The Filar eyepiece micrometer is a combination of a focusing eyepiece and reticle but features a moveable line rule. The Filar micrometer avoids the necessity to estimate fractions of a division on a stage micrometer which may render inaccurate readings.

The mobile line inside the micrometer is designed to cross the field of view, traversing a fixed scale, by means of a precision screw mechanism that is operated by rotating the external knob. In general measuring applications, a single line is aligned with one end of the specimen feature to be measured and a reading of the calibrated knob is noted.

Since installing the filar eyepiece on one side of a binocular head does not work well in practice, our MA414 includes a monocular head so that one can just quickly swap heads when measurement tasks are required.