What is the concept of the so-called defocus line in photography?

Bilinear is a photographic term, which refers to unsatisfactory defocusing imaging effect. Generally speaking, a good blur should be uniform and continuous according to the transition from distance to focus, and there is no obvious edge residue in defocus imaging. "Out-of-focus bilinear", the blurred outline is not uniformly defocused, but separated by several lines, forming something similar to small bubbles, which often interferes with the theme of the picture and affects the expressive force of the composition subject and the image (of course, there are exceptions in everything, and there are also examples of using bilinear to do special effects in photography.

For SLR cameras, this imaging quality is only related to the quality of the lens itself. Experience has proved that this kind of blur has a very important influence on the final expressive force of the image, so it has become an important reference for evaluating the quality of the lens.

I found two photos from the Internet for your reference. Pay attention to the background The first defocus image is beautiful, and the second one is obviously bilinear.