objektivdiameter

Objektivdiameter, or objective diameter, is the diameter of the primary light-collecting element of an optical instrument. It is one of the most important specifications for telescopes, microscopes, and camera lenses because it largely controls how much light the instrument can gather and, in conjunction with wavelength, limits the finest detail that can be resolved.

In telescopes, the objective diameter corresponds to the diameter of the primary lens or mirror. A larger diameter increases light-gathering power roughly with the square of the diameter, improving faint-object visibility. It also sets the diffraction limit; for a circular aperture the ideal angular resolution is approximately 1.22 times wavelength divided by the diameter (in radians). In practice, atmospheric seeing often dominates resolution for ground-based systems, so gains from increasing diameter may be partially negated by turbulence. Obstructions such as secondary mirrors reduce throughput and contrast.

In microscopy and photography, the term similarly describes the entrance aperture of the objective. In microscope objectives, brightness and resolution are more directly governed by the numerical aperture and refractive index of the immersion medium, with NA roughly equal to n sin α. The objective diameter can affect the mount size, mechanical stiffness, and maximum NA for a given focal length, but it is not the sole determinant of resolving power.

Practical considerations include manufacturing cost, weight, detector size, and optical aberrations. Larger diameters require higher quality polishing, coatings, and alignment; they are more subject to flexure and thermal expansion. In telescope design, the choice of diameter involves trade-offs between light-gathering power, resolving power, portability, and cost.