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Just as in the field of photography, there are also binoculars
with variable magnification
factors, known as zoom binoculars. The specification for zoom
binoculars is, for example, 10-30x60 instead of 8x42, which is the
specification for standard binoculars. The example above of the
specification for zoom binoculars means that the magnification can
be increased in stages from 10 to 30, as desired.
Yet zoom binoculars, as opposed to zoom objectives in photography, are not very popular. This is because binoculars are made up of two equal monocular scopes that are connected together to work as a single entity. The magnification of each monocular is exactly the same (and does not change) as long as the two monoculars are perfectly aligned (collimation) and the magnification of the two monoculars is perfectly synchronised. The two monoculars work together to produce one single image (with exactly the same magnification from each monocular) to the eyes of the viewer. Zoom binoculars, on the other hand, must somehow maintain synchronisation of the changing magnification in each monocular. This is accomplished through the zoom mechanism (which consists of moving lens elements with each eyepiece) linking the two monoculars by means of a flexible metal band that passes through the ocular arms, connecting the zoom mechanisms of the two monoculars. However, since a lot of optic elements are involved in the zoom mechanism, it is virtually impossible to get the magnification in both monoculars exactly the same when the viewer uses the zoom. This causes a slight distortion of the image, which does not happen in the case of ordinary binoculars of high quality, and which may be rather disappointing to the viewer.
Since zoom binoculars use moving lens elements that have some degree of slack (no matter how small the tolerances may be), the alignment or collimation will not be perfect, with all its negative consequences (See 'Collimation'). In brief, it is a matter of weighing up everything and making compromises to produce the best possible binoculars within a limited budget. It is worth noting that budgets are always limited, even with expensive binoculars!
It is also good to bear in mind that besides the magnification factor, other features of binoculars, such as the exit pupil, the twilight factor and the brightness by definition, also change. This is not really a problem but it is worth taking into consideration.
The marketing departments of suppliers are currently bringing zoom binoculars with extremely high magnification factors onto the market. However, zoom binoculars are not suitable for everyday use.
It can be concluded that the production of zoom binoculars is so complicated because the optical system is very complex. That is why standard binoculars with a fixed magnification factor are generally the more popular and logical choice. Zoom binoculars are not practical anyhow because binoculars with extremely high magnification factors cannot be handled without the use of a tripod. However, if variable magnification is required, you can always opt for binoculars with interchangeable oculars. These offer you the best of both worlds, that is to say, the versatility of variable magnifications but without the performance limitations of zoom binoculars. So zoom binoculars are not what you would recommend your friend to buy!