Tilted plane focus

Limits to focus in imaging

Focus is relative to spatial depth. Selective focus in photography is usually associated with depth of focus. A pinhole generates an image of ‘infinite’ relative focus from a point just outside the opening out to infinity. Lenses focus more selectively so that for objects near the lens the distance between lens and sensor or film is increased, and is shortened for more distant objects to a point beyond which all is in focus. In telephoto lenses this point may be tens or hundreds of metres from the camera. Wide-angle lenses distinguish differences in depth only up to a short distance beyond which all is in focus.

Depth of field

Depth of field is an effect that permits bringing into focus objects at varying distances from the camera and at varying depth between each other into the field of view. A short lens as explained above will bring objects into focus that are relatively close to the camera, but it will also keep focus at greater distances between each other. A telephoto lens will be very shallow in its gamut of focus.

Reducing the size of the aperture of the lens deepens the focus. Down to a pinhole size this will increase in effect , though the closer the objects are to the camera the shorter the distance between focussed objects.

Plane of focus

Because focus depends on the distance between lens and sensor/film plane, focus in the space in front of the camera is not on a point but on a plane which is parallel to the film plane. However it should be noted that spherical construction of lenses rather than the ideal parabolic construction, which is rarely and expensively achieved, means that this plane is slightly concave, the more so in simple single element lenses and increasingly so with lenses of lower quality construction and materials. Compound lenses are built to correct this ‘spherical aberration’ or ‘curvature of field’.

Tilted plane

What if there were a way to vary the distance between the lens and film/sensor plane across the field? This would permit focussing on objects at varying distances from the camera. One means of achieving this is to tilt the lens in relation to the film/sensor plane. This will mean that individual points on the picture plane will focus on different points in depth, with the effect that the plane of sharp focus will tilt.

The use of this technique (which is based on the principle of the Scheimpflug rule) at full aperture will permit the photographer to select at least three points in depth in the scene in front of the camera on which to focus. Furthermore increasing the size of the aperture and decreasing the depth of field effect will force points other than those in focus to be increasingly blurred, thereby increasing the contrast between the sharp and blurred areas. In this way the scene in front of the camera can be modulated in depth, with sharpness and softness being placed exactly where the photographer needs to place emphasis. A view camera permits full control over this technique.

Tilted plane focus on smaller formats

It is possible to achieve similar effects on the 35mm camera. The lens (preferably a long lens of around 80mm) of a manual single-lens reflex camera (SLR) is set to infinity, shutter speed set for the correct exposure at widest aperture. Then the lens is removed and held against the lens mount opening and tilted. It is possible to see the titled plane effect through the viewfinder. Despite the fact that the lens is separated from the body there is little flare or fogging, and this can be avoided through the positioning of a foam rubber 'donut' between lens and camera body. A commercial option is the Lensbaby but it does not permit the same degree of manipulation of the lens to off-axis positions and tilts required for the technique. A better but more expensive option is to use tilt/shift lenses (e.g. Canon 24mm TS-E)

References

* Robin Gower (1991) "Professional Photography", Australia, October, p. 15
* Greg Neville (1990) 'A World of Fragments and Isolated Parts', "The Age" Melbourne, 9 August 1990, p.14
* Glenda Thompson (1990) 'The Bulletin/Mumm Cordon Rouge Champagne Photographic Awards', "The Bulletin", Sydney, 6 November, p. 94-98

See also

* Tilt-shift photography
* View camera