My job more often is shooting pictures and sending reports, not bullets. Frankly I have limited knowledge about photography, but am on a mission to assemble a solid photo system. Requirement is ability to shoot images of recognisable facial features at 150m or more, if possible. The magnification of telephoto lenses is measured by Milimeters (100-500mm in case of the Canon lens above), which don't make much sense. As mid-tier lens cost ~$2.5k, I want to be sure, this is the correct choice.
Maybe some of you also have experience with telephoto lens and could share experience what a 500mm lens would look like, compared to rifle scope / spotting scope. 10x? 20x? 40x? This question blows fuses on photography forums, so turn to you, brothers.