From: David J Taylor on
"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
news:034d5d44$0$1331$c3e8da3(a)news.astraweb.com...
[]
> But how do you get aperture-related focus shift if you're not stopping
> down for the exposure?

Focus sensors use the rays from the centre portion of the ray bundle
coming out of the back end of the lens (effectively f/2.8 or f/4.0 - for
example), and may therefore focus on a different axial location than where
the rays from the extreme of the lens (f/1.8, say) converge, if the lens
has spherical aberrations.

David


From: David J Taylor on
"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
news:034d2bde$0$1369$c3e8da3(a)news.astraweb.com...
[]
> 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?

I already explained that the focus sensor may only have a working aperture
of f/4, for example, so the aperture /is/ different.

David

From: Doug McDonald on

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

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.

Doug McDonald
From: Wilba on
David J Taylor wrote:
> Wilba wrote:
> []
>> But how do you get aperture-related focus shift if you're not stopping
>> down for the exposure?
>
> Focus sensors use the rays from the centre portion of the ray bundle
> coming out of the back end of the lens (effectively f/2.8 or f/4.0 - for
> example), and may therefore focus on a different axial location than where
> the rays from the extreme of the lens (f/1.8, say) converge, if the lens
> has spherical aberrations.

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?


From: Wilba on
David J Taylor wrote:
> Wilba wrote:
> []
>> 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?
>
> I already explained that the focus sensor may only have a working aperture
> of f/4, for example, so the aperture /is/ different.

That's nothing to do with the only form of aperture-related focus shift that
I'm familiar with - http://diglloyd.com/diglloyd/free/FocusShift/index.html.

If you are talking about some other phenomenon, please provide a link to a
good description.

I can't see how what you're talking about would be relevant anyway, since
the AF system is calibrated to produce a good focus under exactly those
conditions.