EF 50/1.8 AF Experiment?

Chris Malcolm wrote:
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.

Thanks for your effort. Unfortunately, there is no answer to my question
within it. :- )

Ofnuts wrote:
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

It's a good document which has influenced my thinking.

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.

Thanks for sharing your thoughts. Unfortunately, they don't answer my
question.

Alan Browne wrote:
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

Sure. These were shot a while back for comparison with PD AF shots to show
the calibration error I had at the time -
http://www.users.on.net/~alanw/Usene...ewFullsize.jpg (3.6MB)
http://www.users.on.net/~alanw/Usene...ewFullsize.jpg (3.4MB).

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?

On 09-12-20 22:07 , Wilba wrote:
Alan Browne wrote:
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

Sure. These were shot a while back for comparison with PD AF shots to show
the calibration error I had at the time -
http://www.users.on.net/~alanw/Usene...ewFullsize.jpg (3.6MB)
http://www.users.on.net/~alanw/Usene...ewFullsize.jpg (3.4MB).

Other than the general softness in 1.8, it looks like it focused
accurately or at worst a hair in front.

2.8 looks fine, perhaps a bit behind.

That softness may fall outside the AF hysteresis band.

I shot my tests further back (for other reasons). Will repeat
eventually closer in (with the 50mm @ f/1.7).
http://photo.net/photodb/photo?photo...372792&size=lg

Wilba wrote:
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?

Huh? I thought all Strines were called Curly (bald) or Blue (redhead).

WTF???

Good on ya, Wilba.. I think stateside we might spell it with an h at the
end, assuming your spelling is more how it's pronounced- or does that
spelling append a different meaning? Is your given name Wilbur?

--
john mcwilliams

Alan Browne wrote:
Wilba wrote:
Alan Browne wrote:
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

Sure. These were shot a while back for comparison with PD AF shots to
show
the calibration error I had at the time -
http://www.users.on.net/~alanw/Usene...ewFullsize.jpg (3.6MB)
http://www.users.on.net/~alanw/Usene...ewFullsize.jpg (3.4MB).

Other than the general softness in 1.8, it looks like it focused
accurately or at worst a hair in front.

2.8 looks fine, perhaps a bit behind.

Those shots were focussed using contrast detection. Any error you might
perceive is of the order of the typical shot-to-shot variation.

That softness may fall outside the AF hysteresis band.

More information please.

I shot my tests further back (for other reasons). Will repeat eventually
closer in (with the 50mm @ f/1.7).
http://photo.net/photodb/photo?photo...372792&size=lg

Yeah, hard to tell much with that much DOF.

John McWilliams wrote:
Wilba wrote:
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?

Huh? I thought all Strines were called Curly (bald) or Blue (redhead).

WTF???

Yeah, we're not all called Bruce.

Good on ya, Wilba.. I think stateside we might spell it with an h at the
end, assuming your spelling is more how it's pronounced-

Right, just like it looks - alba, amoeba, caramba, samba, tuba, etc.

or does that spelling append a different meaning? Is your given name
Wilbur?

It's a nickname from 1969, after the cartoon character Wilbur the Worm. It
stuck so I made it my own by changing the spelling. It matches my character
better than my given name. :- )