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From: Wilba on 20 Jan 2010 19:16
Chris Malcolm wrote:
> Wilba wrote:
>> David J Taylor wrote:
>>> Wilba wrote:
>>>> David J Taylor wrote:
>>>>> the fact that there is a dead-band where focus will be deemed to
>>>>> be good enough if you are anywhere within that band. Approach
>>>>> from different starting points, infinity or close up, and you will
>>>>> stop at a different position, always within that dead-band, though
>>>>> (at least in theory).
>>>> Like Paul said, my AF system focuses precisely and repeatedly at the
>>>> of a band, never in the middle. But if you put the subject in that
>>>> the system will confirm focus.
>>> Exactly what you would expect when the focus system has a dead-zone
>>> or hysteresis, and when you are within that dead-zone the focus system
>>> reports "in-focus".
>> Yeah but, "likely stop at a different position, always within that
>> dead-band", is not true in this case. My focus does not stop _within_
>> dead-band, it always stops at one or other of the ends of it. It's quite
>> precise, not fuzzy as you'd expect from a hysteresis band.
> Why would you expect fuzziness in a hysteresis band? I've seen plenty
> of systems with hysteresis with very precise boundaries to the
> hysteresis band.
I don't, but David seems to, unless I'm reading him wrong.
> Note by the way the the in-focus band defined by the acceptable focus
> error band isn't a hysteresis band. But there's no reason why it
> should have fuzzy edges either.
Whatever. Not my term.
From: Wilba on 20 Jan 2010 19:21
Chris Malcolm wrote:
> Wilba was originally describing something very like that, but since
> other cameras behave the opposite way in that situation I wasn't
> convinced he hadn't got it back to front, and didn't want to get
> involved in that argument until it got a lot clearer what had in fact
The AF behaviour and the beep test finding the same focus points are still
as originally described, only the directionality of the beep test that has
From: David J Taylor on 21 Jan 2010 04:52
"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
>> Why would you expect fuzziness in a hysteresis band? I've seen plenty
>> of systems with hysteresis with very precise boundaries to the
>> hysteresis band.
> I don't, but David seems to, unless I'm reading him wrong.
I don't. I don't think I have ever used the word "fuzziness".
However, given that there is noise in any electronic or optical system,
and given that there will be tolerances in any mechanical system, I would
not be surprised if given a fixed starting point, the AF system did not
stop at /precisely/ the same finishing point every time.
From: Pete on 21 Jan 2010 11:01
Chris Malcolm wrote
> Different kind of directional. When a system is to seek a target and
> stop when it gets there there can be a problem of dithering or hunting
> close to the target due to tiny movements over the threshold, possibly
> due to noise. So some kind of delay between on-target and off-target
> conditions is imposed, often by giving some kind of snap action over
> the target, so that it has to run past the target to be triggered as
> target reached, and then under-run it by some amount to be triggered
> as off-target.
> It's a kind of snap action. In the past often implemented with
> snap-over springs on a mechanical switch. Now often implemented by
> control software.
Thanks very much for your input to this thread: I read it all yesterday,
looked up the references (plus many others) and tried to understand how AF
I thought I was doing well until I started thinking about the distance from
the exit pupil to the sensor/film plane. E.g. a long distance gives the
"narrow angle of attack" required by sensors to reduce vignetting - that
implies a wide baseline would be inappropriate in this case. I guess that
means I've missed a fundamental point (as happens quite often).
The reason for my great interest is attempting to understand an AF problem,
under very obscure lighting, with a non-Canon system: f/1.4 and f/1.8 lenses
refused to focus anywhere near the subject; f/2.8 and f/4 lenses were always
spot on. Tests shots made on each lens from wide-open to f/5.6 showed the
problem did not vary with shooting aperture. I certainly don't want to bore
everyone to death trying to understand it all. Indeed, I'm happy to have
learnt which lenses to avoid under certain lighting.
From: Wilba on 21 Jan 2010 17:47
David J Taylor wrote:
> Wilba wrote:
>> Chris Malcolm wrote:
>>> Why would you expect fuzziness in a hysteresis band? I've seen
>>> plenty of systems with hysteresis with very precise boundaries to
>>> the hysteresis band.
>> I don't, but David seems to, unless I'm reading him wrong.
> I don't. I don't think I have ever used the word "fuzziness".
But you did say, "you will likely stop at a different position, always
within that dead-band", which doesn't happen with this system (it always
stops at the ends of the beep band).
> However, given that there is noise in any electronic or optical system,
> and given that there will be tolerances in any mechanical system, I would
> not be surprised if given a fixed starting point, the AF system did not
> stop at /precisely/ the same finishing point every time.
Correct. Even CDAF has shot-to-shot variation. PDAF is a touch more
variable. But we're not talking about micrometric precision, just not
focussing between the end points of the beep band. The beep test shots
match, so it seems that PDAF triggers focus confirmation on the transition,
on entering or leaving the beep band.