a dead-zone between
the in-focus indication turning on and it disappearing. You might expect
to find the the correct point of focus somewhere between those two points.

As we seem to have drifted into things which you say are irrelevant,
perhaps a succinct restatement of your original problem might help.

Cheers,
David

From: Chris Malcolm on
Wilba <usenet(a)cutthisimago.com.au> wrote:
> David J Taylor wrote:
>> Wilba wrote:
>>> []
>>> Na, I don't buy it. I can't find any write up apart from speculation in
>>> this thread (which seems to me to be based on a misunderstanding
>>> of the original mention of the effective aperture of the AF sensor),
>>> that describes what you're talking about in relation to aperture-related
>>> focus shift. Can you give me some references?
>>
>> I already pointed you to the Wikipedia article showing how lens
>> aberrations could cause a shift of focal point as the f/number was
>> changed:
>>
>> http://en.wikipedia.org/wiki/Spherical_aberration
>>
>> Look at this diagram:
>>
>> http://upload.wikimedia.org/wikipedia/commons/9/92/Spherical_aberration_2.svg
>> and see how the rays from the edge of the lens are brought to a
>> focus nearer the lens than those from the centre part of the lens.
>> Of course, a good lens design will try to minimise this effect.
>> Depending on the exact lens design, the shift of minimum circle
>> of confusion as the aperture changes may be either towards
>> or away from that at smaller apertures.

> So you're saying that changing the aperture for the exposure can cause the
> focus to shift with a spherically-aberrant lens. Lovely. Wonderful. That's
> aperture-related focus shift as we know it
> (http://diglloyd.com/diglloyd/free/FocusShift/index.html).

> So I ask the exact same question again - if there is no, N-O, none, nil,
> nought, and zero aperture change, how then can you get a focus shift due to
> this phenomenon?

You keep quoting that diglloyd document. It goes into aperture related
focus shift when focussing manually. It doesn't consider the focus
shift problems introduced by AF mechanisms and the fact that they have
fixed apertures which are usually smaller than the max aperture of the
lens. It suggests that if you want to know about that kind of thing
you should pay money to get the fuller more detailed explanatory
document.

> If you are talking about some other kind of aperture change or focus shift,
> I need a name and references to _that_. Thanks.

It's exactly the same kind of focus shift, introduced by the factor
not specifically mentioned in the diglloyd document: the difference
which usually exists between the fixed effective aperture of an AF
phase detection sensor, and the lens aperture which the image exposure in
made at.

>> Remember that the auto-focus on some cameras can be open-loop
>> rather than closed loop, and therefore subject to varying degrees of
>> error. The sensor says the focus is off by so much, the firmware
>> says move the lens by a certain amount, the lens is moved, and that's
>> that. There's no second check to see how much error remains.
>> In continuous-focus mode rather than single-shot-focus mode,
>> performance may differ again.

> This is a closed-loop system, so not relevant.

But the loop is closed on an AF phase detect sensor with a fixed
aperture which is usually less than the widest aperture of the lens in
use.

--
Chris Malcolm