Distorted photo distortion

When shooting a square object, the surrounding area is curled or swollen. It is not easy to use a long focal length lens, but it is obvious to use a wide-angle lens. Imaging defects caused by the deviation of the optical path of the main light. When the optical system corrects spherical aberration, coma, astigmatism and curvature of the image plane, the intersection of the main light and the Gaussian image plane is the image point, which is clear. However, when the intersection height y between the main light and the Gaussian image plane is not equal to the ideal imaging height y of the corresponding object point, the image is deformed, which is different from the original object. The distortion can be expressed as the formula 1.

This is called absolute distortion. Or expressed as a percentage, and Equation 2 is called relative distortion.

The distortion is caused by the fact that the vertical axis magnification cannot be kept constant in the whole field of view. When the distorted optical system images a square mesh object (Figure 2a), if Δ y >: 0, the intersection height y of the main rays is lower than the ideal image height y, and the larger the field of view, the lower it is, forming an image in the shape of a beer barrel (Figure 2b), so it is also called barrel distortion in orthodontics; If δ y

In the general optical system, as long as the image distortion caused by distortion is not perceived by human eyes, it is allowed, and the allowable distortion value is about 4%. However, in some optical systems that need to measure the size of objects according to images, such as aerial measuring lenses, the distortion directly affects the measurement accuracy, so it must be strictly corrected to make the distortion as small as one in ten thousand, or even several in one hundred thousand. Generally speaking, the distortion value varies with the position of the aperture (aperture stop, the same below). For a single thin lens or a thin lens group, the distortion is zero when the diaphragm coincides with it. If the optical structures of the optical systems on both sides of the diaphragm are symmetrical to each other, and the vertical axis magnification of the whole system is β=- 1, then the symmetrical optical surfaces contribute equally to the vertical axis aberrations (coma, distortion, etc.). ), so the vertical axis aberration of the whole optical system is zero. Therefore, optical systems with strict requirements for distortion often adopt symmetrical or nearly symmetrical structural forms.