The Canon Angle Finder C serves two primary purposes. The first and most commonly served purpose is to create a more comfortable viewing angle when shooting with the camera low or at an awkward angle (such as when shooting upward). The second is to help the photographer to obtain precise manual focus through its 2.5x magnification feature.
Supplied with Finder Adapters Ec-C and Ed-C, the Canon Angle Finder C will fit any EOS camera (and many other brand film and digital SLRs) by replacing the standard eyecup. Squeeze the eyecup to release the catch and slide it up - slide the AF-C back into the eyecup's position and it is ready to go.