Ocular Micrometer

Quick Answer: The Ocular Micrometer is a small glass disc engraved with a scale of equal divisions that is placed inside a microscope eyepiece, so the scale is seen over the specimen and, after calibration against a stage micrometer, can be used to measure cells and other small structures.

A Scale Inside the Eyepiece

The disc, also called an eyepiece graticule or reticle, rests on the field stop inside the eyepiece, the plane where the eyepiece brings the image to focus. Placed there, its scale appears sharp and superimposed on whatever specimen is on the stage. The scale itself has no fixed value in micrometres, because the size of the specimen image at that plane depends on the objective in use.

That is why an ocular micrometer is always calibrated with a stage micrometer, a slide carrying a scale of known length. With each objective in turn, the two scales are lined up and the number of eyepiece divisions that match a known length on the slide is counted. Dividing gives the value of one eyepiece division for that objective, which is recorded and reused while the same microscope and eyepiece are in use.

Once calibrated, measuring is quick: rotate the eyepiece to lay the scale across the cell, pollen grain or fibre, count divisions and multiply by the calibration value. Students measure onion epidermal cells, cheek cells, stomata or fungal spores this way. The disc must match the inside diameter of the eyepiece it goes into, so the eyepiece model should be known when ordering.

Specifications

What Students Measure

• Cell length and width in plant and animal cell practicals.
• The size of microorganisms, spores and pollen grains.
• Fibres, particles and crystals in materials and forensic science teaching.
• Changes in cell size during plasmolysis or growth experiments.

Care & Handling

• Handle the disc by its edge with clean fingers or tweezers; a fingerprint on the scale is magnified in the field of view.
• Seat it with the scale side placed as the eyepiece instructions describe, so the lines come into sharp focus.
• Clean it only with a blower and lens tissue.
• Label each eyepiece that holds a calibrated disc and keep its calibration values with the microscope.

Why Choose LabEquip

Biology departments in schools and colleges, and teachers preparing students for practical assessments that include measuring cells, buy an ocular micrometer together with a stage micrometer, since one is of little use without the other. LabEquip lists both in General Lab Products, and the same kind of disc also appears as the Eye Piece Graticule (Micrometer) in the microscope range; share the microscope and eyepiece model through the contact page so that the disc size can be matched.

Frequently Asked Questions

Why does the eyepiece scale need calibrating?

Because its divisions have no fixed real size. The specimen’s image is enlarged by the objective before it reaches the eyepiece, so the length each division represents changes with every objective. Calibration against a stage micrometer finds that length for each objective.

How do I calibrate it?

Put the stage micrometer on the stage and focus on its scale. Rotate the eyepiece so the two scales lie side by side, line up a zero point and find where lines on both scales coincide again further along. Divide the known stage length by the number of eyepiece divisions between those points.

Do I need to recalibrate if I change the objective?

Yes. Each objective gives a different magnification, so each needs its own calibration value. Once measured, the values for a given microscope and eyepiece can be written down and reused.

Where does the disc sit in the microscope?

Inside the eyepiece, resting on the field stop, the ring where the image formed by the objective comes into focus. In that position the scale and the specimen are seen sharply together.

Can it be used in any microscope?

It can be used in most student and research microscopes, provided the disc fits the inside diameter of the eyepiece. Eyepieces designed to take a graticule make fitting straightforward, so look at the eyepiece before ordering.

What is the difference between an ocular and a stage micrometer?

The ocular micrometer sits in the eyepiece and is used for every measurement, but its divisions have no fixed value. The stage micrometer is a slide with a scale of known length, used only to calibrate the eyepiece scale.

Last Updated: September 2026