When the thumb screw is tightened the play is removed and the rail is locked in place; no more having your sight move and losing alignment! Can be removed and reinstalled with good consistency/repeatability, just push it fully into the hotshoe before tightening. No metal to metal contact or risk of damage. Hotshoe Weaver/Picatinny mount. 

Fits into the camera's flash bracket for mounting a red dot rifle sight.
Eliminates the need to use the viewfinder, great for when you're using a long lens with a narrow field of view. Compatible with most OEM hotshoes (ISO 518:2006, Canon/Nikon/Etc).
$35.00 

Holographic Red Dot Sight: Selectable between four different reticles, three levels of brightness, two colors (red/green). Eliminates the need to use the viewfinder, great for long lenses with a narrow field of view. Requires the Red Dot Rail. (imported) $45.00