ElectricStuff has some tips for changing the EOS 300D/Rebel focussing screen to help manual focussing and a tutorial on how you can modify your Canon EF Extension tube for EF-S Lens.
“Canon’s EOS 300D digital SLR comes with a lens in a new variation of Canon’s EF format, EF-S. This has a projection on the rear, which prevents the use of old Canon extension tubes for close-up work. Canon have redesigned the tubes (new ones have a II designation – e.g. EF25 II), however if you already have one of the old EF tubes, this page describes how they can be modified to fit the EF-S lens, to save the expense of upgrading. The older tubes will probably also be available more cheaply second-hand. “
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