Sequence viewing > Index - Cameras - lenses - optics - Resource - © Lloyd Godman

Film - Digital

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The size relationship between different objects, in a scene or between parts of the same object, are initially determined by the position of the camera, not the choice of lens. The only aspect of perspective that the lens determines is the angle of the view or how much is included in the frame.  The shorter the lens, the wider the angle of view, which means more of the scene will be recorded on the film.  The longer the lens, the narrower the angle of view and a smaller section of the scene will be recorded.  By standing in the same place with lenses of different focal lengths, the same photograph can be obtained simply by enlarging a section of the photograph made by the shorter lens and the perspective is relatively the same.


The size of the subject is determined by the projection distance between the lens and the film plane, which is a function of the camera-to-subject distance and the focal length of the lens.

The shape of the subject and the size relationships between different objects in the scene are also determined by the lens-to-film plane projection distances.

When an object or a section of it is centered in front of the lens, the image it projects onto the film plane will fall in the exact centre of the image circle.  The light rays from the centered object cross in the centre of the lens and travel an equal distance to the film plane.  This lens to subject alignment usually provides the most accurate recording of an objects shape.
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As an object moves away from alignment with the centre of the lens, or as its extremities recede from the centered position, its projected image will move to the edges of the image circle.  Inside the camera, light rays from the farthest edges of the object must now be projected a greater distance to the film than the light rays projected through the centre.  The section of the object farthest from the centre of the lens consequently expands due to the greater projection distance, and changes the objects shape in the final image.

This 'distortion' is more noticeable with wider than normal lenses because of the relative difference in projection distances.

Move over image to view roll over image






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