From: Wilba on
Paul Furman wrote:
> Wilba wrote:
>>
>> I'm ready to propose a clarification. Since David has got me thinking in
>> terms of exit pupils, I would now say something like this -
>>
>> The "f/2.8" in "f/2.8 high-precision AF sensor", denotes the size of the
>> exit pupil of a lens required to illuminate that AF sensor. A lens with a
>> smaller exit pupil (e.g. f/4) will not adequately illuminate the AF
>> sensor. An f/2.8 lens will illuminate the AF sensor from the outer extent
>> of it's exit pupil. A lens with a larger exit pupil (e.g. f/1.8) will
>> illuminate wider than the AF sensor, and therefore the light reaching the
>> AF sensor will not come from the outer extent of that lens's exit pupil.
>
> This sounds right.
>
>> The "f/2.8" designation of the AF sensor does not refer to a relationship
>> between a focal length and an actual or virtual aperture of the AF sensor
>> itself, as if it were an objective lens.
>
> I read something about entrance pupils - that's what you use to rotate
> about for perfect panoramas avoiding parallax errors by keeping the actual
> viewpoint constant. When I say 'actual', that's kind of an abstract
> concept. The entrance pupil is located where it appears to be from the
> perspective of the subject. It doesn't matter where the actual aperture
> is, there are lots of tricks that lens designers will do which make the
> aperture appear to be larger/smaller/closer/further than it is in physical
> space - all that matters is where it appears to be from the front of the
> lens. This isn't imaginary; you can put a camera in front of the lens and
> focus on the aperture's edges and that will align with the point outside
> the lens where the effective entrance pupil is functionally located in
> real space. That's what counts. Mark it with chalk with confidence. So it
> isn't always real intuitive, that's my point for this story, but it does
> ultimately make sense.
>
> Now, just to elaborate about how counterintuitive things can get... if you
> use an external aperture like you placed over the front and put it in back
> of the lens, in just the right spot, the view from the front will have the
> aperture appearing to be at infinity. That means if you put a camera in
> front of this setup, and focus on the aperture opening inside the lens, on
> the edge of the aperture blades where the hole appears to be - it will be
> focused on infinity and the horizon will also be in focus. At this point
> the lens has strange properties and there are (almost) no parallax errors.
> You can move the camera around and it will still stitch perfect panos
> because the viewpoint is coming from infinity so it doesn't really change.
>
> What also happens in that scenario is the lens only collects rays that
> travel straight and parallel into it's front element so the field of view
> is no wider than the front piece of glass. Let's say it's an f/2.8 lens
> with a 77mm diameter lens about half 1:1 macro scale, 0.5x magnification.
> Change the focus and you mess up this relationship but move the camera and
> you could keep walking forward along a railroad track capturing closer
> views and stitch that together as an isometric projection of the train
> track never fading in perspective, as a constant width in an infinitely
> tall panorama of the rail.
>
> That describes a 'telecentric' lens, not in the terms you may have heard
> here, as telecentric from the exit pupil to the sensor for 'digital
> lenses' but from the entrance pupil to the subject. These are used for
> inspecting machining and manufacturing; to measure parts precisely
> perspective foreshortening must be eliminated.

Okey dokey! :-D


From: Wilba on
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.)

>>> 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...?


From: David J Taylor on

"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
news:0358458a$0$1371$c3e8da3(a)news.astraweb.com...
> David J Taylor wrote:
>> Wilba wrote:
>> []
>>> Yes, but I'm thinking the "f/2.8" AF sensor is only going to see the
>>> rays that it would see from an f/2.8 exit pupil, so it doesn't see the
>>> rays from the extremes of the f/1.8 exit pupil. No?
>>
>> I don't know - it could depend on the precise optical design.
>
> Have you read http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf?

Yes.

> It describes exactly what I said (AFAICT).

It doesn't exactly describe the 450D - it's from 2005. Perhaps someone
who knows the 450D can comment - I can't, so I am not prepared to make a
definitive statement. I changed my "would" to "could".

Cheers,
David

From: Paul Furman on
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.


> _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

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: Paul Furman on
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.


>>>> 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.

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

all google groups messages filtered due to spam