A group of light rays traveling parallel to the optical axis from an infinitely far point. When these rays pass through a lens, they converge in the shape of a cone to form a point image within the film plane.

A light ray which passes close to the optical axis and is inclined at a very small angle with respect to the optical axis. The point at which paraxial rays converge is called the paraxial focal point. Since the image formed by a monochromatic paraxial ray is in principle free of aberrations, the paraxial ray is an important factor in understanding the basic operation of lens systems.

The brightness of a lens is determined by the F number, but this value only indicates the brightness at the optical axis position, i.e., at the center of the image. The brightness (image surface illuminance) at the edge of the image is called peripheral illumination and is expressed as a percent (%) of the amount of illumination at the image center. Peripheral illumination is affected by lens vignetting and the cos4 (cosine 4) law and is inevitably lower than the center of the image.

Since light is a type of electromagnetic wave, it can be thought of as uniformly vibrating in all directions in a plane perpendicular to the direction of propagation. This type of light is called natural light (or natural polarized light). If the direction of vibration of natural light becomes polarized for some reason, that light is called polarized light. When natural light is reflected from the surface of glass or water, for example, the reflected light vibrates in one direction only and is completely polarized. Also, on a sunny day the light from the area of the sky at a 90º angle from the sun becomes polarized due to the effect of air molecules and particles in the atmosphere. The half-mirrors used in autofocus SLR cameras also cause light polarization.

The focal length of a thin, double-convex, single-element lens is the distance along the optical axis from the center of the lens to its focal point. This center point of the lens is called the principal point. However, since actual photographic lenses consist of combinations of several convex and concave lens elements, it is not visually apparent where the center of the lens might be. The principal point of a multi-element lens is therefore defined as the point on the optical axis at a distance equal to the focal length measured back toward the lens from the focal point. The principal point measured from the front focal point is called the front principal point, and the principal point measured from the rear focal point is called the rear principal point. The distance between these two principal points is called the principal point interval.

A light ray which enters the lens at an angle at a point other than the optical axis point and passes through the center of the diaphragm opening. Principal light rays are the fundamental light rays used for image exposure at all diaphragm openings from maximum aperture to minimum aperture.

Focusing is accomplished by moving one or more lens elements positioned internally, behind the lens’ diaphragm assembly. By moving internal elements, less weight is required to be moved, so focusing can be faster and more responsive. Furthermore, the front of the lens does not move during focusing — ideal for photographers who use filters.

To reduce the length of a telephoto lens, it is necessary to increase the mutual power of the convex-concave groupings. Fluorite’s low index of refraction makes it possible to achieve significant reduction in lens length while maintaining high image quality.
Although the extraordinary optical properties of fluorite were discovered in the 19th century and lens designers have long desired to use it, naturally formed pieces of fluorite large enough for use in lens production are extremely difficult to find. Deciding to solve this problem, Canon took up the challenge of developing synthetic crystals, bringing practical fluorite production technology on-line by the late 1960’s.

Reflection differs from reflection in that it is a phenomenon which causes a portion of the light striking the surface of glass or other medium to break off and propagate in an entirely new direction. The direction of propagation is the same regardless of wavelength. When light enters and leaves a lens which does not have an anti-reflection coating, approximately 5% of the light is reflected at the glass-air boundary. The amount of light direction of propagation. The two elements of a light wave which can actually be detected by the human eye are the wavelength and amplitude. Differences in wavelength are sensed as differences in color (within the visible light range) and differences in amplitude are sensed as differences in brightness (light intensity). The third element which cannot be detected by the human eye is the direction of vibration within the plane perpendicular to the light wave’s direction of propagation.

The resolution of a lens indicates the capacity of reproduction of a subject point of the lens. The resolution of the final photograph depends on three factors: the resolution of the lens, the resolution of the film, and the resolution of the printing paper. Resolution is evaluated by photographing, at a specified magnification, a chart containing groups of black and white stripes that gradually decrease in narrowness, then using a microscope to observe the negative image at a magnification of 50x. It is common to hear resolution expressed as a numerical value such as 50 lines or 100 lines. This value indicates the number of lines per millimeter of the smallest black and white line pattern which can be clearly recorded on the film. To test the resolution of a lens alone, a method is used in which a fine resolution chart is positioned in the location corresponding to the film plane and projected through the test lens onto a screen. The numerical value used for expressing resolving power is only an indication of the degree of resolution possible, and does not indicate resolution clarity or contrast.