From: David J Taylor on
"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
news:0357c8c5$0$1291$c3e8da3(a)news.astraweb.com...
[]
> 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. 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 think I can broadly agree with that.

David

From: David J Taylor on
"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
news:0357b2c2$0$1344$c3e8da3(a)news.astraweb.com...
[]
> 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. I
did ask for a clarification a few messages back.

>> As you know, that lens is quite soft even in the centre of the image at
>> full
>> aperture:
>>
>> http://www.dpreview.com/lensreviews/canon_50_1p8_ii_c16/page3.asp
>>
>> and likely that softness would mask the effects I am discussing.
>
> Both effects have the same cause - spherical aberation, right? So
> they're inextricably linked...

Lack of sharpness could be caused by dozens of effects, of which spherical
aberration is only one.

Cheers,
David

From: David J Taylor on
"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
news:0357b669$0$1274$c3e8da3(a)news.astraweb.com...
[]
> 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 would depend on the precise optical design.

Cheers,
David

From: Paul Furman on
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.

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

all google groups messages filtered due to spam
From: Paul Furman on
David J Taylor wrote:
> "Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
> news:0357b2c2$0$1344$c3e8da3(a)news.astraweb.com...
> []
>> 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.


> I did ask for a clarification a few messages back.
>
>>> As you know, that lens is quite soft even in the centre of the image
>>> at full
>>> aperture:
>>>
>>> http://www.dpreview.com/lensreviews/canon_50_1p8_ii_c16/page3.asp
>>>
>>> and likely that softness would mask the effects I am discussing.
>>
>> Both effects have the same cause - spherical aberation, right? So
>> they're inextricably linked...
>
> Lack of sharpness could be caused by dozens of effects, of which
> spherical aberration is only one.
>
> Cheers,
> David


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

all google groups messages filtered due to spam