From: OldBoy on
"Wilba" <usenet(a)CUTTHISimago.com.au> wrote in message
news:034d2e3f$0$1347$c3e8da3(a)news.astraweb.com...
> 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...?

It's more a switch to higher sensitivity and by that more accuracy (no
hunting).

> 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.

And in-lens stabilization helps also since the image on the AF sensor is
also stabilized :-)

From: Paul Furman on
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.

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

all google groups messages filtered due to spam
From: Wilba on
OldBoy wrote:
> Wilba wrote:
>> 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...?
>
> It's more a switch to higher sensitivity and by that more accuracy (no
> hunting).

Yeah, by using a more accurate (i.e. wider "base") AF sensor than the f/5.6
sensor.

>> 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.
>
> And in-lens stabilization helps also since the image on the AF sensor is
> also stabilized :-)

Yeah, could do. I had it off for all the tests for that reason.


From: Wilba on
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?


From: Paul Furman on
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.

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

all google groups messages filtered due to spam