For use with the Canon EOS-1Ds Mark III Digital SLR. HoodEye links eye to camera for improved viewfinder optics. Easily upgrade your camera eyepiece in seconds. Four HoodEYE models fit all Canon and Nikon Digital SLR cameras. Unique self locking mount system keeps HoodEYE eyecup securely in place. The HoodEYE eyecup rotates to accommodate both right and left eyed shooters. Figuring out what model you need is easy...
Nikon has SQUARE eyepieces or ROUND eyepieces.
Canon has one 22mm eyepiece for the 1DS MARK III.
All the rest of the Canon models use the 18mm eyepiece.