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Longfellow wrote:

> Like all(?) DSLRs, the Canon's have a configurable autofocus point
> assortment, where (I gather) a common configuration is to have them all
> functional at once. Now, I'm not the dimmest bulb in the room, but I
> could never figure out why that configuration might have value; there
> was no way that I could see of controlling where the point of focus was
> going to be.

I only use that mode with AI Servo for subjects in motion. Why Canon makes
their DSLRs use all focus points in the basic modes is a mystery, but it
sure answers a lot of "why are my photos not sharp?" questions.

> Now, sez I, it can't really be that simple, so what am I missing here?
> Can't figure out what I'm not understanding about this, so I'm asking
> the question here.
>
> What am I missing about all this?

Nothing. :- )

The only thing I'd add for completeness is the recommendation to use a
single AF point other than the centre point when DOF is tight enough that
recomposition might put the subject outside of it.

Longfellow wrote:
> Wilba wrote:
>> Longfellow wrote:
>>>
>>> Like all(?) DSLRs, the Canon's have a configurable autofocus point
>>> assortment, where (I gather) a common configuration is to have them all
>>> functional at once. Now, I'm not the dimmest bulb in the room, but I
>>> could never figure out why that configuration might have value; there
>>> was no way that I could see of controlling where the point of focus was
>>> going to be.
>>
>> I only use that mode with AI Servo for subjects in motion. Why Canon
>> makes their DSLRs use all focus points in the basic modes is a mystery,
>> but it sure answers a lot of "why are my photos not sharp?" questions.
>
> I didn't know that Canon's basic modes had a default autofocus point
> pattern.

More than default - fixed, not adjustable, no choice. If I could I'd delete
maybe 80% of the functions of my 450D. Basically, give me an electronic
version of an ME Super with AF, Live View, and E-TTL, and I'd be happy as a
clam.

> But, on reflection, that sounds about what one might expect; basic modes
> are for the-camera-does-it-all shots, which means the camera has to also
> choose the point of focus... yeah, it's about honking big DSLRs as
> jewelry,
> isnt' it.

I dunno, but why buy a sophisticated tool and not learn how to use it? The
problem is the camera makes a dumb choice of which focus point to use
(whatever is closest).

Wilba <(E-Mail Removed)> wrote:
> Longfellow wrote:

>> Like all(?) DSLRs, the Canon's have a configurable autofocus point
>> assortment, where (I gather) a common configuration is to have them all
>> functional at once. Now, I'm not the dimmest bulb in the room, but I
>> could never figure out why that configuration might have value; there
>> was no way that I could see of controlling where the point of focus was
>> going to be.

> I only use that mode with AI Servo for subjects in motion. Why Canon makes
> their DSLRs use all focus points in the basic modes is a mystery, but it
> sure answers a lot of "why are my photos not sharp?" questions.

>> Now, sez I, it can't really be that simple, so what am I missing here?
>> Can't figure out what I'm not understanding about this, so I'm asking
>> the question here.
>>
>> What am I missing about all this?

> Nothing. :- )

> The only thing I'd add for completeness is the recommendation to use a
> single AF point other than the centre point when DOF is tight enough that
> recomposition might put the subject outside of it.

Provided you're not shooting at a wide enough aperture to bring in
aperture related focus drift effects, or your camera isn't one of
those with a speial wider aperture central AF sensor to help with
that, and of course providing the plane on which your camera's AF
sensors are mounted is precisely enough aligned with the image sensor
plane.

If DSLR AF sensors were well enough designed, manufactured, and
calibrated, for the above not to matter, they wouldn't be selling so
much more finely crafted and adjusted AF sytems on their most
expensive models.

My impression is that the cheaper DSLRs have AF systems whose defects
you probably won't be able to find easily with the kit zoom lens that
came with the camera.

With respect to lens reviews I've some to a similar conclusion: that
most reviewers of lenses with very critical focusing due to very
shallow DoF ascribe faults to the lens which are actually faults in
their understanding and use of the camera's focusing systems. Just as
lots of reviewers ascribe what are really differences in in-camera
jpeg processing to be due to the image sensor itself.

--
Chris Malcolm

Chris Malcolm wrote:
> Wilba wrote:

>> The only thing I'd add for completeness is the recommendation to use a
>> single AF point other than the centre point when DOF is tight enough that
>> recomposition might put the subject outside of it.
>
> Provided you're not shooting at a wide enough aperture to bring in
> aperture related focus drift effects, ...

Irrelevant.

> ... or your camera isn't one of those with a speial wider aperture central
> AF sensor to help with that, ...

Impossible.

> ... and of course providing the plane on which your camera's AF
> sensors are mounted is precisely enough aligned with the image sensor
> plane.

Also completely irrelevant.

