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Spotting scopes also use Porro and roof prisms, but the overwhelming majority are Porro prisms, because size is not such an issue with these instruments. There are some exceptions, notably small, compact roof prism scopes designed for maximum portability, but these do not offer interchangeable eyepiece capability. With the exception of large European drawtube scopes, all the premium, full-feature, large-objective scopes on the market today use Porro prisms.
by J.T. Kozak
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