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