From: Wilba on
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.

I thought aperture related focus drift happened if the shot was at an
aperture other than wide open. You're saying when I focus and shoot at the
lens's widest aperture, that's when I get focus drift? How does that work?

>>> 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.

What do I have to do differently because of it?

> 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.

I think that often cuts both ways. :- )


From: Wilba on
Paul Furman wrote:
> Wilba wrote:
>> Paul Furman wrote:

>>> 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.
>
> Yeah, that's the only thing that makes sense, though it's still a bit
> sketchy. In the middle of this page:
> http://www.stacken.kth.se/~maxz/defocuscontrol/
> the crops of dark blurry blobs show harsh rimmed OOF circles and perhaps
> the AF sensor is grabbing those edges. I don't think that's quite right
> but the best I can come up with.

It's a credible idea. :- )

Thanks a bunch for your help!


From: Wilba on
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. :- )


From: Wilba on
David J Taylor 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?
>>
>>> 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. :- )
>
> See the ray diagram here:
> http://en.wikipedia.org/wiki/Spherical_aberration
>
> 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?


From: Wilba on
OldBoy wrote:
> Wilba wrote:
>> Wilba wrote:

>>> 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.
>>
>> 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.
>>
>> 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?
>
> Keep in mind that according to Canon, AF accuracy is within 1 CoC for
> non-pro EOS camera's and within 1/3 CoC for pro EOS camera's.
> (About +/- 0.02 mm resp. +/- 0.010mm at the sensor)

I thought that was about the high-precision AF sensor kicking in with lenses
that have a maximum aperture of f/2.8 or better...?

If it is about the max. aperture of the lens (and whether it's narrower than
f/2.8), then the 18-55 should perform worse, not better. Having thought some
more about this, I'm tending to think that it's probably more of a DOF thing
(harder to see it with deeper DOF) than the effect being absent with the
18-55.