EF 50/1.8 AF Experiment?

Paul Furman wrote:
Wilba wrote:
Paul Furman wrote:
Wilba wrote:
Paul Furman wrote:
Wilba wrote:
Paul Furman wrote:
Wilba wrote:
Paul Furman wrote:
Chris Malcolm wrote:
Wilba wrote:
David J Taylor wrote:
Wilba wrote:

Any thoughts about why the 50/1.8 figures make sense (farside
focus... 0.8mm outside the DOF), but it doesn't work like that
for the 18-55?

As others have said, the focal point will shift with f/number,

How is that a factor when shooting at widest apertures?

Because with a specific lens and AF sensor the focusing takes
place
always at the effective aperture of the AF sensor, whereas the
aperture with which the photograph is taken can vary. Many
cameras
have a max AF sensor aperture of around f6. Some go down as far
as
f2.8. Hence if you're using a lens at f1.8, and it happens to be
a
spherical lens design with aperture related focus drift (as many
of
the golden oldie 50mms are), then this is an important factor.

Yeah, this could still be the explanation. Which direction does
the focus shift when stopping down? -that would provide a clue.
http://diglloyd.com/diglloyd/free/FocusShift/index.html
"With every lens I’ve tested to date, the focus moves farther
away. For example, if focus at f/1.4 is centered at 1.00 meters,
then by f/2.8 it might now be centered at 1.02 meters."

On average (ignoring front/back approach), Wilba's camera is
focusing closer (when 'stopped down' through obstructions to the
AF sensor),
then opening fully to the sensor for taking the pic; the actual
point in focus is further back. That's the opposite of what this
effect would suggest.

My version of this jigsaw puzzle has several significant pieces
missing. :- )

"Obstructions to the AF sensor" - what is that? How is that
"stopped down" (for the AF sensor?)?

What part of the camera+lens system is "opening fully to the sensor
for taking the pic"? That sounds like something you'd say about the
aperture, but that isn't closing at any stage so it can't then
open...?

Completely not getting what your saying. :- )

The view from the AF sensor isn't as clear as the view to the image
sensor so it's effectively stopped down a little. I'm not sure how
exactly.

The view on the ground glass is also. Supposedly anything faster
than perhaps f/2.8 doesn't improve the brightness, etc.

But this focus shift theory seems to predict the opposite behavior
from what you are getting.

But how do you get aperture-related focus shift if you're not
stopping down for the exposure?

It would be opening up for the exposure. Like if you held a smaller
aperture over the front for focusing then removed it to take the shot.

What the flipping heck are you talking about!?! :- )

How can the aperture go wider than its widest?! Why/how/who/when
would you hold a smaller aperture over the front for focussing?

Are we actually talking about the same thing? The only aperture-related
focus shift I know about is like this -
http://diglloyd.com/diglloyd/free/FocusShift/index.html. What you're
saying only sounds like it might make some kind of sense in relation to
something completely different. :- )

:-)

The AF sensors are off to the side, looking through mirrors and/or
prisms or something, like the viewfinder.

Ri-ight.

When you block part of their view, the obstruction acts the same as
aperture blades: making the opening narrower.

What does that?! How's it relevant to _anything_ FFS? :- )

When the actual photo is taken, all the mirrors get out of the way and
the full f/1.8 projects in all it's glory onto the photo sensor.

Yeah... so at that point there's no light from the optical path falling
on the AF sensor. Why does it matter what the AF sensor is doing then?!!
:- )

Just give me any clue, anything, about what this has to do with my
experiements and this kind of focus shift -
http://diglloyd.com/diglloyd/free/FocusShift/index.html - that _could_ be
a factor, _if_ I was stopping down the aperture from its maximum. :- )

The trend this suggests is the opposite of what you are experiencing if
I'm following this right, so consider this a tangent. But if it were a
factor, it would be because there is something blocking the view that the
AF sensors see, so they are locking onto the target based on a stopped
down view.

It's like the AF sensor(s) are sitting at the side of the stage, peering
through the curtains at an angle through a small mirror g.

Ah, OK. As long as it's not related to any known phenomenon of relevance, I
won't let it won't bother me. :- )

Happy New Year (my time) in one minute...

Yeah? We did that last night. yawn :- )

"Wilba" wrote in message
...
[]
But how do you get aperture-related focus shift if you're not stopping
down for the exposure?

Focus sensors use the rays from the centre portion of the ray bundle
coming out of the back end of the lens (effectively f/2.8 or f/4.0 - for
example), and may therefore focus on a different axial location than where
the rays from the extreme of the lens (f/1.8, say) converge, if the lens
has spherical aberrations.

David

"Wilba" wrote in message
...
[]
Sure. So when the aperture *doesn't* change (focus and exposure both
occur at the same aperture), how do you get focus shift in the centre of
the image?

I already explained that the focus sensor may only have a working aperture
of f/4, for example, so the aperture /is/ different.

David

With spherical aberration, for example, rays from the edge of the lens
(i.e. full aperture) focus at a different point from those from the middle
part of the lens, so the focus "point" can shift with aperture. The focus
sensors tend to have a narrower acceptance angle than f/1.8, hence they
will adjust the lens so that rays from nearer the centre will be focussed,
leaving the outer rays focussing at a different, incorrect position.

