AZUQI INTERNATIONAL

Lens "format" is the size CCD the lens was designed for. These can be ?", 1/3", ?", 2/3", or 1".
Larger format lenses can be used on smaller CCD's - see "Trading Down Lenses".

Focal length is the distance from the "principal point" of a lens to its focal point, expressed in millimeters (mm).
Focal length controls the magnification of the image captured and the field of view.
Focal length can be fixed or variable - see "Varifocal & Zoom Lenses."
The chart below covers the basic information about focal length.

Aperture is a measure of the size of the lens opening, which affects its ability to gather light and the depth of field.
The aperture of a lens is specified by the f#, which is the ratio of the focal length to the lens diameter. Thus a larger diameter lens has a smaller f#.
The aperture of a lens can be fixed or adjustable.
This chart covers basic information about depth of field.

The depth of field defines the object distance over which the image is in focus.

Smaller apertures (higher F numbers) give greater depth of field.
Shorter focal lengths (smaller mm numbers) give greater depth of field.
Greater subject distances give greater depth of field, unless a telephoto is being used.

The lens aperture does more than control the amount of light—it also controls the depth of field.
The more the lens is adjusted to a higher F number ("stopped down"), the greater the depth of field. The more the lens is adjusted to a lower F number (which is not called "stopped up"), the lower the depth of field.
Changes in focal length also affect the depth of field.
As focal length decreases (moves toward the wide angle direction), depth of field increases.
As focal length increases (moves toward the telephoto direction), depth of field decreases.
Since a wide-angle lens has a greater depth of field, it can give both a sharp foreground and a sharp background.