[Steve]

I

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.

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

Inverse Square Law
This is the law of diminishing luminance that states that light reduces by the inverse of the distance squared. In other words, if the light travels twice as far from the subject, it reduces by its inverse (2x becomes 1/2) and then squared (1/2 x 1/2) = 1/4. So the amount of light falling on a background twice as far away from the camera as the subject being photographed, it will only have a quarter of the light, hence it will be four times as dark.

People often wonder why the subject they lit so beautifully resulted in the background being so dark and this law explains it.

We can extrapolate from this the following:
2x as far = 1/4 the light
3x as far = 1/9 the light
4x as far = 1/16 the light
5x as far = 1/25 the light

Metering is an important consideration so a basic understanding of this principle can be very helpful.

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.