From: David J Taylor on
"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
news:03565acd$0$1367$c3e8da3(a)news.astraweb.com...
[]
> 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.
As Chris pointed out, the extreme rays are those from the edges of the
exit pupil, not those at the edge of the image.

> 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. 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.

Cheers,
David

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: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.

I have a 50mm f/1.4. I first noticed AF problems when doing low light
portrait shots in pubs, cafes, etc.. If I focussed on the eyes they'd
be out of focus, but the ears would be really sharp. The AF at wide
apertures was very reliably and consistently backfocusing. That's what
started me down the long learning curve of discovering how the AF
worked, and eventually to recalibrating my camera's AF.

--
Chris Malcolm
From: David J Taylor on
"Chris Malcolm" <cam(a)holyrood.ed.ac.uk> wrote in message
news:7qoevbFc72U1(a)mid.individual.net...
[]
> I have a 50mm f/1.4. I first noticed AF problems when doing low light
> portrait shots in pubs, cafes, etc.. If I focussed on the eyes they'd
> be out of focus, but the ears would be really sharp. The AF at wide
> apertures was very reliably and consistently backfocusing. That's what
> started me down the long learning curve of discovering how the AF
> worked, and eventually to recalibrating my camera's AF.
>
> --
> Chris Malcolm

Most interesting, Chris. Did you find that the focussing problems varied
with light level? I can see that I will have to think about testing my
own lens (Nikon 35mm f/1.8). Not that you would use it in low-light
situations, but one suggestion I've seen is to compare auto-focus with the
phase-detect sensor, and auto-focus using LiveView.

I do notice that with my small-aperture zoom lenses you need to be more
careful in low light levels (pre-dawn) to get an accurate focus, but I'd
put this down simple to lack of light on the AF sensors, rather than any
back-focus issue.

Cheers,
David

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:7qoevbFc72U1(a)mid.individual.net...
> []
>> I have a 50mm f/1.4. I first noticed AF problems when doing low light
>> portrait shots in pubs, cafes, etc.. If I focussed on the eyes they'd
>> be out of focus, but the ears would be really sharp. The AF at wide
>> apertures was very reliably and consistently backfocusing. That's what
>> started me down the long learning curve of discovering how the AF
>> worked, and eventually to recalibrating my camera's AF.
>>
>> --
>> Chris Malcolm

> Most interesting, Chris. Did you find that the focussing problems varied
> with light level?

Not much. In low light I found the AF would struggle longer to find
focus, and often fail to find focus. But when it found focus it was
usually as accurate as good light focus.

> I can see that I will have to think about testing my
> own lens (Nikon 35mm f/1.8). Not that you would use it in low-light
> situations, but one suggestion I've seen is to compare auto-focus with the
> phase-detect sensor, and auto-focus using LiveView.

Compare autofocus running in from infinity to point of focus, running
out from very close focus to the same point, and using manual focus
with AF confirmation running in from both directions. If you also have
live view contrast based focus then try approaching that from both
directions too. I found that sometimes one direction would give a much
better focus than the other, especially where the focal length was
short. It's easy to sway several mm back and forth when squinting
through a viewfinder because it has no obvious effect on the
image. But it will affect the AF so use a tripod.

Photographing a clothes peg or large paper clip clipped to a ruler
slanting away from the camera is a good simple way of seeing DoF and
point of sharpest focus.

When you think you've identified a misfocusing problem do several
repeated tests to discover the reliability, and to make sure you've
not been misled by random variations.

If your lens has focus distance markers on it you might like to check
them for accuracy too, while you're about it :-)

--
Chris Malcolm
From: Wilba on
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. And still you don't answer direct questions. Makes it look like you
don't know what you're talking about. :- )