Why can some cameras preview after a long exposure?

Gradient medium gray mirror, one side is visually dark, and the other side is transparent optical glass. Among them, the different degrees and ways of the gradual transition zone from deep to shallow determine its type. They are usually suitable for scenes where the sky is too exposed and the scenery is too dark. ND filter can adjust the amount of light entering the camera. And can realize long exposure shooting in the daytime with strong light. If you don't use a filter, it is difficult to take a long exposure effect with the current camera performance. When shooting seascape, you can take long exposure shooting with shutter speed of 30 seconds, 1 minute and 2 minutes.