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EF 50/1.8 AF Experiment?



 
 
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  #1  
Old December 20th 09, 10:00 AM posted to rec.photo.digital.slr-systems
Wilba[_3_]
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Posts: 572
Default EF 50/1.8 AF Experiment?

The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two
shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2) erratic
focus under difficult conditions (low light, shallow DOF).

Many people claim that 2) is a result of the crudeness of the cheap
focussing motor and electronics in the lens, that those components are not
able to provide the required accuracy and control of motion of the focus
ring.

But I wonder if 2) is actually a result of 1) - if the AF sensors have fuzzy
images to work with, how /could/ the system nail the focus in difficult
conditions?

It would be interesting to see what happens when the AF sensors have sharper
images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to AF when
the DOF preview button is pressed, so I can't test that. External aperture
perhaps?

Any ideas for how these competing hypotheses could be tested? Is there a
consequence of either hypothesis that could be disproved empirically?


  #2  
Old December 20th 09, 12:09 PM 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:
The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two
shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2) erratic
focus under difficult conditions (low light, shallow DOF).


Many people claim that 2) is a result of the crudeness of the cheap
focussing motor and electronics in the lens, that those components are not
able to provide the required accuracy and control of motion of the focus
ring.


But I wonder if 2) is actually a result of 1) - if the AF sensors have fuzzy
images to work with, how /could/ the system nail the focus in difficult
conditions?


It would be interesting to see what happens when the AF sensors have sharper
images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to AF when
the DOF preview button is pressed, so I can't test that. External aperture
perhaps?


Any ideas for how these competing hypotheses could be tested? Is there a
consequence of either hypothesis that could be disproved empirically?


The AF sensors pay no attention to the aperture at which you're going
to take the picture. They do their work before the lens is stopped
down. Their construction gives them an effective aperture of their
own. Often this is around f6. That means that when the largest
aperture of a lens is smaller than that they can't get enough light to
work properly. That's why generally speaking you can't make reflex
lenses autofocus, because for technical reasons their best compromise
aperture is often smaller than that, e.g. 500mm f8.

More expensive DSLRs will also have larger aperture AF sensors at the
central position, e.g. around f3, with which they'll be able to get
focus in lower light with lenses which with max apertures which open
that far. It also improves the focus on very fast lenses with
spherical aberration and corresponding aperture related focus drift,
such as the old spherical type of 50mm f1.4 lenses.

Since the DOF gets very thin indeed at wide apertures and close
portrait type distances, which is often what is going on in a dimly
lit interior, the slightest error in AF will leave the image blurred
at the point you wished to focus on, and sharp nearby. For example in
a portrait you might have focused on the eyes, and find that the eyes
aren't in focus, but the tip of the nose, or the ears, are. The reason
for that is often that when DoF gets so sharp it becomes smaller than
the small residual error in the AF of your camera, i.e. your camera
has a slight front or back focus in the AF sensor plane calibration
which is larger than the DoF at these wide apertures.

If you find a systematic error of this type in your camera than you
either must switch to manual focus, or compensate yourself, e.g. by
holding down focus on the eyes and then simply moving your head back
or forwards a few cm to take up the systematic error.

Usually the more expensive DSLRs have better AF sensors so they can
focus better in lower light. The wider aperture AF sensors are also
able to get a tighter focus for wide aperture low light work because
the AF sensor itself has effectively a shallower DoF. That will also
rein in some of the aperture related focus drift of wide aperture
spherical lenses.

The more expensive DSLRs are also sometimes able to read lens-specific
focus compensation factors from the lens, and use that to trim out
systematic errors in autofocus for that specific lens.

The most expensive DSLRs go one better than that. They have user
trimmable tables of focus compensation for specific lenses in order to
get better focus with the more awkward lenses in the more awkward
situation, in which the AF will have slight lens-specific systematic
focus errors.

--
Chris Malcolm
  #3  
Old December 20th 09, 12:50 PM posted to rec.photo.digital.slr-systems
Ofnuts
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Posts: 644
Default EF 50/1.8 AF Experiment?

On 20/12/2009 10:00, Wilba wrote:
The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two
shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2) erratic
focus under difficult conditions (low light, shallow DOF).

Many people claim that 2) is a result of the crudeness of the cheap
focussing motor and electronics in the lens, that those components are not
able to provide the required accuracy and control of motion of the focus
ring.

But I wonder if 2) is actually a result of 1) - if the AF sensors have fuzzy
images to work with, how /could/ the system nail the focus in difficult
conditions?


