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image camera distance objects

Rochester Institute of Technology

Perspective —Refers to the appearance of depth when a three-dimensional object or scene is represented in a two-dimensional image, such as a photograph, or when the subject is viewed directly.

Types of perspective —The fact that we humans have two eyes is often credited for our ability to perceive depth and judge relative distances in the three-dimensional world with such accuracy. If we close one eye, however, the world does not suddenly appear two dimensional because there are other depth cues, cues that photographers can use to represent the three-dimensional world in two-dimensional photographs.

Linear perspective —Linear perspective is exemplified by the convergence of parallel subject lines in images and by the decrease in image size as the object distance increases. Linear perspective is so effective in representing depth in two-dimensional images that it is often the only type of depth cue provided by artists when making simple line drawings.

Early Egyptian artists drew pictures without linear perspective, where a boat in the foreground and a boat on the horizon were drawn the same size. Brunelleschi, an Italian architect, is credited with devising a mathematically proportional system, which he used in architectural drawings in 1420. In 1435 Alberti, another Italian architect, originated the concept of thinking of the picture plane as a window through which he looked at the visible world, which enabled him to record the images of objects with the correct size-distance relationships. Alberti is also credited with the concept of vanishing points, where parallel subject lines that recede from the viewer meet in the image.

Overlap —If a subject is arranged so that a nearby object obscures part of a more distant object, the viewer is provided with a powerful cue as to which object is closer. Overlap by itself, however, does not provide any information concerning the actual distances or even relative distances of the near and far objects.

Depth of field —Refers to the range of object distances within which objects are imaged with acceptable sharpness. When a camera is focused on an object at a certain distance and closer and more distant objects appear unsharp in the photograph, the viewer is made aware that the objects are at different distances. When the depth of field is very large and objects at all distances appear sharp in the photograph, the viewer may become confused about relative distances unless there are other depth cues such as relative sizes. Photographs in which a pole in the background appears to be growing out of the head of a person in the foreground is an example of confusion resulting from a large depth of field.

Lighting —Depth can be emphasized with lighting that reveals the shape, form, and position of objects. A lack of tonal separation between the dark hair of a model and a dark background, for example, destroys the appearance of distance between the model and the background. Uniform front lighting on a white cube with three visible planes can make it appear two dimensional, whereas lighting that produces a different luminance on each plane will reveal the cube’s true form. With curved surfaces, as on the human face, form and depth are revealed with lighting that produces gradations of tone on the curved surfaces. Shadows on objects on the opposite side from the main light and shadows of objects cast onto other surfaces can emphasize both the form of the objects and their locations.

Aerial haze —The scattering of light by small particles in the atmosphere makes distant objects appear lighter and with less contrast than nearby objects. With a series of mountain ridges that increase in distance, each more distant ridge appears lighter than the one in front of it. Heavy fog is a more dramatic example of the same effect, where a person walking away from the camera can appear to fade and then disappear in a short distance. The appearance of aerial haze, and therefore the apparent distance, can be altered considerably by the choice of film and filter. Since the haze consists of scattered light that is bluish in color, use of a red filter with panchromatic film will decrease the appearance of haze and a blue filter will increase it. Black-and-white infrared film used with a red or infrared filter will decrease the appearance of haze and depth and increase the detail that is visible in the more distant objects. Smoke in a smoke-filled room creates the similar effect of increasing the appearance of depth in interior photographs. Fog machines are used by some photographers and cinema-tographers to achieve a haze effect.

Color —As applied to perspective, red is identified as an advancing color and blue as a receding color. The terms indicate that if a red card and a blue card are placed side by side and there are no other cues as to their distance from the viewer, the red card will tend to appear nearer. If the explanation for this effect is that it is due to association, it can be noted that with aerial haze, the haze becomes more bluish with increasing distance. Also, blue sky is thought of as being at a great distance. Researchers in visual perception have considered the possibility of a physiological explanation based on a difference in focus on the retina in the eye for red light and blue light. It is not uncommon for artists to use red and blue colors to achieve perspective effects in their paintings.

