Canon EF - Manual (Page 57)

the subject appears small, move in closer until it
occupies about one-third of the total viewfinder
area. Push the AE memory lock button and hold it
in. Then you may step back and/or change the
position of the subject in the viewfinder to suit your
compositional taste, while the correct exposure
reading is locked into the camera's Variable Aperture
AE control. Releasing the shutter will produce a
properly exposed photograph.
In the second situation (2) above, the scene cannot
be metered directly. Most scenes are composed of
both light and dark areas which usually average out
to a medium gray. Because all TTL exposure meters
are calibrated to give proper exposure when reading
a neutral gray card (of 18% reflectance), they give
correct exposure for most scenes. In extreme cases in
which the scene is predominantly light or predomi­
nantly dark, the Variable Aperture AE control must
be fooled into providing the correct exposure. One
readily available alternate subject is thepalmof your
own hand held out a foot or so in front of the lens
and placed in the same light as your original subject.
Focus on your subject normally. Then fill the
viewfinder with the out of focus image of your right
palm making absolutely sure that it is in exactly the