Camera Lens Parts: Alignment Index for Mounting
The simplest of all the camera lens parts is the alignment index for mounting. These are sometimes referred to as the alignment dots.
A lens mount consists of several interlocking metal pieces. Parts on the lens must match up perfectly with parts on the camera body. True, a photographer could blindly feel and twist to get the lens to fit, but the easier and better way is to line up the dots. For example, a Canon lens might have a red dot on its rear, where it mounts on the body. The body also has a red dot, generally near the 12:00 position on the ring. A photographer just needs to line up the matching dots to fit the lens into the mount, then twist until it locks.
Different Colors and Shapes for Compatibility
The dots themselves may be different colors or shapes for lenses from different manufacturers. Sometimes, small triangles or squares are used. One thing to note is that for Canon lenses, EF lenses use a red dot, while EF-S lenses use a white square. EF-S lenses will not work on camera bodies that only have a red dot. EF lenses, however, work on camera bodies with either a white square or red dot.