AF sensors work quite well with other lenses that are not that sharp, so
I doubt that the "softness" of the lens at f/1.8 is really a culprit.

It would be interesting to see what happens when the AF sensors have sharper
images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to AF when
the DOF preview button is pressed, so I can't test that. External aperture
perhaps?


Read this excellent explanation of the DSLR phase-detection AF system:

http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf

To make it short, for AF sensors:
- they are designed to work with a minimum aperture (usually f/5.6 or
better)(and don't benefit from a bigger one)
- the more accurate you want the sensor, the wider the design aperture
has to be.

IIRC in the 450D most AF sensors require f/5.6 minimum, and the central
one is doubled with a "bigger" one that requires f/2.8 and is put in
action when the mounted lens reports that it has a maximum aperture of
f/2.8 or better. This allows a more accurate focus with these lenses,
which is required since the aperture of the lens can lead to very
shallow DoF (some entry level DSLR haven't got that second AF sensor and
cannot be efficiently used with lenses opening at f/2.8 or better).

Any ideas for how these competing hypotheses could be tested? Is there a
consequence of either hypothesis that could be disproved empirically?


The 450D is an entry-level camera, so don't expect miracles. With the
50/1.8 the accuracy of its AF system may be a bit pushed to its limits.
And make sure that you are using the central sensor for the the AF.

--
Bertrand, happy owner of a 450D and a 50/1.8
  #4  
Old December 20th 09, 04:00 PM posted to rec.photo.digital.slr-systems
Alan Browne
external usenet poster
 
Posts: 12,640
Default EF 50/1.8 AF Experiment?

On 09-12-20 4:00 , Wilba wrote:
The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two
shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2) erratic
focus under difficult conditions (low light, shallow DOF).

Many people claim that 2) is a result of the crudeness of the cheap
focussing motor and electronics in the lens, that those components are not
able to provide the required accuracy and control of motion of the focus
ring.

But I wonder if 2) is actually a result of 1) - if the AF sensors have fuzzy
images to work with, how /could/ the system nail the focus in difficult
conditions?

It would be interesting to see what happens when the AF sensors have sharper
images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to AF when
the DOF preview button is pressed, so I can't test that. External aperture
perhaps?

Any ideas for how these competing hypotheses could be tested? Is there a
consequence of either hypothesis that could be disproved empirically?


Offhand, even if it is soft wide open, the AF should settle on the
"sharpest" slightly soft contrast, which on average should be sharp enough.

Could you post a simple target wide open?

In fact could you post the one on p18 of this document, shot at a 45 deg
angle. I'd like to compare it to the Minolta 50 f/1.7.

http://focustestchart.com/chart.html#ActualChart
  #5  
Old December 20th 09, 04:12 PM posted to rec.photo.digital.slr-systems
Alan Browne
external usenet poster
 
Posts: 12,640
Default EF 50/1.8 AF Experiment?

On 09-12-20 6:09 , Chris Malcolm wrote:
wrote:
The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two
shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2) erratic
focus under difficult conditions (low light, shallow DOF).


Many people claim that 2) is a result of the crudeness of the cheap
focussing motor and electronics in the lens, that those components are not
able to provide the required accuracy and control of motion of the focus
ring.


But I wonder if 2) is actually a result of 1) - if the AF sensors have fuzzy
images to work with, how /could/ the system nail the focus in difficult
conditions?


It would be interesting to see what happens when the AF sensors have sharper
images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to AF when
the DOF preview button is pressed, so I can't test that. External aperture
perhaps?


Any ideas for how these competing hypotheses could be tested? Is there a
consequence of either hypothesis that could be disproved empirically?


The AF sensors pay no attention to the aperture at which you're going
to take the picture. They do their work before the lens is stopped
down.


It's clear that Wilba understands that. She wants to know if that if
the fact that it is doing the AF function while wide open is affecting
focus accuracy.

  #6  
Old December 20th 09, 08:34 PM posted to rec.photo.digital.slr-systems
LOL!
external usenet poster
 
Posts: 469
Default EF 50/1.8 AF Experiment?

On Sun, 20 Dec 2009 10:00:02 -0500, Alan Browne
wrote:

On 09-12-20 4:00 , Wilba wrote:
The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two
shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2) erratic
focus under difficult conditions (low light, shallow DOF).

Many people claim that 2) is a result of the crudeness of the cheap
focussing motor and electronics in the lens, that those components are not
able to provide the required accuracy and control of motion of the focus
ring.

