Camera movements are all the different ways in which the film plane and the lens may be turned or moved from the normal position.
In a simple camera, the plane of the negative is parallel to the plane of the lens, and the optical axis of the lens passes through the center of the negative at right angles. This arrangement is satisfactory for the general run of photography because such a camera reproduces scenes more or less as the eye sees them (after allowing for the fact that the light-sensitive surface of the eye is curved while the camera sees everything on a flat surface).
But the large professional stand cameras and technical cameras are almost always designed so that the angle of the planes of both the negative and the lens can be varied. The axis of the lens can also be shifted up, down or sideways in relation to the center of the film plane. Every alteration made in this way produces a characteristic change in the appearance of the image.
Rise and Cross Front
This movement is provided by mounting the lens
panel in guides so that it can slide to either side of, or above and
below the normal axis. Only the lens panel is mounted in this way; there
is no need for similar movements of the negative since the object is
simply to displace one in relation to the other.
With the lens displaced in this way, the camera can look at things from above, below, or one side without being tilted at an angle to the planes of the subject. The objection to such tilting is that it produces an unpleasant appearance of distortion because the nearer parts of the subject are reproduced on a larger scale than those farther away.
The classic example of the effect of tilting the camera is the converging of the vertical lines in a photograph taken looking up at a tall building. But the same thing occurs with the horizontal lines when the camera looks at an angle at a line of buildings (when the lines are seen to converge as they run away from the camera). And the same effect can be noticed when the camera looks down on an object like a tall box. (In this case the vertical lines converge towards the bottom of the object.)
Distortion from the same cause affects subjects where there are no obvious vertical or horizontal lines- e.g., in the human figure. Here, in the absence of straight lines for comparison, the trouble is not so obvious, but it is there just the same, and the improvement is often striking when correction is applied.
The rise or cross movements are used in all circumstances like those quoted above, and wherever the camera has to look obliquely at the subject to show parts not properly visible in the normal square-on front view.
In practice, the camera is set up at the viewpoint which covers the subject from the required angle, but instead of being pointed at the subject, it is arranged so that the plane of the negative is parallel to the plane of the subject. The image is roughly focused, and then the cross front is used to slide the lens off center to bring the subject into place on the screen.
As a result of this maneuver the principal plane of the subject remains parallel to the negative plane and so is reproduced without distortion- the top of the box is visible, but the sides remain parallel; the belfry of the church comes into the picture without looking as though it were about to topple backwards, and the line of houses no longer narrows in violent perspective.
Used in this way, the rise and cross movements enable the photographer to control the perspective of subject planes inclined at an angle to the line of sight of the camera.
To provide the vertical and
lateral back movements, the back is hinged or pivoted so that it can be
swung about either side of the vertical and mounted on a base support
which allows it to be turned horizontally about ten degrees each side of
its normal position.
These movements are used to control the perspective of subject planes which are not at right angles to the camera sight line. If the back is swung to bring it parallel to the principal plane of the subject then parallel lines lying in the subject plane considered will appear parallel.
In practice, the camera is set up to look at the subject from the required viewpoint; the image is roughly focused, and the swing back is adjusted until it lies parallel to the plane of the subject in which parallel lines are to remain parallel. After adjusting the lens panel in such a way that it is again parallel to the film plane (or as near parallel as possible) the image is then re-focused and the lens stopped down until the depth of field includes the whole of the subject.
The result is similar to that produced by the rise and cross movement (above) since in each case the plane of the negative is made parallel to the principal plane of the subject to correct the convergence of the important parallels.
In instances such as the above, if the rise and cross movements of the camera front, or the backswing of the camera, are individually insufficient, a combination of these can be used. When correction is made by means of the swing-back, so that the film plane lies parallel to that plane of the subject in which parallel lines are to remain parallel, then overall sharpness is regained by bringing the lens panel into a position parallel to the film plane. Care should always be taken that the camera movements are not used to an extent beyond which the lens is able to cope, and the corners of the image should always be examined carefully on the ground glass screen (with the lens stopped down to the aperture which will be used, if this is practicable).
In practice, if the degree of effective rising front used on, say, a tall building is such that the top of the building is not quite sharp when film plane and lens panel are parallel to the face of the building, this can often be corrected by tilting the lens panel upwards slightly. Such movement must be only slight, however, and it calls for a little stopping down, in order to retain sharpness over the entire building.