Stereophotography —Attempts have been made since the early days of photography to make photographs more realistic through the use of stereoscopic vision. All such attempts involve two (or more) photographic images made from slightly different positions, each of which is then presented to the appropriate eye of the viewer. The stereoscope is an individual viewing device that holds the pictures side by side with a separate viewing lens for each eye. The first stereoscope, designed by Wheatstone in 1832, before the invention of the daguerreotype process, made use of multiple mirrors and pairs of stereoscopic drawings. It is possible, with practice, to fuse the two images of a stereograph without a viewing device. The centers of the two images must not be farther apart than the pupils of the eyes, which varies among individuals but is about 64 mm (2.5 inches) average, since the axes of the two eyes cannot normally be made to diverge. Wedge-shaped lenses in the improved stereoscope introduced by Sir David Brewster in 1849 eliminated eyestrain and made stereophotography practical. With contact-size stereographs, it is recommended that the lenses in the stereoscope have approximately the same focal length as the lenses in the camera, but most were somewhat longer. The imposed limitation on the size of the stereograph images in the viewer is not a problem, because the viewer perceives an image projected into space similar to the effect of looking at the original scene. During the years following the introduction of the Brewster stereoscope, many photographers, including the famed landscape photographer William Henry Jackson, made stereoscopic photographs in addition to conventional photographs. By 1900 stereoscopes were common parlor room fixtures and photographic stereographs were sold by the millions.

A limitation of the stereoscope is that it is an individual viewing device. Various processes have been developed to produce stereoscopic images that could be viewed by more than one person, and even large groups in theaters. The anaglyph consists of superimposed red and green images. By looking through a red filter with one eye and a green filter with the other, viewers see the appropriate image with each eye and perceive a three-dimensional image.

Superimposed images that are polarized at right angles produce depth perception when viewed through polarizing filters that are also rotated at right angles to each other. By using polarizing filters in front of two projectors, color stere-ographic slides or motion pictures can be viewed by groups of people wearing polarized glasses. It is necessary for the screen to have a metallic surface, such as aluminum paint, to prevent the reflected images from being depolarized.

Viewing devices can be eliminated by using a system that combines the two photographs as very narrow alternating vertical strips over which is placed a plastic layer containing a lenticular pattern that refracts the light from each set of strips to the appropriate eye. Stereoscopic cameras with four lenses have been manufactured, a design that represents a refinement of the lenticular stereoscopic process.

Three-dimensional images have been presented on conventional television receivers with a process that requires the viewer to wear glasses that place a neutral density filter over one eye. The filter causes that eye to adapt to a lower light level, which reduces the time resolution and produces a delay in the perception of the image in that eye.

Motion perspective —Identifies a changing image pattern with respect to the angular size and separation of objects in a three-dimensional scene as the distance between the scene and the viewer or camera changes.

Linear perspective —In a static situation it refers to a fixed point of view. A change in distance between a scene and the viewer may be due to movement of the viewer, the subject, or both. It is more convenient, however, to think of the observer as a fixed reference point around which there is a continuous flow of objects, even when it is the viewer who is moving.

When a distant point is selected as representing the direction of movement of the viewer, the parts of the scene appear to move away from this point in all directions. For example, when driving through a tunnel, the opening at the far end expands away from the center at an increasing rate and the edge of the opening passes around the car on all sides as the car emerges from the tunnel. The flow of objects away from the distant vanishing point is most rapid closest to the viewer, because the rate of movement is inversely proportional to the distance between the object and the viewer.

When one approaches a photograph of a three-dimensional scene rather than the scene itself, the effect is entirely different. Now the images of objects at different distances do not change in relative sizes, and the rate of movement of the parts away from the center of the photograph is unrelated to the distance of the objects from the camera. The illusion of depth in two-dimensional photographs is therefore less realistic when the viewer alters the viewing distance (or horizontal position) than when the viewing position remains constant. This is one factor that contributes to the realism of depth in slides and motion pictures that are projected on a large screen where the viewing position remains constant.

The consequences of motion perspective can be seen in motion pictures and television pictures when the camera moves directly into or away from a scene in comparison to using a zoom lens from a fixed position. The use of the zoom lens has an obvious advantage in convenience, and in many situations, the difference in effect is unimportant. But zooming in on a scene tends to reduce the illusion of depth in a manner similar to that of moving in relation to a still photograph because the images of near and far objects increase in size at the same rate.

