Distance in millimeters between the optical center of a lens and the sensor when focused at infinity is known as?

The correct answer is the focal length of the lens. Focal length is defined as the distance between the optical center of the lens and the image sensor when the lens is focused at infinity. It is a crucial characteristic of a lens, influencing both magnification and field of view in photography. A longer focal length results in a narrower field of view and greater magnification, while a shorter focal length provides a wider field of view but less magnification. Understanding focal length is essential for photographers to select and use lenses effectively for their compositions and desired effects.

Depth of field refers to the range of distance in a photograph that appears acceptably sharp, which is influenced by factors like aperture and focal length but is distinct from the focal length itself. The image circle is the circular area projected by the lens onto the sensor; it is related to the lens's design and format compatibility rather than the distance to the sensor. Hyperfocal distance pertains to the nearest point in focus when a lens is set to a specific focal length and aperture, maximizing the depth of field for landscape photography, which is also separate from the definition of focal length.