Well, right, you can do this easily by using various chipped adapters for several lens mounts - but how about permanently modifying your camera to do the same and avoid paying extra for multiple adapters? I love my 300D but I don't own any EF/S lenses, all my arsenal is from 1980s, gorgeous manual focus lenses from Olympus and Carl Zeiss. Canon EF mount has a register distance which lets us use Olympus OM, Pentax M42 and K-mounts, Leica R, and Contax mounts with proper adapter rings. These are vastly available on the net for purchase (ebay). However, Canon EOS auto-focus system needs a lens to speak to itself to work. The lens communicates its focal length and aperture values and the camera activates AF after confirming these. Some clever chaps cracked the code and embedded it in a tiny PIC chip and slapped those on Canon EF adapters for manual lenses. In this hack we'll permanently embed one of these PIC chips in the camera. This hack requires some disassembling and soldering skills. I can rate it at medium hardness. You need: - One AF confirm activating chip for Canon EF mount (ebay keywords: Canon AF confirm chip) - A tiny switch (if you want disabling functionality) - Thin wires - Epoxy (optional) - Philips 0 and flat screwdriver - Soldering iron and solder - Self adhesive tape/ribbon Do not put the battery in the camera when it is disassembled, even if you have it in off position. There are live circuitry in it and you may accidentally short it - burning out some fuses (I'll write another instructable about fixing those, later). AF confirm chip is a tiny pic chip and can be affected from static discharge easily, so be careful while you are working with it. DISCLAIMER: Like all other hacks, this will void your warranty (I don't know if there are any 300D left with warranty anyway). Also involves exposing flash circuitry, if you are careless you may experience a high-voltage electric shock even when the battery is out (which I did briefly). This hack may render your camera totally useless, so if you brick it don't blame me, but put it on ebay for parts (others will like this). I am not responsible for any of these above if you experience a problem. REFERENCE: There are few places where I borrowed wording from Gary Honis's excellent instructions on Canon EOS 300D IR filter removal. Check out his site for an alternative take on disassembling. However, we don't have to deal with the mainboard, so don't strip your camera to barebones like he did.
http://www.instructables.com/id/Hack-Canon-EOS-300D-to-confirm-focus-with-all-lens/