An eyepiece reticule (also known as an eyepiece graticule) is an essential microscope accessory used for measuring specimens, counting particles, and performing comparative observations. To ensure accurate measurements and proper installation, it's important to select the correct reticule size for your microscope eyepiece and fit it correctly.
This guide explains how to identify the right reticule size and install it safely.
What is an Eyepiece Reticule?
An eyepiece reticule is a precision glass disc engraved with a scale, grid, crosshair, or other measuring pattern. It sits inside the microscope eyepiece at the focal plane, allowing measurements to be made directly while viewing a specimen.
Common reticule patterns include:
Linear scales
Crosshair reticules
Grid reticules
Counting chambers
Circular measuring scales
Custom engraved designs
Because the reticule is installed within the eyepiece, selecting the correct diameter is critical.
Step 1: Determine the Reticule Diameter Required
The most common microscope reticule diameters are:
| Reticule Diameter | Common Usage |
|---|---|
| 19 mm | Educational and older microscopes |
| 21 mm | Some specialist eyepieces |
| 24 mm | Most modern laboratory microscopes |
| 25 mm | Certain industrial microscopes |
| 26 mm | Selected research and specialist models |
The required size is determined by the internal reticule holder inside the eyepiece, not the external eyepiece diameter.
Method 1 – Check the Microscope Documentation
The easiest method is to consult the microscope manual or manufacturer's specifications. Many manufacturers specify the required reticule diameter for compatible eyepieces.
Method 2 – Measure the Existing Reticule
If your microscope already contains a reticule, carefully remove it and measure its diameter using a digital caliper.
Method 3 – Measure the Reticule Seat
If no reticule is installed:
Remove the eyepiece from the microscope.
Unscrew the eyepiece retaining ring if fitted.
Measure the internal ledge or recess where the reticule sits.
Select a reticule that matches this diameter.
For best accuracy, use digital calipers capable of measuring to 0.1 mm.
Step 2: Remove the Eyepiece
Before fitting the reticule:
Switch off the microscope.
Remove the eyepiece from the microscope tube.
Work over a clean, dust-free surface.
Wear powder-free gloves if available to avoid fingerprints.
Handle the eyepiece carefully to prevent damage to optical components.
Step 3: Open the Eyepiece
Most microscope eyepieces allow access to the reticule holder by unscrewing either:
The upper eyecup section
A retaining ring
The lower lens assembly
Consult the eyepiece documentation if unsure.
Avoid forcing any components as some eyepieces use fine threads that can be damaged.
Step 4: Insert the Reticule
Place the reticule gently into the designated holder.
Important points:
The engraved side is usually positioned facing downward toward the objective lenses.
Ensure the reticule sits flat in the holder.
Do not force the glass into position.
Remove any visible dust using an air blower before reassembly.
A correctly fitted reticule should remain secure without excessive movement.
Step 5: Reassemble the Eyepiece
Carefully reinstall the retaining ring or eyepiece components.
Check that:
The reticule remains centred.
No dust or debris is trapped inside.
Threads engage smoothly during reassembly.
Once assembled, reinstall the eyepiece into the microscope.
Step 6: Focus the Reticule
Look through the microscope and adjust the eyepiece dioptre ring (if fitted) until the reticule markings appear sharp and clearly defined.
The reticule should remain in focus while viewing specimens.
If the scale appears blurred:
Adjust the eyepiece focus.
Confirm the reticule is seated correctly.
Check for contamination on optical surfaces.
Step 7: Calibrate the Reticule
A reticule scale is not an absolute measurement until calibrated against a stage micrometer.
Calibration should be performed for each objective magnification used.
The process involves:
Place a stage micrometer on the microscope stage.
Align the reticule scale with the stage micrometer scale.
Calculate the actual value represented by each reticule division.
Record calibration values for every objective lens.
Without calibration, measurements obtained from the reticule will not be accurate.
Common Installation Issues
Reticule Will Not Fit
The reticule diameter may be incorrect. Recheck the internal eyepiece measurement before ordering a replacement.
Reticule Appears Out of Focus
The reticule may not be located at the correct focal plane or the eyepiece focus may require adjustment.
Scale Appears Rotated
Some reticules can rotate within the holder. Reposition the reticule before fully tightening the retaining ring.
Dust Visible Inside the Eyepiece
Disassemble the eyepiece and clean using appropriate optical cleaning procedures before refitting.
Need Help Selecting a Reticule?
If you're unsure which eyepiece reticule is compatible with your microscope, our team can help identify the correct size and scale. Simply provide your microscope make, model, and eyepiece details, and we'll recommend the appropriate reticule for your application.
View our reference guide: Reticule Diameters for Common Microscopes
Prefer to watch a video of how to size and fit an eyepiece reticule?
https://www.graticulesoptics.com/knowledge-centre/technical-videos
Contact Us
Have a question about eyepiece reticules or need help selecting the right one for your microscope? Get in touch with our team.