From: Chris Malcolm on
Wilba <usenet(a)> wrote:
> Chris Malcolm wrote:
>> Wilba wrote:
>>> Okay, let's assume you're right, that there is a miscalibration which
>>> results in a consistent offset in the focus (I've accommodated that
>>> possibility in my other reply today). How does what we've been
>>> talking about account for the difference between the two focuses?
>>> One is consistently _here_, and other is consistently _there_.
>>> That's the significant thing. How do you account for the width
>>> of the beep band?
>> In many modern cameras and lenses which communicate electronically the
>> width of the "beep band", i.e., the change in focus over which the
>> subject is deemed to be in focus by the AF system, is set by an
>> acceptable focus error parameter for that lens. In other words focus
>> is considered to be ok not when the focus error is at its smallest,
>> but when it's less than this acceptable error parameter. Very sharp
>> lenses will have a smaller value than softer lenses. The lens chip
>> holds the value, supplied to it at lens calibration time, and it
>> supplies this value to the camera along with its other parameters such
>> as max aperture, focal length, etc..
>> When you send a lens off to be recalibrated one of the things they may
>> do is to rewrite the lens focussing parameter table with values which
>> more accurately reflect the performance of that particular copy of the
>> lens, and which are more appropriate to your specific camera and
>> purposes.

> Right-oh, so there's nothing we can think of so far that directly attributes
> the width of the beep band to any form of ARFD.

Depends what you mean by "directly". A lens with AFRD won't be able to
be as accurately focused by phase detection as one without, so the
lens calibraters might wish to set the acceptable error (width of beep
band) larger.

Chris Malcolm
From: David J Taylor on
"Paul Furman" <> wrote in message
> It's not clear his camera gives a range, just one beep. And he got
> consistent dead-on results from the front. And consistently front
> focused coming from the rear so a fuzz factor band appears not to
> explain it. My Nikons give little bracket arrows when approaching
> acceptable focus, one side or the other or both, then a green dot in the
> middle when it's optimal.
> --
> Paul Furman

What you describe, Paul, is exactly what I suggested might happen - that
the focus stops with a non-zero, but "acceptable" error (i.e. within the
depth-of-focus), and that error is a different error sign according to
which direction focus is approached.

It sounds as if "his camera" has a slight front focus offset, and a
dead-band of about twice that offset. Approach from infinity, and you
arrive at:

offset - dead-band = near-perfect focus

and approach from close-up and you arrive at:

offset + dead-band = noticeable focus error.

If I've understood you correctly, that is.


From: David J Taylor on
"Wilba" <usenet(a)> wrote in message
> Like Paul said, my AF system focuses precisely and repeatedly at the
> ends of a band, never in the middle. But if you put the subject in that
> band, 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".


From: David J Taylor on
"Wilba" <usenet(a)> wrote in message
> The beep band is 3mm deep (around the same size as the DOF), yet the
> system seems capable of achieving optimum focus at 0.1mm increments of
> camera-to-subject distance. Does that fit?

Providing the focus can be stepped in increments rather less than the
dead-band, I don't believe that the precision of the stepping is


From: Chris Malcolm on
Wilba <usenet(a)> wrote:
> Chris Malcolm wrote:
>> Wilba wrote:
>>> Chris Malcolm wrote:
>>>> Wilba wrote:
>>>>> Chris Malcolm wrote:
>>>>>> "Back and front focus errors are common with very fast lenses (and
>>>>>> others). The AF sensors in all DSLRs operate at effective apertures
>>>>>> between f/2.8 and f/7.1, and 'see' the focus point as if the lens was
>>>>>> stopped down to the exact virtual f-stop the sensor in use
>>>>>> imposes. When spherical aberration causes a shift in focus on
>>>>>> stop-down, both wider and smaller apertures will no longer focus on
>>>>>> the targeted plane. This is corrected in camera calibration, general
>>>>>> or lens-specific."
>>>>>> This quote comes from the section "Focus Accuracy" in this article in
>>>>>> the British Journal of Photography.
>>>>> Excellent! So, now that you have some corroboration for that idea,
>>>>> please remind me what you're claiming as a consequence of it.
>>>> In the quote above it says that there are misfocusing problems at
>>>> effective apertures different from the effective focus aperture of the
>>>> AF sensors, defined by the width between them, and that these are
>>>> corrected by calibration. But that calibration can never be
>>>> perfect. When focus is not highly critical that doesn't matter because
>>>> the residual uncorrected imperfections are swallowed in the DoF. But
>>>> when focus is highly critical with very shallow DoF, such as wide
>>>> apertures on high quality lenses, the inevitable imperfection of
>>>> general lens and camera calibrations matters. That's why the very top
>>>> end DSLRs, even though they have the very best AF systems,
>>>> nevertheless allow the user to trim the calibration for each lens, and
>>>> those users who have used it report that it gives useful improvements
>>>> in focus accuracy.
>>>> Of course if your camera has been calibrated for best focus with one
>>>> specific lens, you're happy with the results, and you will never
>>>> acquire another critical focus lens, then you won't ever need to know
>>>> any of this. But since we're having this discussion in public, and
>>>> some of our audience does have more than one lens of critical focus,
>>>> it's worth mentioning these general issues.
>>> That's it?! Okay, since none of that has ever been contested, and it's
>>> never
>>> been shown to be relevant to my test results, unless you have something
>>> that
>>> is, you can retire from the discussion satisfied that you have made your
>>> point. Thank you. :- )
>> Since your test results remain by your own admission completely
>> inexplicable it can't be claimed that none of that will turn out to be
>> relevant when the explanation is found :-)

> Oh no, the results _are_entirely_ explicable.

Ah. Obviously I misinterpreted what you wrote yesterday:

"... I can't find anything whatsoever that can possibly explain my

> The question that remains is
> why the AF system confirms focus asymmetrically, depending on which side you
> start from. Asymmetrical DOF is a very plausible factor, but ARFD (either
> the as-we-know-it version or the effective/virtual AF aperture version) is
> not, AFAICT.

Since the presence of ARFD due to chromatic aberration would limit
lens sharpness and also phase detection AF accuracy it could be what
decided the makers to calibrate the lens with the degree of reported
in-focus width you have experimentally discovered.

Chris Malcolm