Pinholes in A l/M g F 2 were first inferred by the transmittance enhancement in Larruquert group's research. However, no direct proof was reported to verify the existence of the pinholes in A l/M g F 2. In the past 80 years, pinholes were first observed by dark-field microscopy and bright-field microscopy in the transmission mode. They were small and on the order of several hundred nanometers to several micrometers. Essentially, the pinhole was not a real hole, partially because of the lack of the Al element. Increasing the thickness of Al cannot reduce the size of the pinholes. The occurrence of the pinholes was dependent on the deposition rate of the Al film and the substrate heating temperature, and it was independent of the substrate materials. This research eliminates an otherwise easily ignored scattering source, and it will benefit the development of ultra-precise optics, including mirrors for gyro-lasers, the detection of gravitational waves, and coronagraph detection.