From: David J Taylor on
"Chris Malcolm" <cam(a)holyrood.ed.ac.uk> wrote in message
news:7rmu4tFkjaU2(a)mid.individual.net...
[]
> 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.
>
> --
> Chris Malcolm

Would you prefer the term dead-band?

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:7rmu4tFkjaU2(a)mid.individual.net...
> []
>> 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.
>>
>> --
>> Chris Malcolm

> Would you prefer the term dead-band?

A hysteresis band is a kind of dead band, but a dead band isn't
necessarily a hysteresis band, because a hysteresis band is
directional.

--
Chris Malcolm
From: David J Taylor on
"Chris Malcolm" <cam(a)holyrood.ed.ac.uk> wrote in message
news:7ro6seFmv5U1(a)mid.individual.net...
> 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:7rmu4tFkjaU2(a)mid.individual.net...
>> []
>>> 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.
>>>
>>> --
>>> Chris Malcolm
>
>> Would you prefer the term dead-band?
>
> A hysteresis band is a kind of dead band, but a dead band isn't
> necessarily a hysteresis band, because a hysteresis band is
> directional.
>
> --
> Chris Malcolm

Thanks, Chris. I thought that a directional effect was what Wilba was
seeing? Different focus when coming from nearest than when coming from
farthest?

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:7ro6seFmv5U1(a)mid.individual.net...
>> 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:7rmu4tFkjaU2(a)mid.individual.net...
>>> []
>>>> 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.
>>>>
>>>> --
>>>> Chris Malcolm
>>
>>> Would you prefer the term dead-band?
>>
>> A hysteresis band is a kind of dead band, but a dead band isn't
>> necessarily a hysteresis band, because a hysteresis band is
>> directional.
>>
>> --
>> Chris Malcolm

> Thanks, Chris. I thought that a directional effect was what Wilba was
> seeing? Different focus when coming from nearest than when coming from
> farthest?

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.

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
happened.

--
Chris Malcolm
From: David J Taylor on

"Chris Malcolm" <cam(a)holyrood.ed.ac.uk> wrote in message
news:7rp613Fdg5U1(a)mid.individual.net...
> 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:7ro6seFmv5U1(a)mid.individual.net...
>>> 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:7rmu4tFkjaU2(a)mid.individual.net...
>>>> []
>>>>> 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.
>>>>>
>>>>> --
>>>>> Chris Malcolm
>>>
>>>> Would you prefer the term dead-band?
>>>
>>> A hysteresis band is a kind of dead band, but a dead band isn't
>>> necessarily a hysteresis band, because a hysteresis band is
>>> directional.
>>>
>>> --
>>> Chris Malcolm
>
>> Thanks, Chris. I thought that a directional effect was what Wilba was
>> seeing? Different focus when coming from nearest than when coming from
>> farthest?
>
> 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.
>
> 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
> happened.
>
> --
> Chris Malcolm

My background to this is electronic, Chris, in data communication where
the hysteresis is intended to prevent noise from false triggering - it's
in normal logic circuits as well. For example, in serial communication
such as RS-232, a line may have to rise above +3 volts to register has
"high", but it can sink down to zero and still show as "high". It won't
show as "low" until it drops below -3 volts, and then it will need to rise
above +3 volts before it registers as "high" again. You may know all
this, some readers may not. It's not the exact figures which matter, but
that the circuit has a memory and doesn't change state at a single
precisely defined level.

So in the auto-focus case which end of the "in-focus" dead-band it
finished didn't matter, just that it could be different when approached
from infinity or from closest focus. the impulse response of the AF
system system and the dead-band width would determine where it stopped. A
system with overshoot could behave differently than a more heavily-damped
one.

Give me a 'scope and an electronic system over a mechanical one any day!
I feel I can see much better what happens.

Cheers,
David