Objective (optics)

They are used in microscopes, binoculars, telescopes, cameras, slide projectors, CD players and many other optical instruments.

The objective itself is usually a cylinder containing one or more lenses that are typically made of glass; its function is to collect light from the sample.

A typical microscope has three or four objective lenses with different magnifications, screwed into a circular "nosepiece" which may be rotated to select the required lens.

Using an objective and microscope that were designed for different tube lengths will result in spherical aberration.

Particularly in biological applications, samples are usually observed under a glass cover slip, which introduces distortions to the image.

In contrast, so called "metallurgical" objectives are designed for reflected light and do not use glass cover slips.

Basic glass lenses will typically result in significant and unacceptable chromatic aberration.

Alternatively, some objective manufacturers use designs based on ISO metric screw thread such as M26 × 0.75 and M25 × 0.75.

Image projectors (such as video, movie, and slide projectors) use objective lenses that simply reverse the function of a camera lens, with lenses designed to cover a large image plane and project it at a distance onto another surface.

A telescope's light-gathering power and angular resolution are both directly related to the diameter (or "aperture") of its objective lens or mirror.