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?

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

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.

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

Alan Browne wrote:

It's clear that Wilba understands that. She...

Do I sound like a woman?! I'll try to butch it up a bit. hwock ptooey
:- )

On 09-12-20 22:11 , Wilba wrote:
Alan Browne wrote:

It's clear that Wilba understands that. She...

Do I sound like a woman?! I'll try to butch it up a bit.hwock ptooey
:- )

Sorry dude. Wilba sounds feminine.

Now you're in for it, though.

Alan Browne wrote:
Wilba wrote:
Alan Browne wrote:

It's clear that Wilba understands that. She...

Do I sound like a woman?! I'll try to butch it up a bit.hwock ptooey
:- )

Sorry dude. Wilba sounds feminine.

It's an alternative (Australian-sounding) spelling of Wilbur.

Now you're in for it, though.

Eh? What?

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.

I'm not sure why you feel it necessary to make that comment, because
the rest of my post went on to deal with the various circumstances in
which that can be the case, and why. It's a complex topic to which
there is not a simple single answer.

--
Chris Malcolm

On 2010-03-23 14:20:46 +0000, Chris Malcolm said:

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.

I'm not sure why you feel it necessary to make that comment, because
the rest of my post went on to deal with the various circumstances in
which that can be the case, and why. It's a complex topic to which
there is not a simple single answer.

Exactly. This will be an ongoing debate because we do not know the
precise details of the algorithms and hardware used in the AF control
systems. Each hardware supplier had a very good reason to select their
design philosophy - backward compatibility/future possibilities not
being last on their list. Some suppliers have done better than others,
viewed at this point in time. All-in-one cameras circumvent the
compatibility issue therefore the designers could optimize for a single
point in time and change the design completely for the next model.

Of particular interest to me:
Panasonic contrast AF is faster than it should be - I'd like to understand why.

My 50mm f/1.4 and 85mm f/1.8 both produce inconsistent AF under very
harsh conditions whereas my very old 50mm f/1.8 does not. Endless Web
searches and discussions with a few dealers has not resolved this
issue. I conclude: the first two lenses produce a fuzzy image to the AF
sensor due to spherical, coma, and chromatic aberrations; the last
(inferior and cheap) lens gives a better image to the AF sensor.

--
Pete

Pete wrote:

My 50mm f/1.4 and 85mm f/1.8 both produce inconsistent AF under very harsh
conditions whereas my very old 50mm f/1.8 does not. Endless Web searches
and discussions with a few dealers has not resolved this issue. I
conclude: the first two lenses produce a fuzzy image to the AF sensor due
to spherical, coma, and chromatic aberrations; the last (inferior and
cheap) lens gives a better image to the AF sensor.

Interesting. I don't have access to a 50/1.4 or 85/1.8, but my 50/1.8 II
focusses differently depending on which side of the subject it's coming
from, leading to the conclusion that the central "enhanced precision" PD AF
sensor is sufficiently confused by the fuzziness for it to confirm focus
over a range of the order of the DOF.