1. An electron spectrometer comprising: an energy analyzer section that energy-analyzes electrons emitted from a specimen; a micro-channel plate that amplifies the electrons analyzed by the energy analyzer section; a fluorescent screen that converts the electrons amplified by the micro-channel plate into light; a camera that photographs the fluorescent screen; and an effective range calculation section that calculates an effective range of the fluorescent screen within a camera image photographed by the camera, the effective range calculation section performing a process that acquires a plurality of the camera images photographed while causing the energy analyzer section to analyze the electrons with a different center energy, a process that converts the plurality of camera images respectively into a plurality of spectra, and a process that calculates the effective range of the fluorescent screen within the camera image based on the plurality of spectra.