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EF 50/1.8 AF Experiment?



 
 
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  #331  
Old April 2nd 10, 12:59 AM posted to rec.photo.digital.slr-systems
Wilba[_3_]
external usenet poster
 
Posts: 572
Default EF 50/1.8 AF Experiment?

MikeWhy wrote:

I swear by gumb. 40x0.1mm should be cause enough to wrap you in padding
and committed. I can't empathize, but I'm pleased to see an active
curiousity live on in the this world.


I had a spare 1/2 hour and gear that would do it, so why not? :- )

For a short time, depending on how the forum moderators feel about
appropriateness, a no effort nifty-fifty shot is at:
http://photo.net/no-words-forum/00W8Os. Nothing special; this quality of
focus is routine. It was just happened to be up in LR as your last post
popped into my newsreader.


Anyone can do that at f/4. :-D


  #332  
Old April 2nd 10, 01:12 AM posted to rec.photo.digital.slr-systems
Chris Malcolm[_2_]
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Posts: 3,142
Default EF 50/1.8 AF Experiment?

Wilba wrote:
Chris Malcolm wrote:
Wilba wrote:
Chris Malcolm wrote:
Wilba wrote:

The central "enhanced precision" PD AF sensor is sufficiently
confused by the fuzziness caused by spherical aberation for it
to confirm focus over a range of camera to subject distances
of the order of the minimum DOF (widest aperture, closest
focus distance).

The fuzziness caused by spherical aberration is caused by the fact the
perfect lens shape is only nearly spherical. So in a spherical lens
the rays from the lens edge focus at a different point than those from
the centre. But this won't cause any fuzziness in a PDAF sensor
because that works by taking two small samples of rays from two spaced
apart little apertures equally spaced about the centre. The distance
between them is in effect like the baseline of a rangefinder. That
distance apart also corresponds to an aperture of that diameter. So
the PDAF can't work with lenses of smaller aperture than the width
apart of the AF sensor pair.

Therefore in a spherical lens the AF sensor finds exact focus at the
point where the rays from that particular part of the spherical lens
surface focuses. But the image is taken at the specified aperture. So
if it is stopped down from the effective AF aperture the focus will
drift one way. That probably won't matter because typically the DoF is
increased a lot more than the drift.

If on the other hand the image taking aperture is larger than the
effective AF aperture, as it can be with wide aperture lenses, then
the focus drifts the other way, while at the same time the DoF is
narrowing. In this case the resultant focus drift can take the point
of focus outside the DoF.

Yes, that's as I've understood it since before I began this thread.

Can you use the above to explain why an initial gross front-focus
results in optimal focus, and an initial gross back-focus results in
front-focus (with my calibrated 450D and 50/1.8)?

Everything is identical in both cases except for the spatial relationship
of the subject and the plane of focus prior to PD AF being invoked.
It doesn't matter whether I use f/1.8 (the widest aperture of the lens),
or f/2.8 (the rangefinder base aperture of the PD AF sensor), throughout
an individual set of front and back-focussed test exposures, I still get
the
same pattern of optimal focus or front-focus.

How does aperture-related focus shift explain that phenomenon?


That baffles me. It doesn't happen with my 50mm f1.4 lens.


Which bit? _That_ it happens with the 50/1.8, or _why_ it happens?


Both.

--
Chris Malcolm
  #333  
Old April 2nd 10, 01:35 AM posted to rec.photo.digital.slr-systems
Wilba[_3_]
external usenet poster
 
Posts: 572
Default EF 50/1.8 AF Experiment?

Chris Malcolm wrote:
Wilba wrote:
Chris Malcolm wrote:
Wilba wrote:
Chris Malcolm wrote:
Wilba wrote:

The central "enhanced precision" PD AF sensor is sufficiently
confused by the fuzziness caused by spherical aberation for it
to confirm focus over a range of camera to subject distances
of the order of the minimum DOF (widest aperture, closest
focus distance).

The fuzziness caused by spherical aberration is caused by the fact the
perfect lens shape is only nearly spherical. So in a spherical lens
the rays from the lens edge focus at a different point than those from
the centre. But this won't cause any fuzziness in a PDAF sensor
because that works by taking two small samples of rays from two spaced
apart little apertures equally spaced about the centre. The distance
between them is in effect like the baseline of a rangefinder. That
distance apart also corresponds to an aperture of that diameter. So
the PDAF can't work with lenses of smaller aperture than the width
apart of the AF sensor pair.

Therefore in a spherical lens the AF sensor finds exact focus at the
point where the rays from that particular part of the spherical lens
surface focuses. But the image is taken at the specified aperture. So
if it is stopped down from the effective AF aperture the focus will
drift one way. That probably won't matter because typically the DoF is
increased a lot more than the drift.

If on the other hand the image taking aperture is larger than the
effective AF aperture, as it can be with wide aperture lenses, then
the focus drifts the other way, while at the same time the DoF is
narrowing. In this case the resultant focus drift can take the point
of focus outside the DoF.

Yes, that's as I've understood it since before I began this thread.

Can you use the above to explain why an initial gross front-focus
results in optimal focus, and an initial gross back-focus results in
front-focus (with my calibrated 450D and 50/1.8)?

Everything is identical in both cases except for the spatial
relationship
of the subject and the plane of focus prior to PD AF being invoked.
It doesn't matter whether I use f/1.8 (the widest aperture of the
lens),
or f/2.8 (the rangefinder base aperture of the PD AF sensor),
throughout
an individual set of front and back-focussed test exposures, I still
get
the same pattern of optimal focus or front-focus.

How does aperture-related focus shift explain that phenomenon?

That baffles me. It doesn't happen with my 50mm f1.4 lens.


Which bit? _That_ it happens with the 50/1.8, or _why_ it happens?


Both.


And that's what it's all about. :- )

I think you hinted at an explanation a couple of posts ago - "Therefore in a
spherical lens the AF sensor finds exact focus at the point where the rays
from that particular part of the spherical lens surface focuses." If the
rays from that particular part of the lens surface _cannot_be_ exactly
focused, then how likely is it that exact focus in the centre of the image
can be achieved using those rays? I haven't seen a better explanation.


  #334  
Old April 2nd 10, 02:15 AM posted to rec.photo.digital.slr-systems
MikeWhy
external usenet poster
 
Posts: 78
Default EF 50/1.8 AF Experiment?

"Wilba" wrote in message
...
MikeWhy wrote:

I swear by gumb. 40x0.1mm should be cause enough to wrap you in padding
and committed. I can't empathize, but I'm pleased to see an active
curiousity live on in the this world.


I had a spare 1/2 hour and gear that would do it, so why not? :- )


That was 4 months ago.

For a short time, depending on how the forum moderators feel about
appropriateness, a no effort nifty-fifty shot is at:
http://photo.net/no-words-forum/00W8Os. Nothing special; this quality of
focus is routine. It was just happened to be up in LR as your last post
popped into my newsreader.


Anyone can do that at f/4. :-D


My point exactly.

 




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