As usual, you have completely failed to comprehend. :- )

Wilba <(E-Mail Removed)> wrote:
> Chris Malcolm wrote:
>> Wilba wrote:

>>> The only thing I'd add for completeness is the recommendation to use a
>>> single AF point other than the centre point when DOF is tight enough that
>>> recomposition might put the subject outside of it.
>>
>> Provided you're not shooting at a wide enough aperture to bring in
>> aperture related focus drift effects, ...

> Irrelevant.

It affects accuracy of focus when DoF is tight, aperture is large, and
the lens is of spherical design, as many of the good popular
economical "nifty fifty" (50mm) lenses are.

>> ... or your camera isn't one of those with a speial wider aperture central
>> AF sensor to help with that, ...

> Impossible.

Except in the top of the range models from Canon, Nikon, Sony, and
very probably more :-)

>> ... and of course providing the plane on which your camera's AF
>> sensors are mounted is precisely enough aligned with the image sensor
>> plane.

> Also completely irrelevant.

It's not unknown for new cameras to be supplied with slightly tilted
AF sensor planes which throw off the focus of the edge AF sensors on
one side, or both sides in different directions, etc.. When present
it's usually a small enough error that it's not noticeable until you
start using the edge sensors for accurate focus with lenses which have
shallow enough DoF to make the focus errors easily visible. That
usually requires much more critical lenses than the kit zoom. Aligning
the AF sensor plane is part of the AF calibration procedure and like
generic back or front focus is easily corrected by a service
recalibration.

> As usual, you have completely failed to comprehend. :- )

As usual you don't know enough to realise your ignorance :-)

--
Chris Malcolm

Chris Malcolm wrote:
> Wilba wrote:
>> Chris Malcolm wrote:
>>> Wilba wrote:
>>>>
>>>> The only thing I'd add for completeness is the recommendation to
>>>> use a single AF point other than the centre point when DOF is tight
>>>> enough that recomposition might put the subject outside of it.
>>>
>>> Provided you're not shooting at a wide enough aperture to bring in
>>> aperture related focus drift effects, ...
>>
>> Irrelevant.
>
> It affects accuracy of focus when DoF is tight, aperture is large, and
> the lens is of spherical design, as many of the good popular
> economical "nifty fifty" (50mm) lenses are.

How is a gross misfocus better than a possible slight focus error?

>>> ... or your camera isn't one of those with a speial wider aperture
>>> central AF sensor to help with that, ...
>>
>> Impossible.
>
> Except in the top of the range models from Canon, Nikon, Sony, and
> very probably more :-)

How is it possible for a special wider aperture central AF sensor to help
when the subject is moved outside the DOF after focus is locked?

>>> ... and of course providing the plane on which your camera's AF
>>> sensors are mounted is precisely enough aligned with the image
>>> sensor plane.
>>
>> Also completely irrelevant.
>
> It's not unknown for new cameras to be supplied with slightly tilted
> AF sensor planes which throw off the focus of the edge AF sensors on
> one side, or both sides in different directions, etc.. When present
> it's usually a small enough error that it's not noticeable until you
> start using the edge sensors for accurate focus with lenses which have
> shallow enough DoF to make the focus errors easily visible. That
> usually requires much more critical lenses than the kit zoom. Aligning
> the AF sensor plane is part of the AF calibration procedure and like
> generic back or front focus is easily corrected by a service
> recalibration.

How is a possible slight focus error worse than a gross misfocus?

>> As usual, you have completely failed to comprehend. :- )
>
> As usual you don't know enough to realise your ignorance :-)

As usual, your assumption of superiority disables your curiosity and
humility. :- )

Wilba <(E-Mail Removed)> wrote:
> Chris Malcolm wrote:
>> Wilba wrote:
>>> Chris Malcolm wrote:
>>>> Wilba wrote:
>>>>>
>>>>> The only thing I'd add for completeness is the recommendation to
>>>>> use a single AF point other than the centre point when DOF is tight
>>>>> enough that recomposition might put the subject outside of it.
>>>>
>>>> Provided you're not shooting at a wide enough aperture to bring in
>>>> aperture related focus drift effects, ...
>>>
>>> Irrelevant.
>>
>> It affects accuracy of focus when DoF is tight, aperture is large, and
>> the lens is of spherical design, as many of the good popular
>> economical "nifty fifty" (50mm) lenses are.

> How is a gross misfocus better than a possible slight focus error?

No idea what you're talking about.

>>>> ... or your camera isn't one of those with a speial wider aperture
>>>> central AF sensor to help with that, ...
>>>
>>> Impossible.
>>
>> Except in the top of the range models from Canon, Nikon, Sony, and
>> very probably more :-)

> How is it possible for a special wider aperture central AF sensor to help
> when the subject is moved outside the DOF after focus is locked?

