Types
Telescopic sights are classified in terms of the optical magnification and the objective lens diameter, e.g. 10×50. This would denote 10 times magnification with a 50 mm objective lens. In general terms, larger objective lens diameters, due to their ability to gather larger amounts of light, provide a larger exit pupil and hence provide a brighter image at the eyepiece. On fixed magnification sights the magnification power and objective diameter should be chosen on the basis of the intended use.
There are also telescopic sights with variable magnification. The magnification can be varied by manually operating a zoom mechanism. Variable sights offer more flexibility regarding shooting at varying ranges, targets and light conditions and offer a relative wide field of view at lower magnification settings. The syntax for variable sights is the following: minimal magnification – maximum magnification × objective lens, for example, 3–9×40.
Confusingly, some older telescopic sights, mainly of German or other European manufacture, have a different classification where the second part of the designation refers to 'light gathering power.' In these cases, a 4×81 (4× magnification) sight would be presumed to have a brighter sight picture than a 2.5×70 (2.5× magnification), but the objective lens diameter would not bear any direct relation to picture brightness, as brightness is affected also by the magnification factor. Typically objective lenses on early sights are smaller than modern sights, in these examples the 4×81 would have an objective approximately 32mm diameter and the 2.5×70 might be approximately 25mm.
Read more about this topic: Telescopic Sight
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