From: Wilba on
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?


From: RichA on
On Dec 20, 4:00 am, "Wilba" <use...(a)CUTTHISimago.com.au> 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?

Why beat your head on a wall over this? Dump the garbage, buy a D5000
and a Nikon 50mm f1.8.
From: Chris Malcolm on
Wilba <usenet(a)cutthisimago.com.au> 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
From: Ofnuts on
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
From: Wilba on
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. :- )