Professional photographers can use this robust all-metal construction adapter that attaches to your tripod screw hole, allowing for easier adjustment of your zoom and focus. This adapter is not necessary to attach to your DSLR. If you have a high-end lens, it will be able to support the weight of the Eye mirror. Alternatively you can use the more affordable DSLR lens restraint instead of the DSLR adapter if you have a low budget lens.
The DSLR adapter also has its own tripod screw hole, allowing you to attach it to a tripod.