Lenses: Tilt-and-shift lenses
Among the extensive range of lenses designed for EOS cameras are three that do not carry the ‘EF’ designation. EF is short for electro-focus. All EF lenses offer autofocusing with EOS cameras. That means that, by default, this trio of non-EF lenses must be manual focus only.
The lenses are designated ‘TS-E’ - this is an abbreviation of ‘tilt-and-shift for EOS’. The lenses tilt on their axis, and they shift (that is, they slide perpendicular to the axis). The mechanisms needed for these movements mean that it is not possible to include autofocusing, but the benefits offered by tilt-and-shift more than make up for this loss.
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TS-E lenses are available in three different focal lengths − 24mm, 45mm and 90mm. |
Movements
Many cameras from the early days of photography featured a lens panel and a film back connected by a light-tight concertina bellows. Some of these cameras also had a ‘rising front’, which moved the lens upwards perpendicular to the lens axis. This arrangement soon developed into a versatile professional unit called a ‘view’ camera. The flexible concertina bellows unit means that it is relatively easy to make the whole lens panel move not only up and down, but also side to side. It can also be tilted at an angle to the camera back.
Letting the lens panel rise or fall allows you to include the tops of buildings in the frame while controlling the perspective. Tilting the lens panel alters the position of the focus plane, so that it no longer needs to be perpendicular to the camera.
These features were lost with the introduction of 35mm cameras which, for the most part, have the lens mount set parallel to the camera back. It was only with the introduction of the Canon TS-E lenses in 1991 that some of the view camera features became available on 35mm cameras.
Image circles
The actual shape of an image produced by a lens is round, the same shape as the lens itself.
This round image is projected to the rear of the camera, but the rectangular format of the digital or film sensor means that only part of the full image is recorded.
If the diameter of the image circle produced by the lens is similar to the diameter of the film or sensor rectangle, there is no scope for shifting the lens perpendicular to its axis. The moment you do this, corners of the rectangle will fall outside the image circle and will go dark.
The Canon TS-E lenses produce a larger than normal image circle. This means that when the lens is shifted, the film or sensor in the camera remains within the circle.
With conventional lenses, the image circle is just large enough to cover the rectangular sensor or film format. Tilt-and-shift lenses give a much larger image circle. You can, for example, shift the lens to take in the top of a building without having to tilt the camera upwards. This avoids converging vertical lines in the image.
Introducing shift
The maximum shift is 11mm in either direction. At first sight, it appears that you can only shift the lens in two directions. However, the entire lens is on a rotating mount, so the shift can be turned in any direction. (Always reset the shift to zero before rotating.)
11mm does not sound much. Couldn’t you get the same effect by moving the whole camera up or down by 11mm? No. If you raise the camera by 11mm, you are simply moving the centre of the lens 11mm higher up the subject − the change to the image will be insignificant.
However, if you shift the lens by 11mm in relation to the film or sensor, you are moving the entire image by 11mm. The width of a 35mm film frame is only 24mm, so an 11mm shift gives an almost 50% change.
A small amount of shift can overcome the converging vertical lines in an image that occur when you tilt the camera upwards.
Tilting the lens
Most camera lenses are designed so that the lens axis is fixed perpendicular to the film plane. A TS-E lens is different. It has a front section that can be tilted up to 8 degrees away from the axis. The lens also rotates around the axis, so the angle of tilt can be in any direction. Why is this useful?
When a camera lens is focused on a point, everything on a plane the same distance from the camera as that point will also be in focus. That’s why it is called the plane of focus. Anything closer to or further from the plane is out-of-focus. When no tilt is applied to the lens, the plane of focus is parallel to the sensor or film plane.
Tilting the lens alters the plane of focus so that it is no longer parallel to the sensor or film plane. This allows you to create pictures where objects at different distances from the camera are in focus. This is not the same as increasing the depth of field by setting a small aperture.
The most common use for the tilt function is to make a photograph of a receding subject appear sharp from front to back. This is done by tilting the lens so that the plane of focus runs along the surface of the receding subject.
Shooting with a wide aperture in low light gives very little depth of field. However, by tilting the lens forward, the lens axis can be made to follow the plane of the subject. This brings the length of the subject into focus, even though there is no change to the depth of field.