From: Chris Malcolm on
Wilba <usenet(a)cutthisimago.com.au> wrote:
> 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. :- )

Depends what you take "effective aperture" to mean. It could be taken
as effective diameter of lens exploited, which gives the numbers like
f2.8 and f5.6 often mentioned. It could be taken to be effective light
capture area of the AF sensor as a fraction of the lens aperture. But
since it doesn't look at the whole image, that esttimate could in turn
be adjusted by effective change of focal length to account for the
reduction on field of view. And so on.

It all depends on what you want "effective aperture" to mean. Once
you've defined what you mean by "effective aperture" it's just
arithmetic on well known numbers to calculate it.

--
Chris Malcolm
From: Paul Furman on
Wilba wrote:
> 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. :- )

Heh, now I don't know what you're talking about :-)

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

all google groups messages filtered due to spam
From: Paul Furman on
Wilba wrote:
> 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?
>
> What do you see in this arrangement, back-focus or front-focus?
>
> Camera Subject Plane of focus

Back focus.


> And therefore this is...?
>
> Camera Plane of focus Subject

Front focus.

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

all google groups messages filtered due to spam
From: Wilba on
Chris Malcolm wrote:
> Wilba wrote:
>> 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. :- )
>
> Depends what you take "effective aperture" to mean. It could be taken
> as effective diameter of lens exploited, which gives the numbers like
> f2.8 and f5.6 often mentioned. It could be taken to be effective light
> capture area of the AF sensor as a fraction of the lens aperture. But
> since it doesn't look at the whole image, that esttimate could in turn
> be adjusted by effective change of focal length to account for the
> reduction on field of view. And so on.
>
> It all depends on what you want "effective aperture" to mean. Once
> you've defined what you mean by "effective aperture" it's just
> arithmetic on well known numbers to calculate it.

Use the precise meaning I gave - the effective size of an AF sensor's
virtual aperture. If you can tell me the well known diameter of the AF
sensor's virtual aperture, we can work it out. :- )


From: Wilba on
Paul Furman wrote:
> Wilba wrote:
>> 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. :- )
>
> Heh, now I don't know what you're talking about :-)

Isn't this fun!? :- )

See figure 13 of http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf (but
read your way down to it from the beginning so that it makes sense).

The light that reaches the AF sensor effectively comes through a virtual
aperture in the lens ("virtual AF apertures" in figure 13), as we have been
discussing for quite some time. If we knew the diameter of one of those
virtual apertures, we could work out the f-number for it. IIRC, I read
somewhere that it is typically something like f/11. As long as we are clear
that the "f/2.8" in "f/2.8 high-precision AF sensor" does not refer to the
size of the virtual AF aperture.