But I wonder if 2) is actually a result of 1) - if the AF sensors have fuzzy
images to work with, how /could/ the system nail the focus in difficult
conditions?

It would be interesting to see what happens when the AF sensors have sharper
images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to AF when
the DOF preview button is pressed, so I can't test that. External aperture
perhaps?

Any ideas for how these competing hypotheses could be tested? Is there a
consequence of either hypothesis that could be disproved empirically?


Offhand, even if it is soft wide open, the AF should settle on the
"sharpest" slightly soft contrast, which on average should be sharp enough.

Could you post a simple target wide open?

In fact could you post the one on p18 of this document, shot at a 45 deg
angle. I'd like to compare it to the Minolta 50 f/1.7.

http://focustestchart.com/chart.html#ActualChart



Munching popcorn, watching the usenet comedy show, while I enjoy using my
slightly slower but highly accurate contrast-detection focusing cameras.
Just what any intelligent person wants, phase-detection focusing that
focuses slightly faster but never accurately. All those shots, missed
forever.

too ****in' funny

This is such good free entertainment. Beats all the comedy routines on TV.

/me wonders if they ever realize what gigantic fools they continually make
of themselves daily ...

LOL!

  #7  
Old December 20th 09, 09:47 PM posted to rec.photo.digital.slr-systems
Ofnuts
external usenet poster
 
Posts: 644
Default EF 50/1.8 AF Experiment?

On 20/12/2009 20:34, LOL! wrote:
On Sun, 20 Dec 2009 10:00:02 -0500, Alan Browne
wrote:

On 09-12-20 4:00 , Wilba wrote:
The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two
shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2) erratic
focus under difficult conditions (low light, shallow DOF).

Many people claim that 2) is a result of the crudeness of the cheap
focussing motor and electronics in the lens, that those components are not
able to provide the required accuracy and control of motion of the focus
ring.

But I wonder if 2) is actually a result of 1) - if the AF sensors have fuzzy
images to work with, how /could/ the system nail the focus in difficult
conditions?

It would be interesting to see what happens when the AF sensors have sharper
images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to AF when
the DOF preview button is pressed, so I can't test that. External aperture
perhaps?

Any ideas for how these competing hypotheses could be tested? Is there a
consequence of either hypothesis that could be disproved empirically?


Offhand, even if it is soft wide open, the AF should settle on the
"sharpest" slightly soft contrast, which on average should be sharp enough.

Could you post a simple target wide open?

In fact could you post the one on p18 of this document, shot at a 45 deg
angle. I'd like to compare it to the Minolta 50 f/1.7.

http://focustestchart.com/chart.html#ActualChart



Munching popcorn, watching the usenet comedy show, while I enjoy using my
slightly slower but highly accurate contrast-detection focusing cameras.
Just what any intelligent person wants, phase-detection focusing that
focuses slightly faster but never accurately. All those shots, missed
forever.

too ****in' funny

This is such good free entertainment. Beats all the comedy routines on TV.

/me wonders if they ever realize what gigantic fools they continually make
of themselves daily ...


It just so happens that DSLR users are a bit more concerned about focus
than P&S users because:

1) they get to choose which part of the frame is in focus

2) out-of-focus parts aren't obscured by sensor noise or
diffraction-induced blur.

--
Bertrand
  #8  
Old December 20th 09, 10:44 PM posted to rec.photo.digital.slr-systems
Alan Browne
external usenet poster
 
Posts: 12,640
Default EF 50/1.8 AF Experiment?

On 09-12-20 15:47 , Ofnuts wrote:
On 20/12/2009 20:34, LOL! wrote:
On Sun, 20 Dec 2009 10:00:02 -0500, Alan Browne
wrote:

On 09-12-20 4:00 , Wilba wrote:
The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two
shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2)
erratic
focus under difficult conditions (low light, shallow DOF).

Many people claim that 2) is a result of the crudeness of the cheap
focussing motor and electronics in the lens, that those components
are not
able to provide the required accuracy and control of motion of the
focus
ring.

But I wonder if 2) is actually a result of 1) - if the AF sensors
have fuzzy
images to work with, how /could/ the system nail the focus in difficult
conditions?

It would be interesting to see what happens when the AF sensors have
sharper
images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to
AF when
the DOF preview button is pressed, so I can't test that. External
aperture
perhaps?

Any ideas for how these competing hypotheses could be tested? Is
there a
consequence of either hypothesis that could be disproved empirically?

