Ken Durling wrote:
> But what about a 2.8 lens not AF-ing well when stopped down to f/8 or
> slower? That's what I don't get. Isn't that what people have been
> saying? Maybe I misunderstood.
That's the one. ;-)
It isn't a question of whether the AF works with the lens stopped down (I don't
know that I've ever seen such a possibility discussed), it's a question of at
what maximum, wide-open apertures AF works. With the Canon extenders, the loss
of light that results when these are used are passed to the camera (the camera
"knows" that the extender is attached), so that your f/2.8 lens with the 1.4x
extender is seen by the camera and its AF system as an f/4 lens (maximum
aperture, not the exposure for a particular subject)--and the camera's display
will reflect this. The problem arises when you use an extender on an
already-slow lens. For example, using the 1.4x extender on the 28-135 IS changes
it (as far as the camera is concerned) from an f/3.5~5.6 lens to an, um, f/5~8
lens. Since with all EOS bodies other than the 3 and the 1V, AF fails (or is
highly unreliable) at maximum apertures smaller than f/5.6, with this particular
combination AF will (according to spec) fail at whatever point in the zoom range
the maximum aperture falls below the "AF threshold." On the 20-35/3.5~4.5, for
example, the maximum aperture is f/3.5 at 20 mm, f/4 at 24 mm, and f/4.5 at 28
mm, and some similar progression will be apparent with the 28-135 IS (and any
other variable-aperture zoom lens). This will be easier to understand if we
recall that f/stops express a relationship--the ratio between the focal length
of the lens and the physical size of the diaphragm opening--and not the actual
diameter of the aperture opening, so that for a given aperture size, the f/stop
will increase as focal length increases.
Interestingly, the Canon extension tubes do not "tell" the camera that they are
attached.
fcc
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