Why are many pictures of the starry sky round?

It was shot with a super wide-angle fisheye lens.

Fisheye lens belongs to a special super wide-angle lens, and its viewing angle strives to reach or exceed the range that human eyes can see. Therefore, the fisheye lens is very different from the real world scene in people's eyes, because the scenery we see in real life is regular and fixed, and the picture effect produced through the fisheye lens is beyond this category.

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Precautions for shooting with fisheye lens:

The advantage of shooting starry sky with super wide-angle focal length is that it can cover more starry sky, and the scenery/foreground can have a strong visual impact. In addition, it is not easy to compose a composition in the dark, but because it is wide enough, there will not be too much deviation in perspective. Therefore, when shooting the orbit of a star with a super wide-angle focal segment, even if it is not aimed at Polaris accurately, concentric circles can be photographed, and the controllable space is larger.

Shooting the starry sky with a super wide-angle lens also has disadvantages. The first is distortion. If the foreground and shooting position are on the same horizontal line, there will be obvious distortion. Secondly, the wide viewing angle is easily influenced by other artificial light sources in the picture, which leads to an unpleasant halo in the picture. In addition, the super wide-angle lens is not suitable for multiple splicing, and it is prone to serious irreparable splicing cracks.

Baidu encyclopedia-fisheye lens