When sourcing an optical level, the most critical factor is magnification, typically ranging from 20x to 32x. For long-range outdoor work, a 32x magnification is recommended to ensure clarity. You must also verify the standard deviation per kilometer double-run leveling, where a value of 1.0mm or 1.5mm indicates high precision. Additionally, ensure the device features a high-quality compensator (air or magnetic dampened) with a working range of at least ±15' to maintain a level line of sight despite slight tripod vibrations.