From: Chris Malcolm on
Wilba <usenet(a)cutthisimago.com.au> wrote:

> Waaay-back someone said that focussing with an f/1.8 lens using an "f/2.8"
> AF sensor could explain the results I got in the tests that followed the OP.
> I've yet to see a credible description of how that could be.

Whereas others have quite a few. Weird, isn't it?

--
Chris Malcolm
From: Wilba on
Chris Malcolm wrote:
> Wilba wrote:
>> Chris Malcolm wrote:
>>>
>>> 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.
>
> You're making the invalid assumption that all direct questions have a
> sensible answer.

Not necessarily, but I am assuming that someone who wants to be understood
will attempt to understand the other and show that they are trying. :- )

I'm ready to propose a clarification. Since David has got me thinking in
terms of exit pupils, I would now say something like this -

The "f/2.8" in "f/2.8 high-precision AF sensor", denotes the size of the
exit pupil of a lens required to illuminate that AF sensor. A lens with a
smaller exit pupil (e.g. f/4) will not adequately illuminate the AF sensor.
An f/2.8 lens will illuminate the AF sensor from the outer extent of it's
exit pupil. A lens with a larger exit pupil (e.g. f/1.8) will illuminate
wider than the AF sensor, and therefore the light reaching the AF sensor
will not come from the outer extent of that lens's exit pupil. The "f/2.8"
designation of the AF sensor does not refer to a relationship between a
focal length and an actual or virtual aperture of the AF sensor itself, as
if it were an objective lens.


From: Wilba on
Chris Malcolm wrote:
> Wilba wrote:
>>
>> Waaay-back someone said that focussing with an f/1.8 lens using an
>> "f/2.8"
>> AF sensor could explain the results I got in the tests that followed the
>> OP.
>> I've yet to see a credible description of how that could be.
>
> Whereas others have quite a few. Weird, isn't it?

Not really. Complex things are hard to understand and explain, especially in
writing. Misunderstanding is inevitable. I'm not at all sure anyone has
understood the question, let alone the answers. :- )


From: Paul Furman on
Wilba wrote:
> David J Taylor wrote:
>> Wilba wrote:
>>> David J Taylor wrote:
>>>> the point is that there is light from the f/1.8 lens which is not
>>>> used for the focus measurement, and the fact that this unused
>>>> light come from the extremes of the lens ray bundle is critical should
>>>> the lens have significant spherical aberration.
>>> I don't get why is it critical. You're saying that the focus system won't
>>> use the aberant rays? That sounds like a good thing to me.
>> The focus system uses /only/ the rays which are not coming from the
>> centre of the lens exit pupil. These are more likely to be the rays
>> showing spherical aberration.
>
> Yes, but I'm thinking the "f/2.8" AF sensor is only going to see the rays
> that it would see from an f/2.8 exit pupil, so it doesn't see the rays from
> the extremes of the f/1.8 exit pupil. No?

Yep.


>>>> I would expect the "f/2.8" precision focus sensor to have a smaller
>>>> range of "in-lock" indication. I have never said otherwise.
>>> You said -
>>>
>>> "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."
>> To expand on what I wrote:
>>
>> 1 - "I would expect the "f/2.8" precision focus sensor to have a smaller
>> range of "in-lock" indication [than the "f/5.6" standard focus sensor]."
>
> Correct. Canon say their "f/5.6" AF sensors should focus "within the DOF",
> and the "f/2.8" sensors should focus "within the inner 1/3 of the DOF".

I think that's a simplification.

Assuming you are shooting at f/5.6,

-If you have a zoom that only gets as wide as f/5.6, they promise the
focus will at least be just barely inside the DOF.

-If you have an f/2.8 lens, they promise the focus will at least be just
barely inside the DOF of f/2.8 (smaller than f/5.6). That would
guarantee that the focus will be within the inner 1/3 of an f/5.6 shot.
I don't think it gets within the inner 1/3 of an f/1.8 shot or even an
f/2.8 shot.

I suspect these cameras are not promising to always be inside the DOF of
an f/1.8 shot and you are lucky to get that at all. Manual focus in live
view is guaranteed to work though. Well, my D700 doesn't zoom to 100% in
live view so even that is not perfect, although better than most people
can see with their eyes through today's DSLR viewfinders with focus
screens optimized for AF use.

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

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From: Paul Furman on
Wilba wrote:
>
> But the fact remains that I _do_ get images in which the the centre is well
> focussed, so why should I care that they might, theoretically, not be?

To seek out new knowledge.
To boldly go where no man has gone before!
<star trek theme.wav>


>> 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.
>
> ...do I need to modify my test procedure to
> control for this effect? (That's the bottom line.)

No, it looks like you are testing beyond the specifications of the
camera. It's only guaranteed to f/2.8. Any further investigation would
likely prove frustrating <g>.

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

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