Roof vs. Porro Prism
Long-distance surveillance models like the 20-power, porro prism Steiner binoculars are larger, more powerful, and more complex.
Roof prism binoculars are the ones that have two straight "tubes" linked together with an adjustable bridge. Porro prisms are the common ones that have a sort of "Z" type shape in the tubes. And there is a big difference.
Due to the unique construction methods necessary to enable a roof prism binocular to transmit a clear image, they are difficult to produce well and unless the lenses are coated properly, will never have the image quality of a porro model. Since it's many times more difficult to make a high-quality roof prism, for equal amounts of money, you're much more likely to get a higher quality pair of binoculars if you go with the conventional porro model.
At the top of the scale they are about equal, but a roof prism model will never be better than a comparably priced porro prism. So, old Uncle Ed's WWII porro binocs may not be so bad after all. Having said that, today's generations of roof prism models above around $300 are pretty good. But, for the same price, you could probably get a higher quality-and a bit more rugged-binocular in a porro.
Get In Focus
The Steiner family of police and military binoculars includes a variety of types and sizes for different applications.
So which is best? Center or individual focus? In a nutshell, the center focus models (which hold about 90 percent of the market) are fast and convenient. But all those moving parts can often spell problems. It's almost impossible to waterproof them and the collimation can drift out.
For sheer rugged dependability the individual focus models win hands-down. That's why military models and virtually all waterproof models, are individual focus binoculars.