Prism based binoculars are primarily of two types- Porro Prism binoculars and Roof prism binoculars. Porro erection system has been patented in the name of an Italian scientist Ignazio Porro, who had developed his own system of magnifying an image and representing it in a erect form. This method has evolved a lot and is said to have been mastered by the likes of Carl Zeiss. These binoculars work on a Z-shaped prism set-up, resulting in a better depth realization and a wider field of object-viewing. They also fold the elliptical path using the Z-shaped setup, thereby reducing the physical size of the binoculars to less than that of the optical Focal length of the object being viewed.

Binoculars or binocular telescopes, are really two identical symmetrical telescopes mounted together. Because they point in the same direction, they allow the viewer to see a far greater distance than with the naked eye. A binocular has a big advantage over single telescopes as you can see through both eyes and they are easier to hold steadily with two hands.The objective lens diameter generally determines how much light is allowed while the magnification will display the ratio of the focal lengths. An example is 8 X 35 (eight times magnification and 35 mm objective diameter) or 7 X 50 (seven times magnification and 50 mm objective diameter). In these two examples, the first one has a higher magnification ( 8 over 7) but allows less light (35 mm is smaller than 50 mm) than the second one. If you are requiring binoculars for low light use, buy the biggest objective diameter.

Binoculars designed from the Porro prism design are generally brighter than a roof prism design but newer technologies are reducing the differences (the Porro prism binoculars still have the highest value at a lower price).Roof-prism models are generally lighter and smaller than Porro models, but you will usually pay more. If the binoculars are sealed and filled with nitrogen gas, they will not cloud with condensation.All binoculars either use "porro prisms" or "roof prisms". The ones using porro prisms are the traditional design where the eye pieces are off-center to the barrels, whereas the ones using roof prisms the eye pieces are in line with the objective lenses and have a sleek design. Porro prism binoculars are, generally speaking, less expensive and have greater three-dimensional viewing effect, whereas roof prism binoculars are more compact and lighter, but are usually more expensive. Making a roof prism design binocular weatherproof is also easier and more successful.

You should now be familiar with concepts like "porro prisms" and "roof prisms", "diopter", "magnification" and "objective lens" and "exit pupil" and well on your way to understanding binoculars and ready to buy an affordable and quality pair of binoculars.It seems obvious that the bigger the magnification, the better the binoculars. This is not true. The more powerful a binocular, the less bright the image - which will be a problem when focusing on something in the shade of a tree or in dim light in general. More power also has an impact on field of view.A set of technical detail imprinted on any pair of binoculars could be something like this: 8x42, or 10x50. The first number (8, 10) is the magnification number, the "power" of the binoculars. The object is magnified by 8 or 10 times, which means that it appears to be 8 or 10 times closer than seen with the naked eye.The second number (42, 50) is the diameter of the objective lens (in millimeters), i.e. the two big lenses at the front of the two barrels. The bigger the objective lens, the more light is captured, resulting in a clearer and brighter image, which is important when used in fading light or when focusing on something in the shade of a tree. Objective lens opening is also called the "aperture" of the binocular.

Source: Big Porro Binoculars, Compact Porro Binoculars