Julian Loke wrote:
> Did you manage to perform your f/5.6 AF experiment?
> What was the outcome?
Hi Julian,
Here are my unscientific results:
In order to have a lens with a maximum aperture of f/5.6, I attached the EF 2x
extender to a 200/2.8L, mounted on an EOS 3. Using utility wires (conveniently
located opposite my 3rd-floor balcony) against a blank white (overcast) sky, I
found that focusing on horizontal lines was possible with all AF sensors. By
turning the camera to the vertical position, I found that I could focus on the
wires with the center sensor with consistency; beyond that the results were less
conclusive. I did get focus confirmation at least one time using one of the
other cross-type sensors, but not at all with the other sensors. Nonetheless, I
did get pretty accurate focus on some occasions with these other sensors,
although without focus confirmation. Thus, while the AF system would not confirm
focus, it did continue to function for vertical lines (or at least attempted to
do so) at f/5.6. Clearly, the sensors do not simply deactivate at f/5.6; rather,
f/5.6 should be considered outside the limit of when accurate focus can be
consistently achieved. Hence, it may be possible to achieve vertical-line AF at
maximum apertures smaller than Canon specifies, but your results are unlikely to
be consistent.
An observation: Some of the wires have a wrapped metal casing, and the EOS 3
with the 200/2.8L and 2x extender was sufficiently sensitive to detect these
coils, giving me what I interpreted as false-positive results. I therefore made
an effort to choose wires that have only a plain black casing, so that there
were no diagonal lines or textures on which the horizontal-line sensors could
lock.
Finally, I should mention that it was snowing at the time; I trust this did not
affect my results.
fcc
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