This is the Cavision MHE52 viewfinder, a single lens element cap designed to mount over a 3.5 inch LCD, such as the 5DMkII, transforming the LCD into a viewfinder for Live View and video work. It greatly enhances working in sunlight or conditions where the viewfinder can be hard to visualize allowing more accurate composition and focusing.
The camera mounts to the plate in front of the viewfinder and is adjustable to mount snugly to the LCD on most cameras. There is a swing connection so you can quickly swing the viewfinder out of the way if you want to look through the eyepiece, etc.
The viewfinder can be mounted, along with the camera, on a Cavision shoulder mount to provide great stability and good visualization.
Please Note: Height alignment of the LCD Viewfinder is intended for the Canon 5DMkII and may not align properly with other cameras. For use with the Canon 7D and T2i an additional RPC4580 spacer plate must be requested.