Also, using a long, fixed focal-length lens decreases motion perspective because the image of an object moving toward the camera increases in size more slowly. This technique was used effectively in the motion picture Lawrence of Arabia , where the viewer is kept in suspense when the image of a distant horseman (friend or enemy?) very slowly increases in size, even though he is obviously riding rapidly toward the camera. Conversely, use of a short focal-length lens for a car race seems to exaggerate speed due to motion perspective.

Motion parallax —When a person moves sideways, whether just moving one’s head a few inches or looking out the side window of a moving vehicle, the relative positions of near and far objects change. The basic concept is the same as for stereoscopic vision. The two eyes view an object from slightly different angles and the relative position of the background is different for the two eyes, but with continuous lateral movement the foreground-background changes are also continuous. If one fixates a foreground object, the background appears to move in the same direction the person is moving. If one fixates a background object, the foreground appears to move in the opposite direction. During the movement, the person has a strong impression of the distance between foreground and background objects. A similar perception results when viewing moving images produced with a motion-picture or video camera that was moving laterally.

Holography —Holographic images also present different viewpoints to the two eyes, and, within limits, the viewer experiences motion parallax with lateral movement. A hologram is made by recording on a photographic material the interference pattern between a direct coherent light beam and light from another beam from the same source after it is reflected or transmitted by the subject. By viewing the hologram with a beam of coherent light, positioned the same as the direct beam used to expose the hologram, an image is produced that has the same three-dimensional appearance as the original subject.

Linear perspective variables —The relative sizes of the images of objects at different distances is determined by the position of the eye when we look at the objects directly and the position of the camera lens when we photograph the objects. Although lenses of different focal lengths change the size of the entire image rather than the relative sizes of parts of the image, strong perspective is associated with short focal-length wide-angle lenses and weak perspective with long focal-length telephoto lenses. This is in part due to the fact that the focal length of the camera lens determines the camera-to-object distance required to obtain an image of the desired size—small for short focal-length wide-angle lenses and large for long focal-length telephoto lenses. In addition to the relative image sizes, which largely determine the viewer’s impression of the perspective in photographs, psychological factors involved in changes in viewing distance also influence the perception.

Object distance and image size —Two objects of equal size placed at distances of 1 foot and 2 feet from a camera lens produce images having a ratio of sizes of 2:1. If the ratio of the object distances is 1:3, the ratio of image sizes will be 3:1. The relationship between image size and object distance is that image size is inversely proportional to object distance. Linear perspective is based on the relative size of the images of objects at different distances. With movable objects, the image sizes and linear perspective can be controlled by moving the objects. In situations where the subject matter cannot be moved easily, it is necessary to move the camera and/or change the focal length of the camera lens to alter image sizes.

With two objects at a ratio of distances of 1:2 from the camera, moving the camera farther away to double the distance from the closer object does not double the distance to the farther object, therefore, the ratio of the image sizes will not remain the same. If the ratio of object distances changes from 1:2 to 2:3 by moving the camera, the ratio of image sizes will change from 2:1 to 3:2 (or 1.5:1). Moving the camera farther away not only reduces the size of both images, but it also makes them more nearly equal in size. The two images can never be made exactly equal in size no matter how far the camera is moved away, but with very large object distances the differences in size can become insignificant.

The linear perspective produced by moving the camera farther from the objects is referred to as a weak perspective. Thus, weak perspective can be attributed to a picture in which image size decreases more slowly with increasing object distance than expected. The images of parallel subject lines also converge less than expected, with weak perspective. Another aspect of weak perspective is that space appears to be compressed, as though there were less distance between nearer and farther objects than actually exists.

Conversely, moving a camera closer to two objects increases the image size of the nearer object more rapidly than that of the farther object, producing a stronger perspective. For example, with objects at a distance ratio of 1:2, moving the camera in to one-half the original distance to the closer object doubles its image size but reduces the distance to the farther object from 2 to 1.5, therefore increasing the image size of the farther object to only 1.33 times its original size. Moving the camera closer to the subject produces a stronger linear perspective whereby image size decreases more rapidly with increasing object distance, and the space between the closer and farther objects appears to increase. Strong perspective is especially flattering in architectural photographs of small rooms because it makes the rooms appear to be more spacious, and it permits the building of relatively shallow motion-picture and television sets that appear to have normal depth when viewed by the audience. Strong perspective is inappropriate, however, for certain other subjects where the photograph is expected to closely resemble the subject.

