From: Paul Furman on
Wilba wrote:
> Wilba wrote:
>> Wilba wrote:
>>> Doug McDonald wrote:
>>>> You take pictures using manual focus (at f/1.8) with the focus
>>>> indicator (the little dot at lower right in my 30D) as the criterion
>>>> of correct focus. Try to get it centered in the middle of
>>>> the "in focus is indicated" range. Make several tries. If this
>>>> gives better focus than real autofocus, something is wrong with the
>>>> "focus movement prediction" system.
>>>>
>>>> My results say that the indicator is better than real autofocus,
>>>> but not by a lot. This is using only the center spot on text
>>>> 7 feet away.
>>> Ah, this is exactly the kind of idea I need! That experiment attempts to
>>> separate the detection (AF sensor) and actuation (lens motor) functions
>>> within the system. I'll have a play with it today. Thanks a bunch.
>> Well, it sure has been an interesting day. :- )
>>
>> Here are some preliminary results with a 45 degree target. I need to
>> repeat with a different target before I would state the following with
>> confidence, and it could be quite different for other units depending on
>> their calibration. But anyway...
>>
>> Sharpness in the very centre at f/1.8 appeared fine, practically
>> indistinguishable from f/2.8 or f/4. In previous tests I might not have
>> looked hard enough at the very centre of the image, and may have been
>> misled by other sources of variation. So that's suggests a disproof of my
>> "soft at f/1.8" theory.
>>
>> I found that my phase detect AF sensor has sidedness. If I start with the
>> lens focussed closer than the subject, the results are uniformly
>> excellent, whether autofocussing or manually focussing using the AF
>> confirmation (as Doug described above).
>>
>> If I start with the lens focussed behind the subject, and I manually focus
>> using the AF confirmation, focus is always off by the same tiny amount
>> (one click towards infinity in the EOS Utility will bring it into optimal
>> focus).
>>
>> With initial focus behind the subject and PD autofocus, about seven shots
>> out of ten are out by the same one click as the manual focus, and the rest
>> are optimal, like when starting from the nearside. I assume that the good
>> ones come about from the lens overshooting and then the system corrects
>> towards infinity (so it ultimately approaches focus from the nearside).
>>
>> With an external aperture which gives similar exposures to f/2.8 (with the
>> lens at f/1.8), focus is still excellent starting from the near side, and
>> it improves the performance when starting from the far side (but still not
>> optimal).
>>
>> Those results, and the fact that the lens is able to focus perfectly via
>> contrast detect, suggest to me that the "crude and sloppy mechanism"
>> theory is bogus. I suspect the truth is about the performance of the PD AF
>> sensor with this lens, specifically how much latitude it has for
>> confirming a focus. I noticed when manually focussing with a macro rail
>> that I could move the camera quite some (micro) distance and still get a
>> focus confirmation beep.
>
> I've been thinking a lot about the width of the "beep band" and what it
> means, so I measured it with a macro rail. With the lens at its closest
> focus (430mm from the sensor), the far beep is 3.0mm from the near beep. A
> conventional DOF calculation gives around 4.3mm, and a focus from the
> nearside appears to put the subject bang in the middle of that. So that
> means a farside focus would put the subject something like 0.8mm outside the
> DOF, which sounds about right.

|4.3mm| theoretical
->|3mm|<- measured
.8mm
<- 430mm -> [camera]

> I also tried it at that distance with an EF-S 18-55 at about 50mm (f/5.6).
> The beep band is 12mm, the DOF is 13mm, and where you start from makes very
> little difference to the focus achieved (although from the nearside is best
> again). I wish I had another wide-aperture FFL lens to compare.

|13mm| theoretical
->|12 |<- measured

> 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?

Did I get those diagrams right? I'm really not following what 'makes
sense' about the f/1.8 error. Are these distances confirmed with photo
results in a focus test pattern? For one thing, I wouldn't bother with
calculated DOF, especially for close up.

I would predict results kinda like this due to the AF sensors getting a
restricted aperture view of things with a bit more DOF:

->| 4mm |<- where it beeps
|3mm| confirmed test chart dof

Then if you were taking the final shot at f/8, focus shift as a result
of stopping down could produce something like this:

->| 4mm |<- where it beeps
| 30mm | confirmed test chart dof

or, since you are getting better results from in front:

->| 4mm |<- where it beeps
| 30mm | confirmed test chart dof

But without stopping down, I can't explain the difference from behind or
in front... however, that's at least a useful conclusion from all this:
if you start focused from in front, better focus can be achieved. So
maybe that's the only relevant lesson from all this, and it's a good one.

