A light-tight box with film in it doesn’t require a lens to expose film—even a pinhole camera can capture an image. But a lens can gather more light than a pinhole. A lens can focus and clarify. It’s the tool we use to define our image.
Scientists and photographers have studied the lens since the early Twentieth Century. Few will describe any lens as technically perfect, although a lens’ unique optical characteristics make it the best choice for a particular shoot.
Lens function is complex. We know that since all visible objects reflect light rays in all directions, we must gather as many rays as possible and get them to our film without distortion. Simple lenses use a single convex glass element positioned so that the light rays from the subject are bent towards and converge at the film. By carefully placing the lens relative to the film, we successfully record an image.
An iris is an aperture of variable size used to control the intensity of light falling on film. The iris control is usually calibrated in f-stops or T-stops. A change of one f-stop or T-stop is equivalent to doubling or halving the intensity of light falling on the film. T-stops are more accurate because they factor for light loss through the lens glasses.
Simple lenses have a limited ability to focus light. Optical distortions can result when light rays enter from the lens perimeter; those rays have to travel farther to reach the film and will be less focused. Toa limited degree we can solve this problem by narrowing the glass width; the resulting loss in light, however,makes the lens “slower.” For each additional stop of light passage in a given lens, the design becomes more complex—the geometric correction becomes more extreme.
Standard, wide-angle, telephoto, and zoom lenses include the ability to adjust focus and iris. Some lenses include a second element, positioned between the first element and the film. Its concave surface compensates for the distortion of the first element. Each element introduces distortion, typically resulting in internal reflection, or flare. The distance between these elements and the precise grind of the glass is critical, and these factors add significant cost to the lens.
An aperture controls the quantity of light passing through a lens. The best lenses perform well at every aperture setting. As such, the precision of the lens must be consistent at every point within each glass element.
Dr. Max Berek of Leitz established image quality standards before 1914 by capturing “miniature” still photographs on 35 mm film. His “circle of confusion” defined the measure of permissible out–of–focus quality in a ten-inch paper photograph. Though modified, this concept endures.
Color is probably the most complex factor in lens design. Because each color has a specific wavelength measured in nanometers, a particular shade has a unique wavelength. Blue objects and red objects will focus in diJerent places on a film frame, whether the film is black-and-white or color. Getting all colors to converge on a single plane despite their diJerent lengths is fundamental to lens design. In 1938 Kodak pioneered the concept of making glass lenses with exotic types of rare-earth elements and cementing them together in each element to correct the aberration.
Zoom lenses were developed later. A good zoom lens must address each of the potential pitfalls while oJering the utility of variable focal length. In motion picture applications, the light transmission, sharpness and individual color focus must remain unchanged despite the focal length change within a shot.
Focal Length and Focus
Lenses are identified by their focal length in millimeters and maximum aperture in f-stops (e.g., 50 mm/f1.4 lens). The focal length is defined as the distance from the optical center of the lens to the film plane. The f-stop is calculated from the dimensions of the lens.
Focal Length and Angle of View
The focal length of a lens determines the angle of view, or perspective, seen through the lens. Normal lenses provide a perspective that approximates human vision.
Lenses that are shorter than normal provide a wider angle of view—they are wide-angle lenses. Lenses that are longer than normal provide a narrower point of view and magnify the subject—they are telephoto lenses. Wide-angle lenses make background objects appear further away; telephoto lenses compress distance and make the background appear closer. Thus, moving the camera toward a subject (as in a dolly move) results in a look that is very diJerent from a scene captured by zooming the lens from a stationary camera position. The apparent separation from the background, making objects relatively smaller, makes camera movement less noticeable. Thus, using wider lenses for hand-held scenes is preferable.
Scientists and photographers have studied the lens since the early Twentieth Century. Few will describe any lens as technically perfect, although a lens’ unique optical characteristics make it the best choice for a particular shoot.
Lens function is complex. We know that since all visible objects reflect light rays in all directions, we must gather as many rays as possible and get them to our film without distortion. Simple lenses use a single convex glass element positioned so that the light rays from the subject are bent towards and converge at the film. By carefully placing the lens relative to the film, we successfully record an image.
An iris is an aperture of variable size used to control the intensity of light falling on film. The iris control is usually calibrated in f-stops or T-stops. A change of one f-stop or T-stop is equivalent to doubling or halving the intensity of light falling on the film. T-stops are more accurate because they factor for light loss through the lens glasses.
Simple lenses have a limited ability to focus light. Optical distortions can result when light rays enter from the lens perimeter; those rays have to travel farther to reach the film and will be less focused. Toa limited degree we can solve this problem by narrowing the glass width; the resulting loss in light, however,makes the lens “slower.” For each additional stop of light passage in a given lens, the design becomes more complex—the geometric correction becomes more extreme.
Standard, wide-angle, telephoto, and zoom lenses include the ability to adjust focus and iris. Some lenses include a second element, positioned between the first element and the film. Its concave surface compensates for the distortion of the first element. Each element introduces distortion, typically resulting in internal reflection, or flare. The distance between these elements and the precise grind of the glass is critical, and these factors add significant cost to the lens.
An aperture controls the quantity of light passing through a lens. The best lenses perform well at every aperture setting. As such, the precision of the lens must be consistent at every point within each glass element.
Dr. Max Berek of Leitz established image quality standards before 1914 by capturing “miniature” still photographs on 35 mm film. His “circle of confusion” defined the measure of permissible out–of–focus quality in a ten-inch paper photograph. Though modified, this concept endures.
Color is probably the most complex factor in lens design. Because each color has a specific wavelength measured in nanometers, a particular shade has a unique wavelength. Blue objects and red objects will focus in diJerent places on a film frame, whether the film is black-and-white or color. Getting all colors to converge on a single plane despite their diJerent lengths is fundamental to lens design. In 1938 Kodak pioneered the concept of making glass lenses with exotic types of rare-earth elements and cementing them together in each element to correct the aberration.
Zoom lenses were developed later. A good zoom lens must address each of the potential pitfalls while oJering the utility of variable focal length. In motion picture applications, the light transmission, sharpness and individual color focus must remain unchanged despite the focal length change within a shot.
Focal Length and Focus
Lenses are identified by their focal length in millimeters and maximum aperture in f-stops (e.g., 50 mm/f1.4 lens). The focal length is defined as the distance from the optical center of the lens to the film plane. The f-stop is calculated from the dimensions of the lens.
Focal Length and Angle of View
The focal length of a lens determines the angle of view, or perspective, seen through the lens. Normal lenses provide a perspective that approximates human vision.
Lenses that are shorter than normal provide a wider angle of view—they are wide-angle lenses. Lenses that are longer than normal provide a narrower point of view and magnify the subject—they are telephoto lenses. Wide-angle lenses make background objects appear further away; telephoto lenses compress distance and make the background appear closer. Thus, moving the camera toward a subject (as in a dolly move) results in a look that is very diJerent from a scene captured by zooming the lens from a stationary camera position. The apparent separation from the background, making objects relatively smaller, makes camera movement less noticeable. Thus, using wider lenses for hand-held scenes is preferable.
