One of the great things about Canon EOS SLR's is that they have excellent clearance between the mirror and lens mount, which means you can actually mount Leica R, Carl Zeiss, Visoflex, Nikon F (MF and AF), Pentax M42, Contax/Yashica (RTS), and Olympus OM Lenses to a Canon body with a simple relatively cheap adapter. 

This is great for the beginner who wants to learn more about photography and wants excellent quality lenses at a more reasonable learning discount.  While for those who already have Film SLR cameras and might be switching to a Canon for the first time, it means you'll be able to get all the great pictures with the lenses you've already invested in. (See how SLR's are a better investment?)  Some of you may even feel adventures and want to experiment with exotic lenses such as the many famous Leica and Carl Zeiss primes.

From a beginners perspective, or anyone on a budget a cheap Lens adapter makes great sense.  Many high quality Pentax and Nikon F lenses can be found on eBay very cheaply.  The ironic thing is Nikon F lenses that are older Manual Focus ones don't fit newer Nikon bodies so Nikonians are selling them off at great prices for us.

The adapters just convert the mount from whatever lens you want to the EOS mount. Most of theses adapters have no electrical circuitry, and therefore you will not get any auto focus features, and you will have to stop down meter to shoot your photos.

These simple metal adapters convert the mount of your lens to Canon EOS Bodies

Stop Down Metering

The technique is very simple.  What you will need is a lens that has manual aperture adjustments on the lens itself since the camera will have no way to control it.  Set the aperture wide open, i.e. the smallest number on the dial.  You will want your camera in either Aperture Priority (Av) or manual for this to work.

Now use the manual focus to focus on your subject.  When you’re focused you can set your aperture to the desired opening.  The reason for doing this after is it darkens your view, and it is quite hard to focus at an aperture of say f 1/22.  If your camera is in Av mode, when you press the trigger the camera will automatically set the shutter speed for you.  It doesn't matter that your body doesn't know what aperture is set, it knows how much light is getting in.

That wasn't too hard was it?

Here you can see my adapter mounted on the Nikon Lens. Now the mount is just like my other EOS lenses