From: Wilba on
Chris Malcolm wrote:
> Wilba wrote:
>> 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.
>
> How can that be, given that different lens designs have very different
> amounts of this problem? How does the AF system distinguish between a
> spherical and an aspherical design of 50mm lens? And if your answer is
> (which is true of some DSLRs) that the camera reads the lens
> calibration data from the in-lens chip, then how does the lens
> calibration data deal with the different sizes of camera AF sensors
> and their different algorithms?

It doesn't have to. For these tests, the AF system in my 450D only has to
work with my 50/1.8, which it does, with consistently excellent results
given the right initial conditions.

If you're saying your effect is a general PD AF system design and/or
calibration problem, that's fine by me, but it doesn't have anything to do
with my original question or anything else that has come from it, including
the results and conclusions.


From: David J Taylor on
"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
news:00cc0806$0$15663$c3e8da3(a)news.astraweb.com...
[]
> 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.

Cheers,
David

From: Better Info on
On Tue, 5 Jan 2010 08:09:11 +0800, "Wilba" <usenet(a)CUTTHISimago.com.au>
wrote:

>David J Taylor wrote:
>> Wilba wrote:
>>> David J Taylor wrote:
>>>>
>>>> If you are using manual focus there is still any backlash in the system.
>>>
>>> Irrelevant in this case.
>>
>> Unless the focus point is infinitely small, there is a dead-zone between
>> the in-focus indication turning on and it disappearing. You might expect
>> to find the the correct point of focus somewhere between those two points.
>
>I guess you mean that there is a range of focusses over which the system
>will confirm focus. Yes, with my 450D's high-precision AF sensor, that range
>is about as big as the DOF near the closest focus distance. Best focus is
>achieved at one end of that range, making the other end clearly OOF.
>
>> As we seem to have drifted into things which you say are irrelevant,
>> perhaps a succinct restatement of your original problem might help.
>
>Okay. My original problem was that I couldn't figure out how to test the
>"crude mechanism" theory of erratic focus with the EF 50mm f/1.8 II. Doug
>McDonald gave me the idea of the beep test (like trap focussing). Using that
>idea I was able to show three things - the AF system will confirm focus over
>a range about as big as the DOF, where the system confirms focus depends on
>where the lens was initially focussed, and that the smallest focus step the
>lens can make is of the order of a few % of the DOF. Thus the "crude
>mechanism" theory is disproved, and the erratic focus behaviour is
>explained.
>
>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.
>

Or ... just buy cameras that have contrast-detection focusing, the most
accurate method available. And it works at all apertures. Some of us got
smart, long ago. When it comes to photography, accuracy wins over speed any
day. What good is faster focus if you get a blurry result? None. A missed
shot. A lost shot. It now defines itself as a useless feature.

From: Wilba on
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.

Let's be perfectly clear - the "f/2.8" in "f/2.8 high-precision AF sensor"
refers to when the camera switches over to using that sensor. When a lens is
mounted that has an f/2.8 or better maximum aperture, the high-precision
sensor is used rather than the less accurate one that requires a maximum
aperture of f/5.6 or better.

Getting back the idea you've just introduced, for me to make any sense of it
you'd have to give me a clue to _how_ the AF sensor might confirm a focus
over a wider range because of that difference.


From: David J Taylor on
"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
news:00cd4aac$0$15575$c3e8da3(a)news.astraweb.com...
[]
> 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.

I was not talking of any specific implementation, simply that a particular
AF sensor will have a range of lens apertures with which it will work. If
the lens aperture is too small (too large an f/number) the AF sensor
doesn't work. If the lens aperture is too large (too small an f/number)
the focus point indicated may not be correct if the lens has spherical
aberration.

> Let's be perfectly clear - the "f/2.8" in "f/2.8 high-precision AF
> sensor" refers to when the camera switches over to using that sensor.
> When a lens is mounted that has an f/2.8 or better maximum aperture, the
> high-precision sensor is used rather than the less accurate one that
> requires a maximum aperture of f/5.6 or better.

OK, but that still tells you nothing about the actual cone of light used
by the "large aperture" AF sensor.

> Getting back the idea you've just introduced, for me to make any sense
> of it you'd have to give me a clue to _how_ the AF sensor might confirm
> a focus over a wider range because of that difference.

You already answered your own question - the high-precision sensor (in
general terms) will have more light to work with, and a larger cone of
light coming from the lens. Thus if the other system parameters including
the hysteresis are the same, the range of "in-focus" indication can be
narrower. You would need to check the exact details of Canon's system for
a more detailed answer.

Cheers,
David