Offhand, even if it is soft wide open, the AF should settle on the
"sharpest" slightly soft contrast, which on average should be sharp
enough.

Could you post a simple target wide open?

In fact could you post the one on p18 of this document, shot at a 45 deg
angle. I'd like to compare it to the Minolta 50 f/1.7.

http://focustestchart.com/chart.html#ActualChart



Munching popcorn, watching the usenet comedy show, while I enjoy
using my
slightly slower but highly accurate contrast-detection focusing cameras.
Just what any intelligent person wants, phase-detection focusing that
focuses slightly faster but never accurately. All those shots, missed
forever.

too ****in' funny

This is such good free entertainment. Beats all the comedy routines on
TV.

/me wonders if they ever realize what gigantic fools they continually
make
of themselves daily ...


It just so happens that DSLR users are a bit more concerned about focus
than P&S users because:

1) they get to choose which part of the frame is in focus

2) out-of-focus parts aren't obscured by sensor noise or
diffraction-induced blur.


We also like high SNR - in this NG. So it's best not to reply to the
brainless automaton aka the P&S troll.
  #9  
Old December 20th 09, 11:05 PM posted to rec.photo.digital.slr-systems
Ray Fischer
external usenet poster
 
Posts: 5,136
Default EF 50/1.8 AF Experiment?

LOL! wrote:
On Sun, 20 Dec 2009 10:00:02 -0500, Alan Browne
wrote:

On 09-12-20 4:00 , Wilba wrote:
The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two
shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2) erratic
focus under difficult conditions (low light, shallow DOF).

Many people claim that 2) is a result of the crudeness of the cheap
focussing motor and electronics in the lens, that those components are not
able to provide the required accuracy and control of motion of the focus
ring.

But I wonder if 2) is actually a result of 1) - if the AF sensors have fuzzy
images to work with, how /could/ the system nail the focus in difficult
conditions?

It would be interesting to see what happens when the AF sensors have sharper
images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to AF when
the DOF preview button is pressed, so I can't test that. External aperture
perhaps?

Any ideas for how these competing hypotheses could be tested? Is there a
consequence of either hypothesis that could be disproved empirically?


Offhand, even if it is soft wide open, the AF should settle on the
"sharpest" slightly soft contrast, which on average should be sharp enough.

Could you post a simple target wide open?

In fact could you post the one on p18 of this document, shot at a 45 deg
angle. I'd like to compare it to the Minolta 50 f/1.7.

http://focustestchart.com/chart.html#ActualChart


Munching popcorn, watching the usenet comedy show, while I enjoy using my
slightly slower but highly accurate contrast-detection focusing cameras.


I'd ask you to list all of those cameras that do "contrast-detection
focusing" that can shoot 8 frames per second, but you're just a lying
chicken**** little troll.

--
Ray Fischer


  #10  
Old December 21st 09, 02:07 AM posted to rec.photo.digital.slr-systems
Chris Malcolm[_2_]
external usenet poster
 
Posts: 3,142
Default EF 50/1.8 AF Experiment?

Alan Browne wrote:
On 09-12-20 6:09 , Chris Malcolm wrote:
wrote:
The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two
shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2) erratic
focus under difficult conditions (low light, shallow DOF).


Many people claim that 2) is a result of the crudeness of the cheap
focussing motor and electronics in the lens, that those components are not
able to provide the required accuracy and control of motion of the focus
ring.


But I wonder if 2) is actually a result of 1) - if the AF sensors have fuzzy
images to work with, how /could/ the system nail the focus in difficult
conditions?


It would be interesting to see what happens when the AF sensors have sharper
images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to AF when
the DOF preview button is pressed, so I can't test that. External aperture
perhaps?


Any ideas for how these competing hypotheses could be tested? Is there a
consequence of either hypothesis that could be disproved empirically?


The AF sensors pay no attention to the aperture at which you're going
to take the picture. They do their work before the lens is stopped
down.


It's clear that Wilba understands that. She wants to know if that if
the fact that it is doing the AF function while wide open is affecting
focus accuracy.


The simple answer, as the sentence above which you quote indicates, is
no. But the question was raised in the context of AF which becomes
unreliable in dim lighting at high apertures, and there are a number
of technical problems and issues here, some due to the properties of
spherical lenses (or incompletely aspherical ones :-), some due to the
way AF sensors work in different cameras, and some due to not uncommon
small AF calibration errors only apparent with very shallow DoF.

--
Chris Malcolm
 




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