What are the advantages and disadvantages of square and round light-reducing mirrors?

Square ND filter:

Advantages: Vignette control is better than circular ND; it has a filter holder that can insert multiple filters to facilitate shooting of different environmental themes.

Disadvantages: The price is more expensive than the circular ND; it takes up space and is troublesome to carry; when the light is complex, the light leaks easily and requires manual repair.

Circular light-reducing mirror:

Advantages: cheap; light and easy to carry.

Disadvantages: Poor vignetting control; circular ND is not conducive to superposition of multiple ND mirrors.

If you are a photography enthusiast or plan to become a photographer, it is best to buy both. After all, there are pros and cons in different situations. If you are just an ordinary photography enthusiast who needs a slow shutter effect and is willing to post-process, buy a round one. Affordable and easy to carry.

(The key here is to remove the dark corners caused by the circular ND)