From: Wilba on
Doug McDonald wrote:
> Wilba wrote:

>>> With spherical aberration, for example, rays from the edge of the lens
>>> (i.e. full aperture) focus at a different point from those from the
>>> middle part of the lens, so the focus "point" can shift with aperture.
>>> The focus sensors tend to have a narrower acceptance angle than f/1.8,
>>> hence they will adjust the lens so that rays from nearer the centre will
>>> be focussed, leaving the outer rays focussing at a different, incorrect
>>> position.
>>
>> Sure. So when the aperture *doesn't* change (focus and exposure both
>> occur at the same aperture), how do you get focus shift in the centre of
>> the image?
>
> At least in Canon cameras, it is because the focus sensors use at most
> the light of an f/2.8 lens.

Do you mean that you don't get any advantage from an aperture wider than
f/2.8?

I'd say what I'm thinking more like, a Canon high-precision AF sensor needs
f/2.8 or better to match its "base" (as in a rangefinder).

> IF this is the problem cause, the focus should be
> correct at f/2.8 using the central cross autofocus sensor.

Depends what problem you're talking about. I can't imagine how it could
matter to this problem -
http://diglloyd.com/diglloyd/free/FocusShift/index.html. If you're talking
about a different problem, please give me a link to a description so that I
have a clue what it is.

> TYPICALLY, for Gauss lenses like this one, no aspheric elements,
> the central part of the lens (say that part used at f/11) and the very
> outer part focus at the same distance. It is the part at one f-stop from
> maximum (i.e. in this lens near f/2.8) that suffers worst from focus
> shift.
> Thus the problem.
>
> However, this is a small problem.

What _is_ the problem? :- )


From: David J Taylor on
"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
news:034e7f2b$0$1328$c3e8da3(a)news.astraweb.com...
[]
> Let me rephrase my question for clarity -
>
> How do you get aperture-related focus shift
> (http://diglloyd.com/diglloyd/free/FocusShift/index.html), in the centre
> of the image, if you're not stopping down for the exposure?

... because the focus sensor /is/ stopping down when measuring the focus.
It's only looking at a small f/4 (or whatever) cone, not the full f/1.8
cone from the lens. Were you to stop down when taking to the cone the
focus sensor is using, the focus would then be exactly as the focus sensor
measured. Use a larger ray bundle when taking, i.e. by using an f/1.8
lens at full aperture, the rays may focus at a different point, and the
resulting image on the sensor may appear out of focus. Likely to be a
small effect unless the lens has excessive aberrations and the ratio
between focus f/number and taking f/number is significant.

The calibration of the AF system will be at a certain aperture (possibly
more than one aperture on more expensive cameras). Use a different lens
aperture and you are at the mercy of how much the lens focal point shifts
when it works at a different aperture to that which the focus sensor uses.

Cheers,
David

From: Wilba on
David J Taylor wrote:
> Wilba wrote:
>> []
>> How do you get aperture-related focus shift
>> (http://diglloyd.com/diglloyd/free/FocusShift/index.html), in the centre
>> of the image, if you're not stopping down for the exposure?
>
> .. because the focus sensor /is/ stopping down when measuring the focus.
> It's only looking at a small f/4 (or whatever) cone, not the full f/1.8
> cone from the lens. Were you to stop down when taking to the cone the
> focus sensor is using, the focus would then be exactly as the focus sensor
> measured. Use a larger ray bundle when taking, i.e. by using an f/1.8
> lens at full aperture, the rays may focus at a different point, and the
> resulting image on the sensor may appear out of focus. Likely to be a
> small effect unless the lens has excessive aberrations and the ratio
> between focus f/number and taking f/number is significant.

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?

> The calibration of the AF system will be at a certain aperture (possibly
> more than one aperture on more expensive cameras). Use a different lens
> aperture and you are at the mercy of how much the lens focal point shifts
> when it works at a different aperture to that which the focus sensor uses.

Yep, so if the AF system is calibrated at f/1.8 (as mine is), and I expose
at f/1.8 (as I do), I won't get any shift (as I don't). :- )


From: Paul Furman on
Wilba wrote:
> David J Taylor wrote:
>> Wilba wrote:
>>> []
>>> How do you get aperture-related focus shift
>>> (http://diglloyd.com/diglloyd/free/FocusShift/index.html), in the centre
>>> of the image, if you're not stopping down for the exposure?
>> .. because the focus sensor /is/ stopping down when measuring the focus.
>> It's only looking at a small f/4 (or whatever) cone, not the full f/1.8
>> cone from the lens. Were you to stop down when taking to the cone the
>> focus sensor is using, the focus would then be exactly as the focus sensor
>> measured. Use a larger ray bundle when taking, i.e. by using an f/1.8
>> lens at full aperture, the rays may focus at a different point, and the
>> resulting image on the sensor may appear out of focus. Likely to be a
>> small effect unless the lens has excessive aberrations and the ratio
>> between focus f/number and taking f/number is significant.
>
> 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?

overview:
http://www.sensorcleaning.com/whatisanslr.php

detail of pro canon AF system (awfully complicated):
http://a.img-dpreview.com/reviews/CanonEOS5DMarkII/Images/features/af04.gif

more detailed explanation:
http://community.the-digital-picture.com/forums/t/2012.aspx
"Many cameras have a special autofocus sensor that becomes active when
the maximum f-number of the lens is f/2.8 or faster. It has better
performance than the normal autofocus sensors because it has a wider
baseline, as illustrated in this diagram from the Canon 1D3 manual:"

more on the last page of their pdf link:
http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf

Perhaps your 450D lacks this extra sensor, the pdf says the 20D has it.


>> The calibration of the AF system will be at a certain aperture (possibly
>> more than one aperture on more expensive cameras). Use a different lens
>> aperture and you are at the mercy of how much the lens focal point shifts
>> when it works at a different aperture to that which the focus sensor uses.
>
> Yep, so if the AF system is calibrated at f/1.8 (as mine is), and I expose
> at f/1.8 (as I do), I won't get any shift (as I don't). :- )

There is a shift in your tests, it's the other direction though if I was
following correctly. If it turns out to be a more or less random error,
it could be the lack of extra AF sensor for wide lenses.

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

all google groups messages filtered due to spam
From: David J Taylor on
"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
news:00c99031$0$15590$c3e8da3(a)news.astraweb.com...
[]
> 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.svgand see how the rays from the edge of the lens are brought to a focusnearer the lens than those from the centre part of the lens. Of course, agood lens design will try to minimise this effect. Depending on the exactlens design, the shift of minimum circle of confusion as the aperturechanges may be either towards or away from that at smaller apertures.Remember that the auto-focus on some cameras can be open-loop rather thanclosed loop, and therefore subject to varying degrees of error. Thesensor says the focus is off by so much, the firmware says move the lensby a certain amount, the lens is moved, and that's that. There's nosecond check to see how much error remains. In continuous-focus moderather than single-shot-focus mode, performance may differ again.Cheers,David