What does the marking 160 on the barrel of an objective mean?

The number 160 indicates the finite tube length of the objective, expressed in millimeters.

Tube length is the distance between the nosepiece opening (where the objective is mounted) and the top of the tube (where the eyepiece is inserted). For a 160 mm objective, this distance is 160 millimeters.

Tube length affects the optical performance of the objective. If this distance is altered, for example by adding accessories in the light path above the nosepiece, spherical aberration may occur unless those accessories contain corrective optics.

• Always use objectives within their specified tube length to avoid distortion or blur
• When adding accessories, ensure they are optically corrected to maintain image quality