OK, here's a possible explanation for the front/back difference. The
character of the out of focus area in front & back is indeed different.
In terms of bokeh, the background is usually smoother and the foreground
harsh. There are a few specialized 'defocus control' lenses which have
an extra ring to change this relationship. I'm not sure quite how that
would effect the AF confirm but it could be a factor.


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

all google groups messages filtered due to spam
From: Wilba on
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?

> and the focus sensors may only accept the smaller cone of rays rather
> than the full f/1.8 cone.

I don't understand any of that. :- )


From: Wilba on
Paul Furman wrote:
> Wilba wrote:
>> Wilba wrote:
>>> Wilba wrote:
>>>> Doug McDonald wrote:
>>>>>
>>>>> You take pictures using manual focus (at f/1.8) with the focus
>>>>> indicator (the little dot at lower right in my 30D) as the criterion
>>>>> of correct focus. Try to get it centered in the middle of
>>>>> the "in focus is indicated" range. Make several tries. If this
>>>>> gives better focus than real autofocus, something is wrong with the
>>>>> "focus movement prediction" system.
>>>>>
>>>>> My results say that the indicator is better than real autofocus,
>>>>> but not by a lot. This is using only the center spot on text
>>>>> 7 feet away.
>>>>
>>>> Ah, this is exactly the kind of idea I need! That experiment attempts
>>>> to separate the detection (AF sensor) and actuation (lens motor)
>>>> functions within the system. I'll have a play with it today. Thanks a
>>>> bunch.
>>>
>>> Well, it sure has been an interesting day. :- )
>>>
>>> Here are some preliminary results with a 45 degree target. I need to
>>> repeat with a different target before I would state the following with
>>> confidence, and it could be quite different for other units depending on
>>> their calibration. But anyway...
>>>
>>> Sharpness in the very centre at f/1.8 appeared fine, practically
>>> indistinguishable from f/2.8 or f/4. In previous tests I might not have
>>> looked hard enough at the very centre of the image, and may have been
>>> misled by other sources of variation. So that's suggests a disproof of
>>> my "soft at f/1.8" theory.
>>>
>>> I found that my phase detect AF sensor has sidedness. If I start with
>>> the lens focussed closer than the subject, the results are uniformly
>>> excellent, whether autofocussing or manually focussing using the AF
>>> confirmation (as Doug described above).
>>>
>>> If I start with the lens focussed behind the subject, and I manually
>>> focus using the AF confirmation, focus is always off by the same tiny
>>> amount (one click towards infinity in the EOS Utility will bring it into
>>> optimal focus).
>>>
>>> With initial focus behind the subject and PD autofocus, about seven
>>> shots out of ten are out by the same one click as the manual focus, and
>>> the rest are optimal, like when starting from the nearside. I assume
>>> that the good ones come about from the lens overshooting and then the
>>> system corrects towards infinity (so it ultimately approaches focus from
>>> the nearside).
>>>
>>> With an external aperture which gives similar exposures to f/2.8 (with
>>> the lens at f/1.8), focus is still excellent starting from the near
>>> side, and it improves the performance when starting from the far side
>>> (but still not optimal).
>>>
>>> Those results, and the fact that the lens is able to focus perfectly via
>>> contrast detect, suggest to me that the "crude and sloppy mechanism"
>>> theory is bogus. I suspect the truth is about the performance of the PD
>>> AF sensor with this lens, specifically how much latitude it has for
>>> confirming a focus. I noticed when manually focussing with a macro rail
>>> that I could move the camera quite some (micro) distance and still get a
>>> focus confirmation beep.
>>
>> I've been thinking a lot about the width of the "beep band" and what it
>> means, so I measured it with a macro rail. With the lens at its closest
>> focus (430mm from the sensor), the far beep is 3.0mm from the near beep.
>> A conventional DOF calculation gives around 4.3mm, and a focus from the
>> nearside appears to put the subject bang in the middle of that. So that
>> means a farside focus would put the subject something like 0.8mm outside
>> the DOF, which sounds about right.
>
> |4.3mm| theoretical
> ->|3mm|<- measured
> .8mm
> <- 430mm -> [camera]
>
>> I also tried it at that distance with an EF-S 18-55 at about 50mm
>> (f/5.6). The beep band is 12mm, the DOF is 13mm, and where you start from
>> makes very little difference to the focus achieved (although from the
>> nearside is best again). I wish I had another wide-aperture FFL lens to
>> compare.
>
> |13mm| theoretical
> ->|12 |<- measured
>
>> 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?
>
> Did I get those diagrams right?