I can't imagine how any AF sensor can help you when you've switched
focusing off, and I can;t imagine what point you're making by pointing
out that sensors you aren't using aren't being of any use.

>>>> ... and of course providing the plane on which your camera's AF
>>>> sensors are mounted is precisely enough aligned with the image
>>>> sensor plane.
>>>
>>> Also completely irrelevant.
>>
>> It's not unknown for new cameras to be supplied with slightly tilted
>> AF sensor planes which throw off the focus of the edge AF sensors on
>> one side, or both sides in different directions, etc.. When present
>> it's usually a small enough error that it's not noticeable until you
>> start using the edge sensors for accurate focus with lenses which have
>> shallow enough DoF to make the focus errors easily visible. That
>> usually requires much more critical lenses than the kit zoom. Aligning
>> the AF sensor plane is part of the AF calibration procedure and like
>> generic back or front focus is easily corrected by a service
>> recalibration.

> How is a possible slight focus error worse than a gross misfocus?

>>> As usual, you have completely failed to comprehend. :- )

You're quite right. I completely fail to comprehend any of your
questions and answers here. In case you're rushing to tell me that the
reason I don't comprehend is because I'm ignorant, that's not the
problem here. The problem is your ambiguous terse one word or one
sentence rejoinders. That's not a good strategy for having productive
discussions. It is however a very good strategy for maximising your
chances of being able to accuse people of misunderstanding.

--
Chris Malcolm

Chris Malcolm wrote:
> Wilba wrote:
>> Chris Malcolm wrote:
>>> Wilba wrote:
>>>> Chris Malcolm wrote:
>>>>> Wilba wrote:
>>>>>>
>>>>>> The only thing I'd add for completeness is the recommendation to
>>>>>> use a single AF point other than the centre point when DOF is tight
>>>>>> enough that recomposition might put the subject outside of it.
>>>>>
>>>>> Provided you're not shooting at a wide enough aperture to bring in
>>>>> aperture related focus drift effects, ...
>>>>
>>>> Irrelevant.
>>>
>>> It affects accuracy of focus when DoF is tight, aperture is large, and
>>> the lens is of spherical design, as many of the good popular
>>> economical "nifty fifty" (50mm) lenses are.
>
>> How is a gross misfocus better than a possible slight focus error?
>
> No idea what you're talking about.

Exactly! What do you need to be able to understand "use a single AF point
other than the centre point when DOF is tight enough that recomposition
might put the subject outside of it"? AFAICT, whatever ideas you are
pursuing have nothing to do with that.

>>>>> ... or your camera isn't one of those with a speial wider aperture
>>>>> central AF sensor to help with that, ...
>>>>
>>>> Impossible.
>>>
>>> Except in the top of the range models from Canon, Nikon, Sony, and
>>> very probably more :-)
>
>> How is it possible for a special wider aperture central AF sensor to help
>> when the subject is moved outside the DOF after focus is locked?
>
> I can't imagine how any AF sensor can help you when you've switched
> focusing off, and I can;t imagine what point you're making by pointing
> out that sensors you aren't using aren't being of any use.

There you go again! I say "the subject is moved outside the DOF after focus
is locked", and you think "you've switched focusing off". My hovercraft is
full of eels! (http://en.wikipedia.org/wiki/Dirty_Hungarian_Phrasebook)

>>>>> ... and of course providing the plane on which your camera's AF
>>>>> sensors are mounted is precisely enough aligned with the image
>>>>> sensor plane.
>>>>
>>>> Also completely irrelevant.
>>>
>>> It's not unknown for new cameras to be supplied with slightly tilted
>>> AF sensor planes which throw off the focus of the edge AF sensors on
>>> one side, or both sides in different directions, etc.. When present
>>> it's usually a small enough error that it's not noticeable until you
>>> start using the edge sensors for accurate focus with lenses which have
>>> shallow enough DoF to make the focus errors easily visible. That
>>> usually requires much more critical lenses than the kit zoom. Aligning
>>> the AF sensor plane is part of the AF calibration procedure and like
>>> generic back or front focus is easily corrected by a service
>>> recalibration.
>>
>> How is a possible slight focus error worse than a gross misfocus?
>
>>>> As usual, you have completely failed to comprehend. :- )
>
> You're quite right. I completely fail to comprehend any of your
> questions and answers here.

Yes, because you didn't address my original statement. Go back there and get
that, and everything else will make sense.

> In case you're rushing to tell me that the
> reason I don't comprehend is because I'm ignorant, that's not the
> problem here. The problem is your ambiguous terse one word or one
> sentence rejoinders. That's not a good strategy for having productive
> discussions. It is however a very good strategy for maximising your
> chances of being able to accuse people of misunderstanding.

All you have to do is respond to what's said. I'm not going to fall into
your trap of defending against your hijacks. No matter how sophisticated you
make it look, it's still trolling.