From: Paul Furman on
Paul Furman wrote:
> Wilba wrote:
>> Paul Furman wrote:
>>> David J Taylor wrote:
>>>> Wilba wrote:
>>>> []
>>>>> Does this effect explain my test results? (I haven't seen anything
>>>>> so far that tells me so.) Therefore, do I need to modify my test
>>>>> procedure to control for this effect? (That's the bottom line.)
>>>> It's been so long back that I can't recall what your test results were.
>>> Backfocus.
>>>
>>> Barely in the DOF coming from foreground focus (but consistent),
>>> usually outside the DOF coming from initial background focus and more
>>> random.
>>
>> Optimal focus when the initial focus is on the near side of the subject,
>
> It was just barely on the edge of the DOF, not in the middle third.

OK, it was perfect but the range of the beep band was not. On average,
there was a 1.5mm backfocus error.


>> _front_focus_ when coming from an initial far side focus.
>>
>> If you've been thinking back focus all the way through, you need to
>> rethink. :- )
>
> pasted from above for reference (again):
>
> your diagram with some edits for clarity:
> (must use fixed width font or this jpeg:
> http://edgehill.net/1/temp/Clipboard01.jpg)
>
> sensor subject background
> _________________________________________________________________
> |< 430mm >+ sensor to subject distance for optimal focus
> _________________________________________________________________
> |< 430mm >+ starting from near-side focus
> _________________________________________________________________
> |< 433mm >+ far-side focus
> _________________________________________________________________
> | |< 3.0mm >| the "beep band"
> _________________________________________________________________
> | |< 0.75mm >| back-focus error from center of beep band


Should read 1.5mm, not 0.75mm. Doh!


> I removed the theoretical dof & at the bottom gave a 0.75mm distance for
> the back-focus error from the center of the beep zone.
>


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

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From: Wilba on
Paul Furman wrote:
> Wilba wrote:
>> Paul Furman wrote:
>>> Wilba wrote:
>>>> Paul Furman wrote:
>>>>> Wilba wrote:
>>>>>>
>>>>>> 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.
>>>>
>>>> It may be but it's their specification (AFAWCT). If you can prove your
>>>> system doesn't achieve that, Canon will calibrate it for free under
>>>> warranty.
>>>>
>>>>> 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.
>>>>
>>>> More precisely, the _subject_ is at least 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.
>>>>
>>>> No, the spec says the subject should be within the inner 1/3 of the DOF
>>>> if the "f/2.8" AF sensor is invoked. That should be the case if your
>>>> lens's maximum aperture is f/2.8 or better, and at the DOF of the
>>>> maximum aperture.
>>>
>>> I call them for fibbing then.
>>
>> Well, they don't guarantee that all their cameras actually achieve that,
>> only that they should. :- ) (And they seem to when accurately
>> calibrated.)
>
> But yours isn't. I'm not saying it's a bad camera, just not within the
> anticipated spec but probably within the carefully worded spec if you read
> between the lines.

With the 50/1.8, if I start from a significant front-focus, it's perfect
according to the spec. and any other measure. The problem is when I start
with a significant back-focus. It's Jekyll and Hyde. :- )

I suspect, from your message about the "back-focus error", that you're are
fundamentally misunderstanding something about the results of my tests. Your
reply to my reply in that fibre might sort that out.

>>>>> I don't think it gets within the inner 1/3 of an f/1.8 shot or even an
>>>>> f/2.8 shot.
>>>>
>>>> Why not?
>>>
>>> Because it's optimized for f/2.8, not f.1,8.
>>
>> Yeah, but the DOF is thinner... so sharp focus would be easier to
>> nail...?
>
> Not if the AF system can't see much wider than f/2.8.

Um, are you saying that the DOF isn't thinner at f/1.8 than it is at f/2.8?
Or that the thinner DOF at f/1.8 is of no use to the AF sensor if it gets it
from the f/2.8 part of the exit pupil (i.e. not from the extremes)? Or
something else?


From: Wilba on
Paul Furman wrote:
>
> OK, it was perfect but the range of the beep band was not. On average,
> there was a 1.5mm backfocus error.

Now I get you - you're talking about an _average_. It's still an average
1.5mm front-focus, not back-focus.


From: Paul Furman on
Wilba wrote:

> I suspect, from your message about the "back-focus error", that you're are
> fundamentally misunderstanding something about the results of my tests. Your
> reply to my reply in that fibre might sort that out.

Right, follow up in that sub-thread.


>>>>>> I don't think it gets within the inner 1/3 of an f/1.8 shot or even an
>>>>>> f/2.8 shot.
>>>>> Why not?
>>>> Because it's optimized for f/2.8, not f.1,8.
>>> Yeah, but the DOF is thinner... so sharp focus would be easier to
>>> nail...?
>> Not if the AF system can't see much wider than f/2.8.
>
> Um, are you saying that the DOF isn't thinner at f/1.8 than it is at f/2.8?
> Or that the thinner DOF at f/1.8 is of no use to the AF sensor if it gets it
> from the f/2.8 part of the exit pupil (i.e. not from the extremes)? Or
> something else?

The thinner DOF at f/1.8 is of no use to the AF sensor if it gets it
from the fat f/2.8 part of the exit pupil.

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

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From: Paul Furman on
Wilba wrote:
>
> It's still an average 1.5mm front-focus, not back-focus.

The average focus chosen by the AF system is behind the subject. In the
focus shift web page, he auto-focuses on the front eye but the final pic
is focused on the back eye. I'm calling that 'back-focus'.

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

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