Some birders feel strongly that Porro prism binoculars are superior to roof prisms. And we know Porro prism designs are easier and less expensive to manufacture, because the tolerances are not so demanding. But do they give you an inherently better picture?
We put a top roof prism binocular, the Nikon 10x42 Venturer LX, side by side with the Nikon 10x42 Superior E, which is the top roof prism design by the same manufacturer.
We mounted them absolutely steady on a beanbag platform atop a heavy Gitzo tripod so that they could be focused on the same target and the judges could take their time comparing the images.
The two binoculars share the same manufacturer, magnification, objective lens size, and field of view. Also, one would expect these models to represent Nikon's best efforts in optical quality, coatings, and manufacturing tolerances. And both models have received high praise in the birding community. So we thought this would be an informative test.
The results? Although we expected the Porros to be brighter, no one could see any difference in brightness or contrast between the two. We repeated the test on a dark, overcast day, with the same results. However, all the judges reported they saw slightly more detail through the Venturer roofs than through the Superior E Porros. In this individual case, roof bested Porro.
Of course, this test used only one sample of each model binocular. A better test would have compared multiple samples.
Nevertheless, we believe that, whatever optical deficit roof prism designs once had, the improvements in roof prism optics such as phase-corrected coatings have erased them. And in the marketplace, where consumers vote with their wallets, birders who are buying top-end binoculars tend to elect roofs over Porros. This is why almost all the new advances in birding binoculars have been in roof prism designs.
This is part of an article that first appeared in Bird
Watcher's Digest, January, 2000.