Um, I don't understand your diagrams. :- ) Let me try again.

With f/1.8 and a subject in optimal focus at 430mm, a conventional DOF
calculation gives 4.3mm, so lets assume 2.2mm each side (I know we can't
rely on this figure, but it's not a million miles from what I see). Starting
with the camera at 425mm and moving it away from the subject until it beeps
focus confirmation, we get optimal focus again at 430mm. Starting with the
camera at 435mm and moving in, it beeps at 433mm, which is 0.8mm outside the
theoretical DOF and is clearly out of focus.

> I'm really not following what 'makes sense' about the f/1.8 error.

What makes sense is that a focus 3mm beyond the optimal camera to subject
distance would put the subject outside the DOF (3.0 - 4.3/2 = 0.8).

> Are these distances confirmed with photo results in a focus test
> pattern?

Yes, the above is just a mathematical expression of what I'm seeing from
doing the beep tests. When I said, "a farside focus would put the subject
something like 0.8mm outside the DOF, which sounds about right", I meant
that the numbers agree with what I'm seeing - the subject is definitely a
small distance outside the DOF in applicable images.

What's interesting is that the beep tests give the same images as AF. That
tells me that the focus errors that people see with this lens when focussing
are probably not due to a lack of precision in the lens's focus mechanism
(as is commonly believed), but may be due to the fact that the focus sensor
will confirm a focus when the subject is clearly not in focus. That happens
100% of the time when I focus by moving my camera, and something in the
order of 70% of the time when I AF, with the subject further away than where
the lens is initially focussed. (YMMV with a differently calibrated body and
lens.)

> For one thing, I wouldn't bother with calculated DOF, especially for close
> up.

Right, that's just to have some numbers that fit with what I'm seeing.

> I would predict results kinda like this due to the AF sensors getting a
> restricted aperture view of things with a bit more DOF:

I don't follow that, sorry. (All tests and calculations were done at widest
aperture - f/1.8.)

> But without stopping down, I can't explain the difference from behind or
> in front... however, that's at least a useful conclusion from all this: if
> you start focused from in front, better focus can be achieved. So maybe
> that's the only relevant lesson from all this, and it's a good one.

Yes. :- ) (Again, results may be quite different with other combinations
of bodies and lenses.)

> OK, here's a possible explanation for the front/back difference. The
> character of the out of focus area in front & back is indeed different. In
> terms of bokeh, the background is usually smoother and the foreground
> harsh. There are a few specialized 'defocus control' lenses which have an
> extra ring to change this relationship. I'm not sure quite how that would
> effect the AF confirm but it could be a factor.

Very interesting idea, thanks very much. This -
http://www.stacken.kth.se/~maxz/defocuscontrol/ (and the links in it to
http://www.luminous-landscape.com/essays/bokeh.shtml) - gave me enough to
get what you're saying.

That brings us back again to the notion that it is the optical performance
of the 50/1.8 that causes the erratic focus from the farside, not the
perceived crudeness of the focus mechanism.

What's different for me now is that I believe the erratic farside focus is
not about the sharpness wide open, but more likely about the different
"character of the out of focus area" on the nearside and farside of the
plane of focus.


From: Wilba on
Wilba wrote to Paul Furman:
>
> Um, I don't understand your diagrams. :- ) Let me try again.
>
> With f/1.8 and a subject in optimal focus at 430mm, a conventional DOF
> calculation gives 4.3mm, so lets assume 2.2mm each side (I know we can't
> rely on this figure, but it's not a million miles from what I see).
> Starting with the camera at 425mm and moving it away from the subject
> until it beeps focus confirmation, we get optimal focus again at 430mm.
> Starting with the camera at 435mm and moving in, it beeps at 433mm, which
> is 0.8mm outside the theoretical DOF and is clearly out of focus.

Try viewing this text with a fixed width font (e.g. copy into Notepad) -

|< 430mm >| sensor to subject distance for optimal
focus
| |< 2.2mm >|< 2.2mm >| theoretical
DOF
|< 430mm >| nearside
focus
|< 433mm >| farside
focus
| |< 3.0mm >| the "beep
band"
| |<0.8mm>| outside the
DOF


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

>> and the focus sensors may only accept the smaller cone of rays rather
>> than the full f/1.8 cone.

> I don't understand any of that. :- )

It means the AF sensor has a smaller effective aperture than is being
used to take the photograph. If you want the best results from a wide
aperture 50mm of spherical design then you need to understand
this. Alternatively you could buy a modern aspherical design. One of
the great benefits of modern technology is that it enables people who
don't understand what they're doing to do it well.

--
Chris Malcolm