Most AF systems use contrast to determine focus. A focus assist lamp uses a beam of light (or infrared) to read distances. The flash uses a beam but only when it's turned on. When flash is not used, the camera's normal AF system is used.Originally Posted by inky
All EOS EF lenses can be used on Canon film or digital cameras. All third party lenses "for Canon" can be used on all Canon film and digital cameras. The very special EF-S lens that is the "kit lens" for the Digital Rebel can only be used on that single camera. The kit lens is actually very good for $100 but won't work on anything else....can someone tell me, would the lenses i buy to use on the digital rebel work on the 10d or any of the canon 35mm film cameras?