Canon eos rebel t1i - Manual (Page 101)

101
3 Lens Peripheral Illumination Correction
The camera already contains lens peripheral illumination correction
data for about 25 lenses. In step 2, if you select [Enable], the peripheral
light correction will be applied automatically for any lens whose
correction data has been registered in the camera.
With the EOS Utility (provided software), you can check which lenses
have their correction data registered in the camera. You can also
register the correction data for unregistered lenses. For details, see the
Software Instruction Manual (CD-ROM) for EOS Utility.
About the Lens Correction Data
Lens peripheral light correction cannot be applied to JPEG images
already captured.
Depending on shooting conditions, noise might appear on the image
periphery.
When using a third-party lens, setting the correction to [Disable] is
recommended, even if [Correction data available] is displayed.
Lens peripheral light correction is applied even when an Extender is
attached.
If the correction data for the attached lens has not been registered to the
camera, the result will be the same as when the correction is set to
[Disable].
The correction amount applied will be slightly lower than the maximum
correction amount settable with Digital Photo Professional (provided
software).
If the lens does not have distance information, the correction amount will
be lower.
The higher the ISO speed, the lower the correction amount will be.