Lens distortion correction

In photography, we often hear these words, perspective distortion and lens distortion. Many people often confuse perspective with distortion and confuse the relationship between them.

First, what is distortion?

Distortion actually refers to the distortion of the object you shoot relative to the object itself. This photo is out of shape.

For example, the wall should be straight, but it is really curved.

The building was originally straight up and down, but the sideline was curved.

These all belong to lens distortion, which is a kind of aberration. It is the lens that causes the image distortion of the object, but it has no effect on the clarity of the image.

Let's take a look at these two samples just now. The most intuitive feeling is that what should be a straight line is bent. The wall seems to protrude, while the building seems to sink.

We call the protruding distortion effect barrel distortion and the sinking distortion pillow distortion. This kind of distortion is collectively called lens distortion. These distortions are actually caused by the magnification difference of the lens system.

We all know that the thickness of the center and edge of a lens is different. Thick in the middle is called convex lens, and thin in the middle is called concave lens. Lenses are optical devices composed of multiple groups of lenses.

For an ideal optical system, the magnification is constant on the object image plane of a pair of yokes.

However, for the practical optical system, it is almost difficult to achieve the ideal state, because the magnification of the lens changes with the angle between the beam and the main axis.

Generally speaking, light is orthogonal to the main axis and passes through it without distortion. The farther the light is from the main axis, the greater the distortion. When the magnification increases with the increase of incident angle, it is called pincushion distortion. This is what we call pincushion distortion.

Negative distortion when magnification decreases with the increase of incident angle. This is barrel distortion.

Especially when the diopter of the lens is large, this distortion phenomenon is more serious. Distortion like this is related to the inherent characteristics of the lens (convex lens condensing and concave lens diverging), which cannot be completely eliminated and can only be improved. After any lens leaves the factory, its distortion rate is fixed. Generally speaking, the distortion rate of our camera lens is generally controlled at a few tenths of a percent.

We need to remember that barrel distortion is generally produced by wide-angle lenses, while pincushion distortion is generally produced by telephoto lenses. How to solve this problem? Actually, it's very simple. Just make the back of the camera parallel to the front of the building being photographed. If the whole building can't be photographed, either change the wide-angle lens or move back.

Another way to solve the linear distortion problem is to install a camera on the back of the machine. This camera can move the lens up and down or left and right, so that the front of the subject is parallel to the film at a fixed position. This is why architectural photographers almost always use the lens behind the lens.