Most of thermal imaging devices have 3 calibration modes available- manual or silent mode, semi-automatic and automatic mode. In nature, targets appear at different ranges and the environment has different temperatures. With the automatic calibration mode, the image quality is calibrated every couple of minutes without pressing any buttons. With the semi-automatic calibration mode, you need to make a calibration by yourself with clicking the button to initialize the calibration process.
The thermal imaging device with automatic calibration mode is not preferred by some people for hunting as much as the manual or a silent calibration mode because of the “click” sound the device makes while calibrating. Some more expensive thermal imaging devices (like Fortuna) have a special system of an automatic calibration mode that is completely silent and doesn't freeze the image and additionally makes it very useful for hunting.