Changing object distance and focal length —In many picture-making situations it is appropriate to change lens focal length and object distance simultaneously to control linear perspective and overall image size. For example, if the perspective appears too strong and unflattering in a portrait made with a normal focal-length lens, the photographer could substitute a longer focal-length lens and move the camera farther from the subject to obtain the same size image but one with weaker perspective. Since short focal-length wide-angle lenses tend to be used with the camera relatively close to the subject and long focal-length telephoto lenses tend to be used with the camera at relatively large distances, strong perspective is often associated with wide-angle lenses and weak perspective is similarly associated with telephoto lenses, but it is the camera position and not the focal length or type of lens that produces the abnormal linear perspective. The change in linear perspective with a change in object distance is more apparent when an important part of the subject is kept the same size by simultaneously changing the lens focal length.

In situations where a certain linear perspective contributes significantly to the effectiveness of a photograph, the correct procedure is to select the camera position that produces the desired perspective first, and then select the focal-length lens that produces the desired image size. For example, if the photographer wants to frame a building with a tree branch in the foreground, the camera must be placed in the position that produces the desired relationship between the branch and the building. The lens is then selected that produces the desired image size. A zoom lens offers the advantage of providing any focal length and image size between the limits. With fixed focal-length lenses, if the desired focal length is not available, and changing the camera position would reduce the effective-ness due to the change in perspective, the best procedure is to use the next shorter focal-length lens available and then enlarge and crop the image.

Cameras cannot always be placed at the distance selected on the basis of linear perspective. Whenever photographs are made indoors, there are physical limitations on how far away the camera can be placed from the subject. Fortunately, the strong perspective that results from using short focal-length wide-angle lenses at the necessarily close camera positions enhances rather than detracts from the appearance of many architectural and other subjects. There are also many situations where the camera must be placed at a grater distance from the subject than would be desired. This, of course, applies to certain sports activities where cameras cannot be located so close that they interfere with the sporting event, block the view of spectators, or endanger the photographer.

Not all subjects are such that the perspective changes with object distance. Since two-dimensional objects have no depth, photographs of such objects reveal no change in the relative size of different parts of the image with changes in camera distance. Also, photographic copies of paintings, photographs, printed matter, etc., made from a close position with a short focal-length wide-angle lens, and from a distant position with a long focal-length telephoto lens, should be identical.

Viewing distance —Although it might seem that the distance at which we view photographs would have no effect on linear perspective, since a 2:1 ratio of image sizes for two objects at different distances will remain constant regardless of the viewing distance, changes in viewing distance can alter the perception of depth providing that the photograph contains good depth cues. Photographs of two-dimensional objects or subjects that have little depth appear to change little with respect to linear perspective when the viewing distance is changed, whereas those that contain dominant objects in the foreground and background or receding parallel lines that converge in the image can change dramatically.

Seldom do we encounter unnatural-appearing linear perspective when we look at the real world. Such effects tend to occur only when we look at photographs or other two-dimensional representations of three-dimensional objects or scenes. The reason perspective appears normal when we view three-dimensional scenes directly at different distances is that as we change the viewing distance, the perspective and the image size change simultaneously in an appropriate manner. Because we normally know whether we are close to or far away from the scene we are viewing, the corresponding large or small differences in apparent size of objects at different distances seems normal for the viewing distance.

To illustrate how the situation changes when we view photographs rather than actual three-dimensional scenes, assume that two photographs are made of the same scene, one with a normal focal-length lens and the other with a short focal-length lens, with the camera moved closer to match the image size of a foreground object. When viewers look at the two photographs, they assume that the two photographs were taken from the same position because the foreground objects are the same size, but if the perspective appears normal in the first photograph, the stronger perspective in the second photograph will appear abnormal for what is assumed to be the same object distance.

Viewers can make the perspective appear normal in the second (strong perspective) photograph, however, by reducing the viewing distance. The so-called correct viewing distance is equal to the focal length of the camera lens (or, more precisely, the image distance) for contact prints, and the focal length multiplied by the magnification for enlarged prints. This position is identified as the center of perspective.

