Objectives are optical systems that form an inverted image of an object, which can be observed directly, e.g. with a telescope or a microscope.
In addition to its construction characteristics, is distinguished by its luminosity (the ratio of the usable diameter of the lens and its focus), for its focal length and the shape of the image covered.
The achromatic lens is a typical lens found on microscopes; it lends itself to a very broad range of applications where a high level of image sharpness is required.
Among the uses for objectives, there is also a zooming type as can be seen from the diagram below. Note how the position of the central biconcave lens is critical for zooming in or out.