VQiPS: Focus

Guide to Defining Video Quality Requirements

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Focus
The focus function of a camera system is implemented by a lens or series of lenses used to create an image on some sort of media, such as photographic film or other electronic means. A lens can be a simple convex surface, or composed of a number of optical elements in order to correct the many optical aberrations that arise. A lens may be permanently fixed to a camera, or it may be interchangeable with lenses of different focal lengths, apertures, and other properties.

Equipment: Camera Lens
These features can affect the quality of the image or stream the camera lens captures.

Aberration
Lenses do not form perfect images; there is always some degree of distortion or aberration introduced by the lens, which causes the image to be an imperfect replica of the object.

Field of View
The extent of the observable world that is seen at any given moment through the lens.

Focal Length
Determines the field of view, and the apparent size of the objects relative to the image size. If the camera or object is moving, it’s easier to keep the camera on the object if the lens is capturing a wider scene. This ability to capture a wide scene is a function of its “focal length,” which is measured in millimeters, with smaller focal length lenses capturing wider scenes (but making them appear smaller or farther away).

Aperture
Relates to lens opening to reduce or increase light that reaches the image capture surface. Controls the brightness of the image and the fastest shutter speed usable. The ability of lenses to admit light is measured in f-stops, where smaller numbers indicate lenses that admit greater light. The advantage is that more light hitting the sensor will allow the camera to use shorter shutter speeds. This comes at a cost, however: wide aperture lenses (or lenses using a wide-aperture setting) have difficulty keeping both near and distant parts of the scene in focus at the same time (a quality known as depth-of-field).

Depth of Field
The range of distances that appears acceptably sharp in the image.

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