Take great photos of your aquarium from the top down, the way corals are meant to be viewed! This handy lens housing mounts directly to the barrel of your DSLR lens, allowing you to (carefully) dip the housing in the water and get tack sharp photos without annoying lightbulb reflections or surface ripples. Using the three thumbscrews to mount the housing on the lens barrel, you can turn the housing to zoom the lens, which can be difficult with traditional top-down photo boxes.The Porthole features a silicone band for extra grip, and lens standoffs to protect the acrylic from scratches when set face-down.Inner diameter of the housing is approximately 4\” and supports a wide range of lens sizes. The housing length is 7\” overall, and the barrel depth is 6-3/8\” to the inside of the acrylic lens. This allows compatibility with external-focus/zoom lenses and large macro lenses.