What is the reason why the photos taken by the camera always have such black spots?

The SLR viewfinder has small black spots and the photos are in the same place. This should be the lens. I suggest changing the lens and checking it again.

If there are no black spots in the viewfinder, there should be dust or bad spots on the photosensitive element, which need cleaning or maintenance.

Black spots in photos are usually caused by foreign objects on the CCD surface, and the dust attached to the lens is almost negligible. There are foreign bodies such as stains near CCD, and the final portrait will show the shadow of foreign bodies after imaging. The closer the stain is to the CCD, the clearer the shadow is.

Extended data:

When taking pictures, use the longest focal length and set the digital camera to the minimum aperture before shooting. Run the software after taking a photo to see if there are obvious black spots in the photo. If there are black spots, you need to send the CCD to the maintenance point for cleaning, or replace the lens barrel assembly to solve the problem.

Because there are foreign objects on the CCD, the same stains in the same position, the farther the lens goes out, the farther the focus is from the CCD, and the clearer the shadow of the stains. On the contrary, the closer the lens is to the CCD and the closer the focus is to the CCD, the more blurred or even invisible the shadow of the stain is.