This round eyecup fits most cameras with a round eyepeice on the viewfinder of the camera. Designed to keep light from entering the viewfinder and altering exposures. Also designed for comfort when using your camera as well. The rear opening (the smaller one) is 21mm (about 3/4 inch) and the larger opening is 39 mm (about 1.5 inches). Excellent focusing tool for SLR and DSLR cameras. Eye piece fits most all cameras including Canon, Nikon, Sony, Pentax, etc. Specifications: Round camera eyepiece Rear Opening: 21 mm (3/4 in.) Front Opening: 39 mm (1.5 in.) Designed for comfort