Canon EOS 50D Instruction Manual page 93

Differences Between <d> and <1> (Full Auto)
With <1>, many functions such the AF mode, drive mode, and built-in
flash are set automatically to prevent spoiled shots. The functions you can
set are limited. With <d>, only the shutter speed and aperture are set
automatically. You can freely set the AF mode, drive mode, built-in flash,
and other functions.
About Program Shift
In the Program AE mode, you can freely change the shutter speed and
aperture combination (Program) set by the camera while maintaining the
same exposure. This is called Program shift.
To do this, press the shutter button down halfway, then turn the <6>
dial until the desired shutter speed or aperture value is displayed.
Program shift is canceled automatically after the picture is taken.
Program shift cannot be used with flash.
If the "30"" shutter speed and the maximum aperture
blink, it indicates underexposure.
Increase the ISO speed or use flash.
If the "8000" shutter speed and the minimum aperture
blink, it indicates overexposure.
Lower the ISO speed or use an ND filter (sold
separately) to reduce the amount of light entering the
lens.
d: Program AE
93