Canon SPEEDLITE 540EZ Service Manual page 24

Part 2: Technical Information
2) AF auxiliary light emission conditions for different EOS cameras
The 540EZ's AF auxiliary light function automatically emits light according to
commands from the EOS camera. Depending on the camera model, the emission
conditions follow one of the three patterns described below:
(1) When the 540EZ is used with the EOS-1N, the emitted AF auxiliary light
corresponds to the five focusing points.
(2) When the 540EZ is used with other EOS models equipped with multiple
focusing points (EOS 5, 10, Kiss), the camera's built-in AF auxiliary light
function has priority and the 540EZ's AF auxiliary light is not emitted.
(3) When the 540EZ is used with the EOS-1 or other EOS camera with a single,
central focusing point, only the center AF auxiliary light is emitted.
3) AF auxiliary light blockage with EF lenses
When the EOS-1N + 540EZ or EOS-1 + 540EZ is used with one of the following
five lenses — @ EF200/1.8L, @ EF400/2.8L, @ EF500/4.5L, ® EF600/4L, or
© EF1200/5.6L — the AF auxiliary light is blocked by the lens barrel and thus
cannot be used.
There is absolutely no problem using the AF auxiliary light with any other lens,
even at the closest focusing distance, except for extreme close-up shooting with
macro lenses.