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Eyepiece Reticles, Stage Micrometers & Calibration Factors

Eyepiece reticles, stage Micrometers and calibration factors. Calibration factor is essential for some reticles.

These are discs marked with scales or patterns, which fit into the eyepiece at the plane of the intermediate image. This is the position of the specimen image produced by the objective lens of the microscope. Therefore, the scale or pattern is viewed superimposed on this image.

In Huygenian eyepieces of older design the reticle is placed on the field aperture at the internal plane of focus.

With modern Huygenian designs the reticle is held firmly within a recessed cell by a retaining ring, all within the body of the eyepiece.

It should be noted however, that some eyepieces of Huygenian design need the reticle to be fitted by a qualified technician. In such cases contact the equipment manufacturer or distributor. In Kellner or Ramsden eyepieces the reticle is held by a retaining ring at the external plane of focus.

We can supply most reticle in 16, 19 and 21mm diameters within two weeks, other diameters are normally supplied within 4 weeks. When ordering please state the diameter of reticle required. Nominal glass thickness is 1.5mm. Image reading correct through the glass.

Eyepiece reticle to Customer Specifications

When sending details to us for a special eyepiece reticle it is essential that you include the following information noting, however, that the most common 8 to 10x eyepieces are the default and only the diameter is absolutely required:

  1. External diameter of the reticle disc.
  2. Calibration of microscope. It is essential that you give calibration details of the objective magnification you intend to use on your microscope. See following section for information on calibration. e.g. If you have a x2 objective magnification and you want each division of the reticle to represent 1mm in terms of the specimen, then each division will have to be 1mm x 2 (size of objective) = 2mm.
  3. Line thickness. This will vary depending on the power of your eyepiece and we will be happy to offer advice. As a guideline it is suggested that, with a x10 eyepiece, lines should be from 10 to 12 microns.

Calibration of Microscopes (Calibration Factor Calculation)

In general, the total magnification of your microscope can be determined by simply multiplying the objective power by that of the eyepiece.

Objective Eyepiece Total Magnification
x20 x10 200

For most work, especially with modern optics, this is sufficient. However, where actual size is critical it is often necessary to modify the dimensions, so when ordering eyepiece reticle the following information is needed:

  • A calibration factor.
  • Objective magnification.
  • Eyepiece magnification.
  • Reticle diameter.
  • Make and model of microscope.

To calibrate the instrument, fit the microscope with an eyepiece scale and appropriate stage micrometer. Compare the length of the stage micrometer with the eyepiece scale. An exact calibration factor can then be calculated.


Using a 40x objective, a circle in the eyepiece requires a diameter of 4000 microns (4mm) to coincide with or read a 100-micron circle on the stage. The factor is therefore defined as 4.

With the stage micrometer and eyepiece reticle in place, the microscope is focused in the normal way. The eyepiece scale now becomes superimposed upon the enlarged image of the stage micrometer.

Move the stage micrometer until the zeros on each scale are coincident. Further along another coincident point will be found. The relationship between the two points can now be seen and calculated.

a) Number of units =
Number of stage micrometer divisions
Number of eyepiece scale divisions

On scale shown a) there are 24 stage micrometer divisions which align with 8 eyepiece scale divisions. Therefore each eyepiece division is:- 24/8 = 3 units

b) In this case the 17 divisions on the stage micrometer line up with divisions 1 to 8 on the eyepiece scale. Thus 17/7 = 2.42857 units. If the unit of the stage micrometer is 10 microns, then each division = 24.2857 microns. In practical terms this figure may be rounded to 24 or 24.3

c) It is possible that there is no second point of coincidence. Then on microscopes with an adjustable tube length coincidence can be obtained by lengthening or shortening the tube. Where there is no tube length adjustment, all measurements will be approximate.

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