Physics

# What is Afocal System? Its Uses

Last Update: February 4, 2024.

## 1. What is Afocal System?

An optical system that generates no net converging or departure of the beam, or one that has an endless effective focal length, is an afocal system in optics. The use of a pair of optical elements to build this kind of system. With the distance between them being equal to the total of their individual focal lengths.

An optical telescope imaging a star is a straightforward illustration of an afocal optical system. The light incoming is at infinity, and the image it generates is also at infinity. Although the system doesn’t change a collimated beam’s divergence, it does change the beam’s width, which magnifies the image. This telescope’s magnification is set by

Two focal systems come together to produce one focal system. The front focal point of the second system and the rear focal point of the first system are identical. Rays parallel to the axis in object space and rays parallel to the axis in picture space are conjugate with one another. Beam expanders, binoculars, and telescopes are examples of common afocal systems.

The longitudinal and transverse magnifications are preset. Mapped onto equispaced planes are reference page or page planes. The longitudinal magnification causes a shift in the relative axial spacing.

The Gaussian and Newtonian equations are unable to find conjugate planes since the principal points for an afocal system. They are unable to describe it because the magnification is constant.

## 2. Uses of Afocal System:

For example, beam enhancers, infrared and forward-looking infrared systems, camera zoom lenses and telescopic lens attachments like tableside converters, and photographic setups combining cameras and telescopes are all examples of afocal systems for use in laser optics (Afocal photography).

Afocal photography, also known as afocal imaging or afocal projection, is a type of photography in which the camera’s lens is placed over the eyepiece of another image-forming device, like an interferometer or an optical telescope, to simulate the human eye.

## 2.1. Afocal photography:

Afocal photography is also called afocal imaging or afocal projection. It is a type of photography in which the camera’s lens is placed over the eyepiece of another image-forming device. It is like an optical telescope or an optical microscope, to simulate the human eye.

Any system, such as telescopes and microscopes that can create a virtual image of parallel light is compatible with afocal photography. Afocal photography setups are effective because there is no net convergence or divergence in the light path between the two devices because the imaging device’s eyepiece creates collimated light and the camera’s lens is focused at infinity.  In this system, the gadget is focused on the target, and the camera is positioned as closely as possible to the eyepiece. The system’s disadvantage is its high focal ratio, which will result in a dark image and some vignettes. The field of view will also be restricted by a high focal ratio.

## 2.2. Teleside Converter:

A tableside converter often referred to as a telephoto conversion lens or a front-mount teleconverter[2], is an additional lens that is put on the front of a camera lens to extend the lens’s effective focal length. They are used to enlarge the image on cameras and video cameras that don’t have adjustable lenses. They often have an afocal Galilean design, which allows them to adjust the effective focal length between one and three times without raising the focal ratio by changing the width of the incoming beam of light without changing the beam’s divergence.

## 2.3. Beam Expanders:

A focusing system is the beam expander. Such a beam expander transforms a collimated input laser beam into a collimated output beam with a larger beam radius when seen in the context of wave optics. The beam radius can also be reduced by rotating the object. Such a beam expander may occasionally be employed within a laser resonator, for instance, to increase the mode radius of the laser crystal.