A type of flare occurring when the sun or other strong light source is included in the scene and a complex series of reflections among the lens surfaces causes a clearly defined reflection to appear in the image in a position symmetrically opposite the light source. This phenomenon is differentiated from flare by the term "ghosting" due to its ghost-like appearance. Ghost images caused by surface reflections in front of the aperture have the same shape as the aperture a ghost image caused by reflections behind the aperture appears as an out-of-focus area of light fogging. Since ghost images can also be caused by strong light sources outside the picture area, use of a hood or other shading device is recommended for blocking undesired light. Whether or not ghosting will actually occur when the picture is taken can be verified beforehand by looking through the viewfinder and using the camera's depth-of-field check function to close down the lens to the actual aperture to be used during exposure.

Using the depth of field principle, as a lens is gradually focused to farther subject distances, a point will eventually be reached where the far limit of the rear depth of field will be equivalent to "infinity." The shooting distance at this point, i.e., the closest shooting distance at which "infinity" falls within the depth of field, is called the hyperfocal distance. The hyperfocal distance can be determined as follows:

	Hyperfocal distance =	f2	f:  focal length F:  F number d:  minimum circle of confusion diameter
		d • F

Thus, by presetting the lens to the hyperfocal distance, the depth of field will extend from a distance equal to half the hyperfocal distance to infinity. This method is useful for presetting a large depth of field and taking snapshots without having to worry about adjusting the lens focus, especially when using a wide-angle lens. (For example, when the EF 24mm is set to f/11and the shooting distance is set to the hyperfocal distance of approximately 1.5m/4.9ft, all subjects within a range of approximately 70cm/2.3ft from the camera to infinity will be in focus.)

The diameter of the sharp image circle formed by a lens. Interchangeable lenses for 35mm format cameras must have an image circle at least as large as the diagonal of the 24 x 36mm image area, and EF lenses generally have an image circle of about 43.2mm. TS-E lenses, however, are designed with a larger image circle of 58.6mm to cover the lens’ tilt and shift movements.

The distance from the lens' rear principal point to the film plane when the lens is focused on a subject at a certain distance.

The ratio (length ratio) between the actual subject size and the size of the image reproduced on film. A macro lens with a magnification indication of 1:1 can reproduce an image on film the same size as the original subject (actual size). Magnification is generally expressed as a proportional value indicating the size of the image compared to the actual subject. (For example, a magnification of 1:4 is expressed as 0.25x.)

A superb new technology that allows the lens to sense movement from "shake" or vibrations, and instantly apply an optical correction by moving a group of lens elements. The improvement in steadiness can be seen even in the viewfinder, and most users find they can shoot hand-held or on a monopod at shutter speeds about two stops slower than previously possible and consistently get sharp images. More information

A numerical value indicating the degree of refraction of a medium, expressed by the formula n=sin i/sin r is a constant which is unrelated to the light ray's angle of incidence and indicates the refractive index of the refracting medium with respect to the medium from which the light impinges. For general optical glass, "n" usually indicates the index of refraction of the glass with respect to air.

Focusing is performed by moving one or more lens groups positioned between the front lens group and the diaphragm.