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