Transcript
Day One – Extended Leave Assignments Lenses ASSIGNMENT: Read the following pages about Lenses. Then, write a summary discussing what you see are the most aspects and functions of lenses. Also, update your notes to illustrate the most important aspects of the producer in Pre Production.
LENSES Lenses are used in all fields of photographic art. Their primary function is to project a clear image of the viewed scene on the film or, in the case of digital photography and television, on the electronic imaging device. As discussed in chapter 5, the lens is one of the three major parts of the camera. In studio cameras the lens is often considerably larger than the camera itself. Section 6.1, What Lenses Are, covers the basic optical characteristics of lenses and their primary operational controls. The performance characteristics of lenses, that is, how they see the world, are explored in section 6.2, What Lenses See.
Learning Objectives 1.Recognize the various types of zoom lenses 2. Understand the optical characteristics of zoom lenses, such as focal length, iris, ƒ-stop, and depth of field. 3. Use the zoom lens for optimal aesthetic effects.
Glossary Chapter 4 aperture
Iris opening of a lens, usually measured in ƒ-stops.
auto-focus
Automated feature whereby the camera focuses on what it senses to be your target object.
calibrate
To preset a zoom lens to remain in focus throughout the zoom.
compression
The crowding effect achieved by a narrow-angle (telephoto) lens wherein object proportions and relative distances seem shallower.
depth of field
The area in which all objects, located at different distances from the camera, appear in focus. Depth of field depends on the focal length of the lens, its ƒ-stop, and the distance between the object and the camera.
digital zoom lens
A lens that can be programmed through a small built-in computer to repeat zoom positions and their corresponding focus settings.
digital zooming Simulated zoom by cropping the center portion of an image and electronically enlarging the cropped portion. Digital zooms lose picture resolution. ƒ-stop
The calibration on the lens indicating the aperture, or iris opening (and therefore the amount of light transmitted through the lens). The larger the ƒ-stop number, the smaller the aperture; the smaller the ƒstop number, the larger the aperture.
fast lens
A lens that permits a relatively great amount of light to pass through at its maximum aperture (relatively low ƒ-stop number at its lowest setting). Can be used in lowlight conditions.
field of view
The portion of a scene visible through a particular lens; its vista. Expressed in symbols, such as CU for close-up.
focal length
The distance from the optical center of the lens to the front surface of the camera’s imaging device at which the image appears in focus with the lens set at infinity. Focal lengths are measured in millimeters or inches. Shortfocal-length lenses have a wide angle of view (wide vista); long-focal-length (telephoto) lenses have a narrow angle of view (close-up). In a variable-focal-length (zoom) lens, the focal length can be changed continuously from wide-angle (zoomed out) to narrow-angle (zoomed in) and vice versa. A fixed-focal-length (or prime) lens has a single designated focal length.
focus
A picture is in focus when it appears sharp and clear onscreen (technically, the point where the light rays refracted by the lens converge).
iris
Adjustable lens-opening that controls the amount of light passing through the lens. Also called diaphragm or lens diaphragm.
macro position
A lens setting that allows it to be focused at very close distances from an object. Used for close-ups of small objects.
minimum object distance (MOD)
How close the camera can get to an object and still focus on it.
narrow-angle lens
Gives a close-up view of an event relatively far away from the camera. Also called long-focal-length or telephoto lens.
normal lens
A lens or zoom lens position with a focal length that approximates the spatial relationships of normal vision.
rack focus
To change focus from one object or person closer to the camera to one farther away or vice versa.
range extender
An optical attachment to the zoom lens that extends its focal length. Also called extender.
selective focus
Emphasizing an object in a shallow depth of field through focus while keeping its foreground and/or background out of focus.
servo zoom control slow lens
Zoom control that activates motor-driven mechanisms. A lens that permits a relatively small amount of light to pass through at its maximum aperture (relatively high ƒ-stop number at its lowest setting). Can be used only in well-lighted areas.
wide-angle lens A short-focal-length lens that provides a broad vista of a scene. zoom lens
A variable-focal-length lens. It can gradually change from a wide shot to a close-up and vice versa in one continuous move.
zoom range
The degree to which the focal length can be changed from a wide shot to a close-up during a zoom. The zoom range is often stated as a ratio; a 20:1 zoom ratio means that the zoom lens can increase its shortest focal length twenty times.