The correct viewing distance for a contact print of an 8 × 10 inch negative exposed with a 12-inch focal-length lens is 12 inches. Since we tend to view photographs from a distance about equal to the diagonal of the photograph, the perspective would appear normal to most viewers. If the 12-inch focal-length lens is replaced with a 6-inch focal length wide-angle lens, the print would have to be viewed from a distance of 6 inches for the perspective to appear normal. When the print is viewed from a comfortable distance of 12 inches, the perspective will appear too strong. Conversely, the perspective of a photograph made with a 24-inch focal-length lens will appear too weak when viewed from a distance of 12 inches. It is fortunate that people do tend to view photographs from standardized distances based on their size rather than adjusting the viewing distance to make the perspective appear normal, for that would deprive photographers of one of their more useful techniques for making interesting and effective photographs.

Wide-angle effect —In addition to the association of short focal-length wide-angle lenses with strong perspective, they are also associated with the wide-angle effect. The wide-angle effect is characterized by what appear to be distorted image shapes of three-dimensional objects near the edges of photographs. This effect is especially noticeable in group portraits where the heads near the sides seem to be too wide, those near the top and bottom seem to be too long, and those near the corners appear to be stretched diagonally away from the center.

The image stretching occurs because rays of light from off-axis objects reach the film at oblique angles rather than at a right angle as occurs in the center of the film. If the subject consists of balls or other spherical objects, the amount of stretching can be calculated in relation to the angle formed by the light rays that form the image and the lens axis. Thus, at an off-axis angle of 25 degrees, the image is stretched about 10%, and at 45 degrees the image is stretched about 42%. (The image size of off-axis objects changes in proportion to the secant of the angle formed by the central image-forming ray of light and the lens axis. The reciprocal of the cosine of the angle may be substituted for the secant.) Normal focal-length lenses, where the focal length is about equal to the diagonal of the film, have an angle of view of approximately 50 degrees, or a half angle of 25 degrees.

Why don’t we notice the 10% stretching that occurs with normal focal-length lenses? It is not because a 10% change in the shape of a circle is too small to be noticed, but rather that when the photograph is placed at the correct viewing distance, the eye is looking at the edges at the same off-axis angle as the angle of the light rays that formed the image in the camera. Thus, the elliptical image of the spherical object is seen as a circle when the ellipse is viewed obliquely. The image would also appear normal even with the more extreme stretching produced with short focal-length wide-angle lenses as long as the photographs were viewed at the so-called correct viewing distance. Because the correct viewing distance is uncomfortably close for photographs made with short focal-length lenses, people tend to view them from too great a distance, where the stretching is obvious.

If two-dimensional circles drawn on paper are substituted for the three-dimensional balls, the circles will be imaged as circles in the photograph, and there will be no evidence of a wide-angle effect. The distinction is that when one looks at a row of balls from the position of the camera lens, the outline shape of all of the balls will appear circular, whether they are on- or off-axis, but off-axis circles drawn on paper will appear elliptical in shape because they are viewed obliquely. The compression that produces the ellipse when the circle is viewed from the lens position is exactly compensated for by stretching when the image is formed, because the light falls on the film at the same oblique angle at which it leaves the drawing.

Viewing angle —As was noted above, even when viewers are directly in front of photographs they look obliquely at the edges of the picture, which alters the shape of images near the edges. There are situations where it is necessary to view the entire picture at an angle, as when sitting near the sides or the front of a motion picture theater. At moderate angles, viewers tend to compensate mentally for the angle so that they perceive a circle in the picture as a circle even though the shape is an ellipse at the oblique viewing angle, a phenomenon known as shape constancy. At extreme viewing angles, viewers may have difficulty even identifying the subject of the photograph. Some early artists were fascinated with slant perspective, where the image had to be viewed at an extreme angle to appear realistic. Oblique aerial photographs also display slant perspective, which must be rectified if the photographs are to be used for mapping purposes.

View camera perspective controls —In addition to accommodating lenses of varying focal lengths, which allows the camera to be placed at different distances from the subject with corresponding variations of linear perspective, view cameras have tilt and swing adjustments on the front and back of the camera that provide considerable control over the shape and sharpness of the image. Tilt refers to a vertical rotation around a horizontal axis and swing refers to a horizontal rotation around a vertical axis. The tilt and swing adjustments on the lensboard control the angle of the plane of sharp focus vertically and horizontally, but do not alter the shape of the image. The same adjustments on the back of the camera are normally used to control the shape of the image, although they can be used to control the angle of the plane of sharp focus also if they are not needed to control image shape. It is the change in image shape, such as the convergence of the image of parallel subject lines or the relative sizes of the images of objects on opposite sides of the photograph, that represents a change in linear perspective.

