Portraits Affected With The Different Angle Of View And Focal Length Ratio

Camera lens focal lengths are usually specified in millimetres (mm), but some older lenses are marked in centimetres (cm) or inches.
When a photographic lens is set to "infinity", its rear nodal point is separated from the sensor or film, at the focal plane, by the lens's focal length. Objects far away from the camera then produce sharp images on the sensor or film, which is also at the image plane.
To render closer objects in sharp focus, the lens must be adjusted to increase the distance between the rear nodal point and the film, to put the film at the image plane. The focal length (f ), the distance from the front nodal point to the object to photograph (S1), and the distance from the rear nodal point to the image plane (S2) are then related by: 1/S1 + 1/S2 = 1/f
As S1 is decreased, S2 must be increased. For example, consider a normal lens for a 35 mm camera with a focal length of f = 50 mm. To focus a distant object (), the rear nodal point of the lens must be located a distance S2 = 50 mm from the image plane. To focus an object 1 m away (S1 = 1000 mm), the lens must be moved 2.6 mm further away from the image plane, to S2 = 52.6 mm.