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Image.

Two-dimensional reproduction of a subject formed by a lens. When formed on a surface, i.e. a ground-glass screen, it is a real image; if in space, i.e. when the screen is removed, it is an aerial image. The image seen through a telescope optical viewfinder, etc. cannot be focused on a surface without the aid of another optical system and is a virtual image.

Incident light.

Light falling on a surface as opposed to the light reflected by it.

Infinity.

Infinite distance. In practice, a distance so great that any object at that distance will be reproduced sharply if the lens is set at its infinity position, i.e. one focal length from the film.

Interspersed Aspect Ratio

A basic requirement of certified photofinishers and certified photofinishing equipment; specifies the three system print formats - C, H and P - that users select during picture-taking must be available at photofinishing.

Interchangeable lens.

Lens designed to be readily attached to and detached from a camera.

Inverted telephoto lens.

Lens constructed so that the back focus (distance from rear of lens to film) is greater than the focal length of the lens. This construction allows room for mirror movement when short focus lenses are fitted to SLR cameras.

Iris.

Strictly, iris diaphragm. Device consisting of thin overlapping metal leaves pivoting outwards to form a circular opening of variable size to control light transmission through a lens.

ISO Speed

The international standard for representing film sensitivity. The emulsion speed (sensitivity) of the film as determined by the standards of the International Standards Organization. In these standards, both arithmetic (ASA) and logarithmic (DIN) speed values are expressed in a single ISO term. For example, a film with a speed of ISO 100/21° would have a speed of ASA 100 or 21 DIN. The higher the number, the greater the sensitivity, and vice versa. A film speed of ISO 200 is twice as sensitive as ISO 100, and half that of ISO 400 film.

IX Information Exchange

The ability of Advanced Photo System film to communicate with devices, and devices to communicate with film; can be accomplished optically or magnetically using a thin magnetic layer on the film that records digital data.