Optical device for coupling to a camera lens to form a visual observation system

An optical device adapted for coupling to a camera lens so as to form a visual observation system, said optical device comprising:

• a housing;
• a connecting unit disposed at said housing and including a camera lens bayonet for connecting to the camera lens;
• a first lens set disposed in said housing, and having a negative optical power;
• a prism unit located at an image side of said first lens set in said housing, said prism unit having a light-entrance surface and a light-exit surface, an image formed by light beams exiting said light-exit surface being erected with respect to an inverted image formed by light beams entering said light-entrance surface;
• and
• a second lens set located at an image side of said prism unit in said housing, said second lens set having a positive optical power;
• said optical device satisfying:
• 3 mm<L1<15 mm,
• 27 mm<p<44 mm
• in which, L1 represents a distance from a lens contact surface of said camera lens bayonet which is away from said first lens set to an object side surface of said first lens set, and p represents a distance from said object side surface of said first lens set to a camera lens imaging plane of the camera lens, the camera lens imaging plane being located within an optical path of said prism unit.