Main Points
The primary function of the lens is to produce a sharp optical image on the front surface of the camera’s imaging device.
There are various ways to classify zoom lenses: as studio and field lenses and according to zoom range and lens format.
A range extender (an additional lens element) extends the telephoto power of a zoom lens (permits a closer shot) but reduces the range at the wide-angle end.
All television cameras are equipped with zoom (variable-focal-length) lenses.
Some HDTV (high-definition television) camcorders can be modified to accept prime lenses.
The major optical characteristics of lenses are focal length, focus, light transmission (iris, aperture, and ƒ-stop), and depth of field.
The focal length of a lens determines how wide or narrow a vista the camera can show and how much and how close or far away the object seems to be from the camera (viewer). Zoom lenses have a variable focal length, whose major positions are wide-angle, normal, and narrow-angle (telephoto).
A wide-angle lens (zoomed out) gives a wide vista. A narrow-angle lens (zoomed in) gives a narrow vista but magnifies the object so that it appears closer to the camera than it actually is. A normal lens (zoom position toward the midrange of the zoom) approximates the angle of human vision.
A picture is in focus when the projected image is sharp and clear.
A zoom lens needs to be preset (calibrated) so that focus is maintained over the zoom range. If a lens is properly focused when zoomed in, it should remain in focus when zoomed out and in again.
The lens iris, or diaphragm, controls the amount of light passing through the lens. It consists of a series of thin metal plates that form a hole known as the aperture, or lens opening.
The ƒ-stop is a standard scale indicating how much light passes through the lens. Low ƒ-stop numbers indicate large apertures; high ƒ-stop numbers indicate small apertures.
Studio cameras have a remote iris control, which is operated by the VO (video operator) from the CCU (camera control unit).
The area in which objects at different distances from the camera are in focus is called depth of field. The depth of field depends on the focal length of the lens, the aperture (ƒ-stop), and the distance from camera to object.
The two basic operational controls for the zoom lens are the zoom control and the focus control. On ENG/EFP cameras and camcorders, both can be operated either manually or automatically by servo control.
Auto-focus is an automated feature whereby the camera focuses on what it senses to be the target object. HDTV lenses have a focus-assist feature whereby the camera operator selects the target area.
The performance characteristics of wide-angle, normal, and narrow-angle lenses (zoom lenses adjusted to these focal lengths) include field of view, object and distance distortion, movement, and depth of field.
A wide-angle lens (zoom lens in the wide-angle position, zoomed all the way out) offers a wide vista. It gives a wide field of view with the camera relatively close to the scene.
A wide-angle lens distorts objects close to the lens and exaggerates proportions.
A wide-angle lens is ideal for camera movement. It minimizes camera wobbles and makes it easy to keep the picture in focus during camera movement.
The normal lens gives a field of view that approximates that of normal vision. The normal lens (zoom lens in the midrange position) does not distort objects or the perception of distance.
When a camera is moved with the lens in the midrange zoom position, camera wobbles are emphasized considerably more than with a wide-angle lens. The shallower depth of field makes it harder to keep the picture in focus.
A narrow-angle lens (zoom lens in the telephoto position, zoomed all the way in) has a narrow field of view and enlarges the objects in the background. Exactly opposite of the wide-angle lens, which increases the perceived distance between objects, the narrow-angle lens seems to compress the space between objects at different distances from the camera. It slows the perception of object speed toward and away from the camera.
The magnifying power of a narrow-angle lens prevents any camera movement while on the air. Narrow-angle lenses have a shallow depth of field, which makes keeping in focus more difficult but allows for selective focus.
