Q.What is a porro prism? A.
A porro prism is an older style of prism used in binoculars and other optical instruments. A porro prism is larger and bulkier prism than a roof prism and because of this, the body of a porro prism binocular will also tend to be bulkier. You can always recognize a porro prism binocular by its shape. In a porro prism binocular, the eyepiece and the front lens are always offset and are never in a straight line. Optically, however, a porro prism is very efficient. It is also less expensive to make than a good roof prism. This makes a quality porro prism binocular every bit as good optically as a quality roof prism and at a much lower cost. If you want to get the most optics for your dollar, especially in a binocular under $200, a porro prism is your best choice.
Q.What is a roof prism? A.
A roof prism is a smaller prism shaped like a little house with a roof. The main advantage of a roof prism is its size and shape. It is smaller and more compact than a porro prism and can be installed in a much sleeker housing than a porro prism. This makes a roof prism binocular a nicer binocular to hold and handle for most people. You can always recognize a roof prism by its shape. In a roof prism binocular, the eyepiece and the front lens are always in a straight line or straight tube.
On the down side, a roof prism is much more expensive to make to the same optical standards as a good porro prism. In fact, not all roof prisms are created equal. Optically, the sharpest roof prisms are phase corrected (PC) and the brightest roof prisms will have silver coated mirrors (there are no mirrors in a porro prism to lose light). Alignment of the prisms is also far more critical in a roof prism.
This need for precise alignment in a roof prism has a beneficial side effect. A roof prism is solidly housed in a cage and the focusing mechanism is usually inside the binocular instead of outside as in many porro prism binoculars. This makes a good roof prism binocular more rugged and smoother focusing than the average porro prism binocular.