What is the most essential lense for the Canon Rebel XT for doing wedding photography?

You would probably want a couple of lenses and it would depend on how much money you want to spend. The most important would be a medium telezoom with a fast aperture, and I would recommend the EF 70-200mm f/2.8L IS USM. [that 18-55mm will do nothing for you] It has a constant fast aperture which is ideal for use where a flash would not be prudent and still have bright images. And with image stabilization you can hand hold down to 1/20-1/40 sec shutter speed, wide open if you're a steady shot. It's an expensive lens, but well worth the money. I shoot with a nikon D80 and I'm saving up for a nikkor version of the same lens
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Wedding Photography Essex