What do the numbers on the barrel of the microscope objective mean? What about the letters DIN and JIS?

Microscope objective lenses will often have four numbers engraved on the barrel in a 2x2 array. 

The upper left number is the magnification factor of the objective. For example, 4x, 10x, 40x, and 100x.

The upper right number is the numerical aperture of the objective. For example 0.1, 0.25, 0.65, and 1.25.

The lower right number (if given) refers to the thickness of the glass cover slip (in millimeters) assumed by the lens designer for best performance of the objective. Example: 0.17.

The lower left number is the tube length in millimeters. This is related to the standardization of microscopes and the particular standard used for the manufacture of your microscope. Most microscopes employ the Deutsche Industrie Norm, or DIN standard configuration. The Japanese Industrial Standard (JIS) is less commonly used. DIN microscopes begin with an object-to-image distance of 195 mm, and then fix the object distance at 45 mm. The remaining 150 mm distance to the eyepiece field lens sets the internal real image position, which is defined as 10 mm from the end of the mechanical tube (which gives the 160 mm tube length). DIN standard eyepieces have an international standard 23 mm diameter. DIN standard objectives often times have "DIN" etched on the side and have a standard 0.7965 in diameter thread, 36 TPI, 55° Whitworth threading. Celestron microscopes are made to DIN standards. The tube length for the DIN standard is 160 mm, while for the JIS it is 170 mm.

NOTE: JIS objectives can be used on a DIN microscope and vice versa  The threads on both types are interchangable. However, since the optical distances are different, there will be a difference in magnification. A JIS objective used on a DIN microscope will have a slightly lower magnification than the rated magnification since the DIN tube is shorter. And a DIN objective used on a JIS microscope will have a slightly higher magnification than the rated magnification since the JIS tube is longer.

Sometimes objectives have a color ring to aid in identifying the magnification: black (1x), brown (2x), red (4x), yellow (10x), green (20x), turquoise (25x), light blue (40x), dark blue (60x), white (100x).

Updated 12/18/13