The lens is mounted in a panel which can be turned about
both horizontal and vertical axes; it can thus be tilted down or up or
turned from side to side. Ideally, the turning axes should pass through
the rear nodal point of the lens. If this condition is fulfilled, the
lens may be turned (within the limits of its field) without changing the
position of the image.
Front swing is used to make the image of the principal plane of the subject coincide with the plane of the film. It is not necessary when the film plane, lens, and subject planes are all parallel- e.g., for a square-on photograph of a flat-fronted building.
When the principal plane of the subject is not at right angles to the camera sight line, some parts of the subject are nearer the lens than others. If the middle of the subject is focused sharply, then the ends will be out of focus. The ends can be brought into sharp focus if the lens is stopped down far enough to include them both in the depth of field. But this means losing light and prolonging the exposure.
The other way is to move one side of the lens forward to bring the near parts of the subject into focus, and the other side back to focus the distant parts- i.e., to swing the front of the camera in the same direction as the slope of the subject plane.
There are two principal uses for front swing. It can be used simply as a focusing device for giving a sharp picture of a sloping subject without the need for stopping down, or to increase the scope of the vertical back-swing.
If the swing front is used simply to bring the focal plane to coincide with the film plane when the back of the camera has been swung parallel to a sloping subject plane, the result is exactly as though the picture had been taken by a camera with a large amount of rise or cross front movement.
The same effect can be produced in a camera with a long format by focusing the picture on one half of the plate only- e.g., by taking a picture of a tall building on the bottom half of the plate. This is equivalent to using a camera of half the negative format with a very generous amount of rising front, or the equivalent combination of front and back swing.
Camera movements are used both independently and in conjunction.
The swing back and the rise and cross front movements may be used simultaneously to control perspective- generally to produce a more pleasing perspective although they may also be made to produce deliberate distortion.
The swing front alone has no effect on perspective; its purpose is simply to make the plane of sharp focus of the image coincide with the plane of the negative material. The object here is to avoid having to stop down the lens to bring both near and far ends of an inclined subject plane within the depth of field.
Front swing is often used alone- e.g., to bring foreground and background into focus without stopping down too much. It may always be used alone, in fact, for improving the depth of field over an inclined subject plane where there is no need to correct the perspective of the subject.
Back swing, on the other hand, is only rarely used without some degree of front swing. This is because applying back swing to compensate for camera tilt throws the plane of the negative out of register with the focal plane of the lens. Unless some front swing is added the photographer is forced to stop the lens right down and give a needlessly long exposure.
The amount of rise or cross front movement is decided by the position of the image on the focusing screen. The swing back can be set parallel with the principal plane of the subject by eye or measurement. The angle of the swing front can only be decided by careful examination of the sharpness of the image on the screen, preferably through a focusing magnifier.
Camera Movements in Practice
A photograph fully corrected by the use of camera movements is made as follows:
- The photographer selects the viewpoint which shows the subject to the best advantage for his purpose. Ideally this should be done by eye alone since it is the viewpoint and not the lens that decides the picture's perspective.
- The camera is set up with the baseboard level and back vertical and the image is roughly focused on the screen. If the image is too small, then the lens is changed for one of a longer focus; if it is too large, for one of shorter focus.
- At this stage lateral back swing is applied to bring the back of the camera parallel to the subject plane- e.g., the front of a building being photographed obliquely from one side. If the whole camera has been inclined at an angle (e.g., to look up at a tall building) vertical swing is used to bring the back of the camera vertical again. Lateral swing is adjusted by eye but vertical swing can if necessary be checked with a plumb line.
- The front swing is now adjusted to give the best distribution of sharpness over the plate. If the lens panel can be swung far enough to bring it parallel to the back and to the principal plane of the subject, then the whole of the principal plane can be covered sharply without stopping down the lens.
It may happen, however, that at extreme angles of combined front and back swing, the lens is so far off the common axis that it no longer covers the whole of the negative area. When this happens the front swing is reduced until there are no dark corners on the focusing screen- i.e., until the lens is once again covering the plate.
This change, of course, will also swing the plane of sharp focus so that it no longer coincides with the plane of the focusing screen- i.e., it will no longer be possible to get the image sharp at each side when it is sharp in the center. So the lens must be stopped down to restore sharpness over the whole of the screen.
The image will now be fully corrected- i.e., all vertical lines will be parallel to the sides of the focusing screen; the horizontal perspective will be acceptable; the relative proportions of near and far parts of the subject will appear natural, and the image will be sharp wherever sharp definition is called for.
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