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 4 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 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 left number is the tube length in millimeters. This is related 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 195mm, and then fix the object distance at 45mm. The remaining 150mm distance to the eyepiece field lens sets the internal real image position, which is defined as 10mm from the end of the mechanical tube (which gives the 160mm tube length). DIN standard eyepieces have an international standard 23mm diameter. DIN standard objectives often times have "DIN" etched on the side and have a standard 0.7965" diameter thread, 36 TPI, 55° Whitworth threading. Celestron microscopes are made to DIN standards. The tube length for the DIN standard is 160mm, while for the JIS it is 170mm.
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.
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.
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).