To illustrate how changing the angle of the film plane in a view camera affects the linear perspective, assume that the camera is set up to copy a subject consisting of a grid of parallel vertical and horizontal lines. With the back of the camera parallel to the subject, the vertical and horizontal subject lines will be parallel in the image. Tilting with top of the camera ground glass (or film) away from the subject will cause the vertical subject lines to converge toward the bottom of the ground glass, or the top of the picture since the image is inverted on the ground glass. Tilting the ground glass will have no effect on the shape of the horizontal lines, however. When the camera back is tilted in this way, it is in a similar position to the back of a conventional camera that has been tilted up to photograph a tall building, where the vertical building lines converge toward the top of the picture. Thus, it is the angle of the film plane that controls convergence of vertical subject lines, not the angle of the body and lens of the camera. If a view camera is tilted up to photograph a tall building, it is only necessary to tilt the back of the camera perpendicular to the ground and parallel to the vertical lines of the building to prevent convergence of those lines in the photograph. If the photographer wanted to exaggerate the convergence of the vertical lines, perhaps to make the building appear taller, the camera back would be tilted in the opposite direction, increasing the angle between the building lines and the film plane.

Although this explanation has been written in terms of the convergence of parallel subject lines, it could just as well have been in terms of the relative size of objects at different distances from the camera. With the same subject, we can think of the width of the building at the top and the bottom. If a camera is tilted up and no adjustment is made in the angle of the back, the top of the building, which is farther from the camera than the bottom, will be narrower than the bottom in the photograph. With the back of the camera tilted so that it is perpendicular to the ground, the top and bottom of the building will have the same width in the image.

The same principle applies to the control of the convergence of horizontal subject lines by swinging the back of the view camera. The only time horizontal subject lines will be parallel in a photograph is when the film plane is parallel to the horizontal subject lines. When photographing a box-shaped object, such as a building, it is common practice to show two sides of the object. If the back of the camera is swung parallel to the horizontal lines on one side of the object to prevent convergence of those lines, the angle between the camera back and the other side of the object will be increased, which will exaggerate the convergence of the horizontal lines on that side of the object. Even though converging vertical lines are just as natural as converging horizontal lines when we look at objects directly, professional photographers consider converging vertical lines to be less acceptable than converging horizontal lines in photographs.

Trick perspective —Because the three-dimensional world is represented with two dimensions in photographs, it is possible to fool the viewer concerning the perspective of the subject of the photograph. One of the most common accidental illusions occurs when the main subject and the background are equally sharp in the photograph, and an object in the background appears to be sitting on or to be a part of the object in the foreground. Motion-picture photographers make use of the same concept in the glass shot, where part of a scene, which is commonly an imaginary and dramatic large expanse, is painted on glass. The painting is then photographed in combination with action, which is seen through clear areas on the glass. When skillfully done, the painting is accepted by the viewer as part of the three-dimensional world.

Projected backgrounds and traveling mattes are also used so that actors can be photographed under controlled conditions, usually in a studio, commonly on a mockup of a supposedly moving vehicle such as a car, train, plane, or ship, with a projected image of a distant scene appearing in the background.

A trick perspective effect is achieved in the famous Ames room whereby objects appear twice as tall when placed in the right-hand corner as when placed in the left-hand corner. Even though both corners appear to be at the same distance, the left-hand corner is actually twice as far away. The effect is achieved with a ceiling that slopes down and a floor that slopes up from left to right so that, due to linear perspective, the room appears to be rectangular in shape from one position. From any other position the illusion is destroyed.

Scale models have been used by architects to show what a proposed building will look like, and when photographed skillfully, the model can appear to be a full-size building. Motion-picture photographers commonly make use of scale models to depict disaster events, such as a large ship capsized in a storm, where the actual event would be prohibitively expensive or impossible to photograph.

Still photographers have long combined parts of different photographs, such as the image of a clothing model with a photograph of the Egyptian pyramids, to save travel time and money and to produce dramatic effects. With computer software programs that are now available, sophisticated and undetectable special effects can be achieved easily with video and still video images.

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