Sure. So when the aperture *doesn't* change (focus and exposure both occur
at the same aperture), how do you get focus shift in the centre of the
image?



At least in Canon cameras, it is because the focus sensors use at most
the light of an f/2.8 lens. IF this is the problem cause, the focus should be
correct at f/2.8 using the central cross autofocus sensor.

TYPICALLY, for Gauss lenses like this one, no aspheric elements, the central part
of the lens (say that part used at f/11) and the very outer part focus at the
same distance. It is the part at one f-stop from maximum (i.e. in this
lens near f/2.8) that suffers worst from focus shift. Thus the problem.

However, this is a small problem.

Doug McDonald

David J Taylor wrote:
Wilba wrote:
[]
But how do you get aperture-related focus shift if you're not stopping
down for the exposure?

Focus sensors use the rays from the centre portion of the ray bundle
coming out of the back end of the lens (effectively f/2.8 or f/4.0 - for
example), and may therefore focus on a different axial location than where
the rays from the extreme of the lens (f/1.8, say) converge, if the lens
has spherical aberrations.

Let me rephrase my question for clarity -

How do you get aperture-related focus shift
(http://diglloyd.com/diglloyd/free/Fo...ft/index.html), in the centre of
the image, if you're not stopping down for the exposure?

David J Taylor wrote:
Wilba wrote:
[]
Sure. So when the aperture *doesn't* change (focus and exposure both
occur at the same aperture), how do you get focus shift in the centre of
the image?

I already explained that the focus sensor may only have a working aperture
of f/4, for example, so the aperture /is/ different.

That's nothing to do with the only form of aperture-related focus shift that
I'm familiar with - http://diglloyd.com/diglloyd/free/FocusShift/index.html.

If you are talking about some other phenomenon, please provide a link to a
good description.

I can't see how what you're talking about would be relevant anyway, since
the AF system is calibrated to produce a good focus under exactly those
conditions.

Doug McDonald wrote:
Wilba wrote:

With spherical aberration, for example, rays from the edge of the lens
(i.e. full aperture) focus at a different point from those from the
middle part of the lens, so the focus "point" can shift with aperture.
The focus sensors tend to have a narrower acceptance angle than f/1.8,
hence they will adjust the lens so that rays from nearer the centre will
be focussed, leaving the outer rays focussing at a different, incorrect
position.

Sure. So when the aperture *doesn't* change (focus and exposure both
occur at the same aperture), how do you get focus shift in the centre of
the image?

At least in Canon cameras, it is because the focus sensors use at most
the light of an f/2.8 lens.

Do you mean that you don't get any advantage from an aperture wider than
f/2.8?

I'd say what I'm thinking more like, a Canon high-precision AF sensor needs
f/2.8 or better to match its "base" (as in a rangefinder).

IF this is the problem cause, the focus should be
correct at f/2.8 using the central cross autofocus sensor.

Depends what problem you're talking about. I can't imagine how it could
matter to this problem -
http://diglloyd.com/diglloyd/free/FocusShift/index.html. If you're talking
about a different problem, please give me a link to a description so that I
have a clue what it is.

TYPICALLY, for Gauss lenses like this one, no aspheric elements,
the central part of the lens (say that part used at f/11) and the very
outer part focus at the same distance. It is the part at one f-stop from
maximum (i.e. in this lens near f/2.8) that suffers worst from focus
shift.
Thus the problem.

However, this is a small problem.

What _is_ the problem? :- )

Ofnuts wrote:

Bertrand, happy owner of a 450D and a 50/1.8

Bertrand, would you be willing to spend a few minutes doing a test for me?

I have come up with a simple direct way to test the "crude mechanism"
theory, but it requires something like a macro focus rail to move the camera
in increments of say 0.5mm. Is that something you can do?

Anyone else able to help? Thanks.

On 02/01/2010 02:16, Wilba wrote:
Ofnuts wrote:

Bertrand, happy owner of a 450D and a 50/1.8

Bertrand, would you be willing to spend a few minutes doing a test for me?

I have come up with a simple direct way to test the "crude mechanism"
theory, but it requires something like a macro focus rail to move the camera
in increments of say 0.5mm. Is that something you can do?

Anyone else able to help? Thanks.

I'd be willing to help, but I haven't got a macro rail... I have to
think if I can cobble up something to move the subject instead.

What do you have in mind?

--
Bertrand

"Wilba" wrote in message
...
[]
Let me rephrase my question for clarity -

How do you get aperture-related focus shift
(http://diglloyd.com/diglloyd/free/Fo...ft/index.html), in the centre
of the image, if you're not stopping down for the exposure?

... because the focus sensor /is/ stopping down when measuring the focus.
It's only looking at a small f/4 (or whatever) cone, not the full f/1.8
cone from the lens. Were you to stop down when taking to the cone the
focus sensor is using, the focus would then be exactly as the focus sensor
measured. Use a larger ray bundle when taking, i.e. by using an f/1.8
lens at full aperture, the rays may focus at a different point, and the
resulting image on the sensor may appear out of focus. Likely to be a
small effect unless the lens has excessive aberrations and the ratio
between focus f/number and taking f/number is significant.

The calibration of the AF system will be at a certain aperture (possibly
more than one aperture on more expensive cameras). Use a different lens
aperture and you are at the mercy of how much the lens focal point shifts
when it works at a different aperture to that which the focus sensor uses.

Cheers,
David