Binocular Design Governs Shape & Size
Binoculars are really just two telescopes side-by-side, one for each eye. That's why we refer to a "pair" of binoculars.
The varying shapes and sizes that are available are governed by the design of the binoculars.
To better understand why they're different, below is a quick breakdown of the different design types, with examples of what they look like.
But before delving into that, it's useful to understand that binoculars are described by two numbers separated by an "x", e.g. 8x30.
The first number always refers to magnification and means how many times the image is magnified. In our example, the image is magnified eight times, so
will appear to be eight times closer than when viewed with the naked eye.
The second number means the diameter of the objective (front) lens in millimetres. In our example, our 8x30 binoculars have front lenses that are each 30mm in diameter.
Zoom binoculars are described by three numbers, e.g. 7-24x50. The first number (7) means the smallest magnification, the second number (24) means the biggest magnification while the third number (50) indicates the diameter of the objective (front) lens in millimetres.
1. Roof Prism Binoculars
In this design, two small prisms are located in the roof of the binocular in line with the eyepiece and the objective (front) lens.
This allows a slim, streamlined shape resembling two straight pipes. Roof prism binoculars are thus narrower, lighter and easier to
operate than the more bulky Porro Prism glasses.
The higher specification models are favoured by bird watchers, hunters and nature viewers in particular.
Some common sizes include: 7x20, 9x20, 8x25, 8x28, 10x25, 12x25, 8x32, 10x32, 12x32, 8x42 and 10x42. See examples below:
Porro Prism Binoculars
Porro prism binoculars are generally wider, heavier and more cumbersome to handle than the roof prism models.
They are wider than the roof prism design because they have larger prisms and bigger objective lenses.
The image sharpness and quality is as good as an equivalent roof prism binocular.
Some common porro prism binocular sizes include: 7x35, 8x30, 8x40, 8x42, 10x30, 7x50, 10x50, 12x50 and 16x50.
They are generally not as expensive as the equivalent top-end roof prism models. See examples below:
Reverse Porro Prism Binoculars
"Compact" and "ultra compact" (pocket-size) binoculars are usually based on the reverse porro prism design, although there are also compact models of conventional
roof prism design.
Unlike the standard porro prism design, compact and ultra compact binoculars have their eyepieces positioned wider apart than the objective lenses.
As a result, their objective lenses seldom exceed 30mm in diameter.
Compact binoculars are popular because they're small and light and thus easy to carry
and stow away. However, the image quality is generally inferior to larger binoculars, particularly in poor light, because of the small objective lens.
Some common sizes include: 8x30, 10x30, 8x25, 10x25, 12x25, 8x21, 8x24. Zoom models are also available. See examples below:
As with zoom lenses on cameras, the image can be brought closer or further by "zooming" a lever or ring. Although convenient
for some applications, this convenience comes at a price as zoom binoculars are generally much heavier, have a smaller field of view and can be
more costly than conventional binoculars of similar build quality.
Some common sizes include: 7-16x21, 8-20x24, 7-15 X2 5, 9-27x25, 7-21x40, 8-24x30 and 8-24x50. See examples below:
As with zoom binoculars, image-stabilized models have borrowed the technology from photographic lenses. The Canon brand is world-renowned
for its image-stabilized lenses and they've transferred this to their binoculars. Two sensors analyse any shake or tremble,
while a microprocessor shifts the prisms to adjust the the angle of the incoming light, so minimising vibrations.
This allows powerful binoculars to be hand held with minimal image shake, something that can be a problem with high magnification binoculars.
See also the Binoculars Category in our camera store for more binoculars at unbeatable prices.