This number identifies a finite tube length objective. 160 millimetres is the distance from the opening of the nosepiece (where the objective is screwed in) to the top of the observation tube (where the eyepiece is inserted). If this distance is lengthened, e.g. by insertion of accessories in the light path above the nosepiece, spherical aberration will result, unless optically corrected lenses are included in the accessory.

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