Tilt-shift lens is a special lens, in which the internal optics can be tilted and shifted relative to the image sensor, and the image perspective is changed, and the common tilt-shift lens can achieve three kinds of physical movement, namely tilt and offset and rotation, which can achieve special shooting effects.
The first is the oblique motion of the lens, in which the lens moves around the pivot at an angle to its original axis, which is the movement that causes the front part of the lens to form an up-down or side-to-side tilt. This movement allows the focal plane to be controlled, enabling the main subject to be fully focused at a wide aperture.
The second is the offset movement of the lens, where the lens moves parallel to the original axis, allowing the front part of the lens mount to pan up and down or left and right as a whole. With this function, you can change the shooting range to achieve the same effect as raising or lowering the shooting position.
The third is the rotational movement of the lens, which can rotate the entire lens when the lens is mounted on the fuselage. In this way, you can change the direction of operation of the offset and tilt angle according to the shape of the subject.
As machine vision continues to penetrate into different subdivided industries, the number of scenarios that require tilt-shift imaging is also increasing in the field of industrial inspection. Most of the reasons for using tilt-shift imaging in industrial inspection are that there is not enough space to shoot vertically, or that the measured object is not in the same plane at the same time.
Many people call tilt-shift lenses also Sham lenses, because tilt-shift imaging is to conform to Sham's law. This law was proposed by Captain Scham of the Austrian Army. He invented this correction technique in order to use his "perspective plotter" for aerial photography. The subject plane, the lens plane, and the image plane intersect in a straight line. Sham's theory stems from corrections made during zooming in on aerial and land surveys, which were filmed by cameras with fixed fuselages. Today's technology allows us to make this correction with an adjustable tilt-shift camera, while also solving the problem of too little depth of field by changing the position of the sharp plane. In practice, this means that at least one of the bases must be adjusted horizontally (pitch) or vertically (swing) depending on the position of the scene until the three planes intersect in a straight line. At first, this may seem a bit complicated, but adjusting a sharp plane is one of the simplest and most commonly used techniques for professional photographers.
Photographers like to use tilt-shift lenses to capture miniature scenes, or to increase the depth of field.
Going deeper, there are actually two cases of tilt shifting, one is the optical axis tilting, and the other is the optical axis translation. Many SLR tilt-shift lenses have both of these functions. The optical axis tilt is to solve the clarity, and the optical axis translation is to solve the difference between the distal and proximal magnifications. For example, if you want to shoot a building, if you don't use a tilt-shift lens, you will inevitably be near and far away, causing the top of the building to look smaller and the whole building to appear cone-shaped.
If a tilt-shift lens is used, parallax, which is the difference between magnification at the far and near ends, is eliminated.
Tilt-shift imaging for industrial inspection is much more complex than photography. First of all, most of them are tilt-shift applications with optical axis tilt to solve the problem of sharpness when oblique shooting. Without optical axis translation, trapezoidal distortion cannot be corrected.
However, tilt-shift trapezoidal distortion is linear and can be corrected by algorithmic interpolation. However, the general industrial tilt shift is only suitable for detection, not for measurement. Because in addition to the tilt-shift trapezoidal distortion of the image, the optical distortion of the lens itself, the superposition of the two brings great difficulty to the calibration. The following is an in-depth discussion of how to achieve oblique imaging of optical axes in industrial tilt-shift. There are too few application cases of optical axis translation to be discussed here.
According to Scham's law, in order to move the tilt of imaging, the optical axis cannot be perpendicular to the chip. According to the Sham schematic, it can be seen that the larger the shooting angle, the greater the optical axis deflection. The closer the working distance, the greater the optical axis deflection. This requires that industrial tilt-shift lenses must be able to rotate at different angles to accommodate different shooting angles and working distances. However, it is difficult to rotate the optical axis of C-mount industrial lenses because there is no space.
There is a concept called optical back focus (BFL), which refers to the distance from the sensor to the last optical face of the lens. The shorter this distance, the lower the cost of the lens. Therefore, most C-mount industrial lenses on the market have very short back lenses. The C-mouth flange is only 17526mm, so it is almost impossible to design a rotation device on the C-mount lens, and only a fixed angle tilt shift can be done. Because different projects require different angles, most tilt-shift lenses need custom angles, and manufacturers cannot make serialized products, with long customization cycles and high proofing costs.
In response to the above problems, the engineers came up with another solution. Since the lens can't be rotated, simply make the rotation component on the camera, and change the camera C mount to a custom interface, and the problem of rotation space is solved.
With the combination of this tilt-shift component and a custom camera, all C-mount industrial lenses can achieve 10° tilt-shift imaging. Many industrial camera manufacturers have adopted this solution.
Some friends may say that a tilt-shift lens can be achieved with a connector, why is it so complicated. There is only one case that is relatively simple, that is, the rear focus of the lens is relatively long, and there is room to add a pivot adapter, such as a telecentric lens and a macro lens. For this type of lens, a tilt-shift adapter can be designed to achieve a 15° tilt shift by adding it, but this type of tilt shift is only suitable for situations with a relatively small field of view.
In addition, there is no need to do tilt shift for line scan lenses, because the imaging area is a line, and there is no difference in the working distance between the distal and the near end.