I am a former naval officer who frequently used slide rules and wheel-like calculators to determine the ranges of ships. While our periscopes went away from rangefinder optics, I am somewhat 'old school' and use a Leica rangefinder camera. This is a wonderful tool for checking that my aperture is adequate for my desired depth of field. I am so glad that I purchased it, because my head was starting to hurt from all the mental math I was doing before. It is a very useful tool that becomes intuitive after reading the instructions and actually using once or twice.