Canon EOS 600 - Manual (Page 83)

Use the Canon Speedlites 420EZ and 300EZ as
fill-in flash in outdoor settings as well as for nor­
mal flash when shooting at night or in a dimly-lit
room. These units feature a built-in AF auxiliary
light function for dark situations, so you can per­
form AF flash photography.
Fill-in flash:
This technique utilizes a flash unit as an auxil­
iary light to prevent subject underexposure in
backlit situations. Generally, the exposure level
between the main subject illuminated by the
flash and the background in ambient light
must be balanced to avoid unnatural effects
With the Canon Speedlites 420EZ and 300EZ
there is no need to balance this exposure level
because it is automatically controlled.