Rokinon Tilt-Shift 24mm F3.5

What is Tilt-shift?

This page presents a simplified explanation of what a tilt-shift (sometimes called "perspective control") lens can do, as well as some workarounds to duplicate the effects with a regular lens.

The principles of perspective control were developed in 1904 by an Austrian military officer, in order to correct aerial images.

A tilt-shift lens can, in fact, perform two distinct tasks: the tilt and the shift. These will be discussed separately.

Shift

When a camera's sensor is not parallel to the subject being photographed, there is a distortion of perspective, with lines converging toward the vanishing point, as seen in the image below.

While this can be used creatively, oftentimes it is not the desired effect.

Shift refers to the ability by a lens to offset the optical path in order to preserve perspective. It is well illustrated by the following images, where the grid represents a building.

Camera straight
Camera tilted
Lens shifted

Images courtesy of Wikipedia

In the first image, the camera is held vertically, and the sensor plane is on the same vertical orientation as the building. The image of the building is straight, but it cannot be seen in its entirety.

The second image illustrates what most photographers will do (as seen on the example above). Tilting back the camera lets the photographer fit the whole building in the frame, but the shape is deformed. Parallel lines do not appear parallel any more.

The third image shows the effect of a shift lens. By shifting the glass in relation to the sensor, lines remain straight and it is possible to fit the whole building into the frame.

Because of the relation between positions of the subject, the optical elements and the sensor, shifting a lens does not have the same effect as moving the camera to a higher point.

Shift lenses are often used in architecture photography, where it's easy to see their usefulness. They can also be used for other applications where precise composition is paramount, such as for macro photography. FInally, they can help to create a stitch of several images without inducing perspective distortion.

Tilt

A regular lens creates an area of focus parallel to the sensor, as illustrated in the image below. What a tilt lens does is displace the in-focus plane, changing its shape and orientation.

Regular lens
Lens tilted down

The space between the blue lines illustrates the in-focus area. With a regular lens, that space has a uniform thickness, parallel to the sensor plane. This is what is called the depth of field (DOF). With a tilted lens, however, the in-focus area has the shape of a wedge (with the point closer to the camera).

This wedge can be oriented up, down, or even left or right (if the lens allows rotation of the tilting components). The angle of the tilt will determine the angle of the in-focus area. For instance, the three images below show the same cropped scene with the lens tilted up, down, and oriented straight forward. The white marble was used as the focal point and as a reference for composition.

Lens tilted up	Lens straight	Lens tilted down

As you can see, tilting the lens dramatically changes the DOF. It also slightly alters the perspective, which is a side effect of such a movement.

Changing the angle of the in-focus area can have multiple uses. In landscape photography, it can be used to make sure both the foreground and background are in focus, without closing down the aperture. It can also do the opposite and create an impression of "fake miniatures" by decreasing the perceived DOF (especially effective if the subject is seen from above). The in-focus area can also be oriented such that a very narrow part of it covers the subject, creating a very shallow DOF normally only possible with very large apertures.

Limitations

There are a few limitations associated with tilt-shift lenses.

First, because the optical elements can move around, there can be no coupling with the camera's screw-drive, and no autofocus. While a manufacturer could conceivably use an AF motor in the lens (such as SDM of DC), it would be impractical in most situations, since the focus point would be extremely hard to guess for the camera's AF system.

Second, because of the odd angles at which light is likely to hit the sensor, the metering system can be more easily fooled. In part because of this, metering is fully manual and should ideally be done before any tilt or shift is performed.

Since a tilt-shift lens allows its optical components to move in relation to the camera's sensor, its image circle must be larger than that of a regular lens. Nonetheless, vignetting is likely to occur at the extremes of the movement range, especially on full frame cameras.

Workarounds

There are a few workarounds available for photographers who want to imitate the effect of a tilt-shift lens without using one.

First, owners of recent Pentax DSLR cameras can take advantage of the Composition Adjustment feature found on higher-end bodies. This uses the camera's SR system to move the sensor and duplicate the effect of a shift lens, to some extent. The images below compare the effect to that of a fully shifted Rokinon lens.

	Camera tilted up
Normal	Sensor shift	Lens shift	Lens and sensor shift

As can be seen from the images above, the sensor shift is much more limited in range, when compared to a fully shifted lens. It can help in a pinch but is not a full replacement.

Second, the effect of a tilt lens can be partially imitated in post-processing. By selectively adding blur, it is possible to come close to the original effect.

Extending the in-focus area is possible by using focus stacking. This method uses multiple pictures focused at varying distances, manually (or automatically, via software) stacked to use the sharp part of each. This technique does extend the effective DOF, but the effect is not identical since with a tilt-shift lens, the in-focus area is a wedge. If that wedge is tilted to create a long, almost straight in-focus area, the top and bottom of that zone won't be in focus. Stacking images will create a very deep field where everything is sharp (which might be desirable or not, depending on the application).

Another trivial workaround is simply to use a wider lens, if it provides a sufficient field of view when the camera is parallel with your subject.