Why is there a lot of light spots when a subtracter is used to expose the sunset for a long time?

Because it is backlight photography, it is normal to produce light spots. Too many light spots in photos like this are caused by too many glass surfaces of the lens, which refract each other. Only a high-quality lens will reduce the flare. The speed of taking this photo should not be too slow, otherwise the clouds will blur and affect the appearance. It is recommended not to use a reducing mirror, reduce the ISO and minimum aperture (on the premise of quality first), and you can shoot.