There are three different types of focusing mechanism commonly found on binoculars.
Center Focus (Porro Prism)
In the center focus Porro prism the eyepieces are connected to a threaded rod in the central hinge. An internally threaded knurled wheel or cylinder causes the rod to move, thus moving the eyepieces. The right-hand eyepiece is usually independently focusable (Figure 2.26a) to accommodate the differences in focus of the a
Right eyepiece dioptre adjustment. a: Porro prism. b: Roof prism.
b observer's eyes; this facility is called a "dioptre adjustment." The advantage is that the eyepieces can be focused simultaneously, which is a consideration for general terrestrial use, but not for astronomy. The disadvantages are that there is almost always some rocking of the bridge, which leads to difficulty in achieving and maintaining focus, the focusing system is difficult to seal so dirt can enter, and the optical tubes are extremely difficult to waterproof, resulting in increased likelihood of internal condensation (Figure 2.27).
Center Focus (Roof Prism)
Like the Porro prism, with the center focus system there is an external focus wheel and an independent helical focuser (dioptre adjustment) for the right eyepiece (Figure 2.26b), but the similarity ends there. The mechanism is internal, and focusing is achieved by changing the position of a focusing lens between the objective lens and the prism assembly (Figure 2.1). It has the dual advantages of permitting simultaneous focusing of both eyepieces and allowing relatively simple dust- and waterproofing. The disadvantage is that there is an extra optical element that must
Figure 2.27. The bridge rocks on this center-focus Porro prism binocular.
Figure 2.28. Roof prism center focus.
be accurately made, which absorbs a tiny amount of light, and whose movement during focusing alters the field of view slightly (Figure 2.28).
Eyepieces with independent focus each have a helical focuser. This is much more robust than a center focus system and is easier to make dirt- and waterproof. The best quality astronomical (and marine and military) binoculars have independent focusing. The disadvantage is that the eyepieces cannot be focused simultaneously, but this is not an issue for astronomical observation, where refocusing is not necessary once good focus has been attained (Figure 2.29).
Figure 2.29. Independent focus—ideal for astronomy.
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