Stereoscopic Rangefinders were instruments that offered the operator a binocular view of the object being ranged upon and required him to try to adjust his eyes focal length until the obect was at the same apparent depth as markings visible alongside. This type of rangefinder was used in many navies. Most notable amongst them were the variety of Zeiss models the German navy employed for Fire Control.
The quality of the data obtained by visual rangefinding depends on the skill of the operator, the range to the object (short ranges are much more accurately measured), and often the size, nature and level of maintenance offered to the instrument employed. The atmospheric conditions, vibration of the platform, degree of operator fatigue and amount of available light also play a role in how accurately ranges would be measured.