Canon EOS 30D - Manual (Page 146)

Setting the Trimming
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Exit the menu.
Press <0>.
X The Print setting screen will reappear.
X On the upper left, you can see the
trimmed image area that will be
printed.
Image area to be printed
Depending on the printer, the trimmed image area might not be printed
as you specified.
The smaller you make the trimming frame, the grainier the picture will
look. If the picture will be too grainy, the trimming frame will turn red.
While trimming the image, look at the camera’s LCD monitor. If you look
at the image on a TV screen, the trimming frame might not be displayed
accurately.
The trimming frame shape will be different depending on the [Paper
settings], [Image] / [Paper], [Page layout] / [Borders] settings.