From: Paul Furman on
Wilba wrote:
> Paul Furman wrote:
>> 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.
>
> The average doesn't matter, since it's binary - alway either one or the
> other.

Whatever, that's not the issue.


>> 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'.
>
> Right. That's the opposite of what's happening for me, it's front-focus.

No, this describes your results.
Draw the two eyes and camera.
Now draw your setup.
They are the same.
Back Focus.

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

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From: Paul Furman on
Wilba wrote:
> Paul Furman wrote:
>> Wilba wrote:
>
>>>>>>>> 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.
>
> OK, that's what I don't understand. :- )

Sorry, I reworded it in a more confusing way. Ignore that & try this
version:

The wider rays from f/1.8 won't be seen by the AF sensor. It's view
comes through a system that only gathers narrower f/2.8 angled rays.

The wider rays will get clipped, bouncing around side surfaces, never
reaching the AF sensor.

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

all google groups messages filtered due to spam
From: Wilba on
Paul Furman wrote:
> Wilba wrote:
>> Paul Furman wrote:
>>>
>>> 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'.
>>
>> Right. That's the opposite of what's happening for me, it's front-focus.
>
> No, this describes your results.
> Draw the two eyes and camera.
> Now draw your setup.
> They are the same.
> Back Focus.

Just to be sure, what do you see in this arrangement, back-focus or
front-focus?

Camera Subject Plane of
focus

And therefore this is...?

Camera Plane of focus Subject


From: Wilba on
Paul Furman wrote:
>
> The wider rays from f/1.8 won't be seen by the AF sensor. It's view comes
> through a system that only gathers narrower f/2.8 angled rays.
>
> The wider rays will get clipped, bouncing around side surfaces, never
> reaching the AF sensor.

I guess you're saying that the effective size of an AF sensor's virtual
aperture doesn't change with the lens aperture, which is fair enough,
because that's part of the explanation of why there is no ARFD occurring in
the AF system. Still, my system's performance meets that standard (from an
initial near side focus), so I'm not concerned about that aspect.

It would be interesting to get a better idea of the effective size of an AF
sensor's virtual aperture (I think I read somewhere that it might be
something like f/11, but I can't recall where). As long as we're clear that
"f/2.8" doesn't refer to that. :- )


From: Wilba on
Wilba wrote:
> Paul Furman wrote:
>> Wilba wrote:
>>> Paul Furman wrote:
>>>>
>>>> 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'.
>>>
>>> Right. That's the opposite of what's happening for me, it's front-focus.
>>
>> No, this describes your results.
>> Draw the two eyes and camera.
>> Now draw your setup.
>> They are the same.
>> Back Focus.
>
> Just to be sure, what do you see in this arrangement, back-focus or
> front-focus?
>
> Camera Subject Plane of
> focus
>
> And therefore this is...?
>
> Camera Plane of focus Subject

Oops, got a wrap. Try this -

What do you see in this arrangement, back-focus or front-focus?

Camera Subject Plane of focus

And therefore this is...?

Camera Plane of focus Subject