The t-adapter is used for cameras that use an offset tripod mount. Ideally the tripod mount should be inline the with center of the lens. Most point and shoot cameras as well as some Canon DSLR battery grips have a tripod mount that is sightly offset. The t-adapter helps to compensate for this offset. The t-20 is good for offsets up to 20mm (as with most cameras) and the t-30 if good for offsets up to 30mm.
since 2004