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Idiots Guide - Eyepiece Reticles

  • The terms ocular and eyepiece mean the same thing.

  • The terms reticle, reticule and graticule mean the same thing

  • It is not normal to have a calibration certificate for an eyepiece reticle. The reason for this is that the reticle is installed in the eyepiece and all of the optics on the microscope can effect the size measured by the reticle. It is normal practice to use a stage micrometer to calibrate the complete microscope. See also Technical Note - Microscope Calibrating with a Stage Micrometer

  • Size conversions and equivalents. 

1mm = 1000µm
1inch = 1000thou
1mm = 39.4thou
0.1mm = 100µm
0.1inch = 100thou
1thou = 0.0254mm (25.4µm)
0.01mm = 10µm
0.01inch = 10thou
 
0.001mm = 1µm    

 

  • It is far better to increase the objective magnification than use a reticle with finer details.

 

A recent example had a customer that wanted to have a reticle with a horizontal scale to measure spores of 2-5µm in diameter. They said they normally used a 10x objective. We answered as follows:-

‘In this case, our NE1, which has a 10mm scale subdivided into 0.1mm divisions will measure 0-1mm when used with a 10x objective lens. Each division will represent 0.01mm (10um). This will not be good enough to measure spores in the 2um - 5um region. If you use a reticle with a smaller scale in the eyepiece it is very difficult to read so the answer is for the customer to use the 40x objective. Now the 10mm scale will measure 0-0.25mm and each division will represent 2.5um. If the customer is able to use the 100x objective then the measurements can be even more accurate as each division will represent 1um’.

 

  • The diameter of the reticle is important to ensure it fits the eyepiece you have. Pyser has compiled a database of various manufacturers, eyepieces and the size of reticle required to fit them.Click on this link for this database.

  •  
  •  When we select an eyepiece reticle we are only concerned with the objective magnification.

 

The mathematics of microscope magnification is very simple – all you do is divide the size of the feature on the reticle by the objective magnification to get the size that it will actually measure at the stage. Thus a 10mm length scale when used with a 10x objective will measure 1mm at the stage. The same 10mm length reticle when used with a 40x objective will measure 0.25mm (250µm at the stage).

If you want to look at another way, if you have a 10µm feature at the stage and are using a 40x objective lens this will be magnified to 400µm (0.4mm) at the reticle image plan. The next section gives more sizing information.

The eyepiece merely magnifies the image on the reticle.

 

  • Table Relating Stage Size and Objective Magnification with Size at Reticle Image Plane.

 

The two columns on the left hand side show the size of the specimen or feature at the stage. Along the top are typical objective magnifications. The figures in the main part of the table show the magnified size at the image plane where the reticle is positioned. For instance, a 10um feature when used witha 10x objective lens will be 100um at the reticle (simply 10um x 10). So, if you want to measure something that is about 50µm using a 40x objective lens this will actually be 2mm at the reticle image plane. A reticle with a 10mm scale in 0.1mm divisions (Our NE1) or one with a 5mm scale in 0.05mm divisions (Our NE5) may be suitable.

 

 Measurement at Stage (um) (microns)  Measurement at Stage (mm) Objective Lens Magnification
1X 1.25X 2X 5X 10X 20X 40X 100X
1um
0.001mm
1um
1.25um
2um
5um
10um
20um
40um
100um
5um
0.005mm
5um
6um
10um
25um
50um
100um
200um
500um
10um
0.010mm
10um
12.5um
20um
50um
100um
200um
400um
1mm
20um
0.020mm
20um
25um
40um
100um
200um
400um
800um
2mm
50um
0.050mm
50um
60um
100um
250um
500um
1mm 2mm 5mm
100um
0.1mm
100um
125um
200um
500um
1mm 2mm 4mm 10mm
200um
0.2mm
200um
250um
400um
1mm
2mm 4mm 8mm 20mm
500um
0.5mm
500um
600um
1mm 2.5mm 5mm 10mm 20mm 50mm
1000um
1mm 1mm 1.25mm 2mm 5mm 10mm 20mm 40mm 100mm
10 000um
10mm 10mm 12.5mm 20mm 50mm 100mm 200mm 400mm 1000mm

   

  • Colour of reticle lines does not matter in a microscope eyepiece. All lines, whatever their colour, block the light from the specimen/stage so the pattern will always appear dark against a brighter background.

 

  • More information available:-

 

Measuring with a Microscope

 

 

           Eyepiece Reticles 50mm