Optical Microscopy of Meteoritic Metal

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WHAT IS THE MAGNIFICATION?

Factors Affecting Total Magnification.  All images were collected using a Reichert Polyvar microscope coupled with a Nikon Coolpix 4500 camera.  The total magnification of this system depends on several factors:

1) The objective power (variable at 5, 10, 20, 50, 100, and 150x).

2) The eyepiece power (fixed at 10x).

3) The Polyvar magni-changer setting (variable at 0.8, 1, 1.25, and 2x).

4) The Coolpix 4500 optical zoom setting (variable over a range of f-numbers).

The total magnification of a multi-lens system is calculated by multiplying the powers of all component lenses, and the various combinations of factors 1,3, and 4 above lead to hundreds of possible magnifications.  However, the microscope optics far outperform the camera optics, and the best images are obtained at the lowest camera zoom settings.  The camera zoom should be set at the lowest possible setting that eliminates vignetting.  For my Polyvar microscope + Nikon Coolpix camera system, this setting corresponds to a camera f-number of 3.5.

Range of Useful Magnifications.  Every microscope has a range of useful magnifications.  Magnifications that fall outside of this range result in the enlargement of images without a corresponding increase in spatial resolution (this is called empty magnification).  The objective power is the most critical factor in defining this range, and the minimum and maximum magnifications necessary for resolving detail are given by:

Minimum Magnification = 500 x N.A.

and

Maximum Magnification = 1000 x N.A.

where N.A. represents the numerical aperture of the objective lens.  The optical properties of the Polyvar objectives are:

5x

NA=0.1

10x

NA=0.2

20x

NA=0.4

50x

NA=0.8

100x

NA=0.95

150x

NA=0.95

It follows that the Polyvar microscope has a range of useful magnifications of 50 - 950x.  Higher magnifications may be useful for enlarging features of interest but do not contribute toward resolving power.

Field of View.  Many researchers publish micrographs along with with a footnote reporting the total magnification.  Unfortunately, this method will lead to confusion if the image size is reduced or enlarged during any stage of the publication process.  A much better approach is to report the dimensions of the field of view (FOV) because this characteristic is independent of image's final print size.

For my Polyvar + Coolpix system, the FOV at various objective and magni-changer settings were measured using a reference micrometer:

Objective Setting (O) Magni-Changer Setting (M) FOV (Microns)
 

5

 

 0.8 2750
1 2200
1.25 1760
(2) (1100)
 

10

 

 0.8 1375
1 1100
1.25 880
(2) (550)
 

20

 

 0.8 688
1 550
1.25 440
(2) (275)
 

50

 

 0.8 275
1 220
1.25 176
(2) (110)
100 0.8 137.5
1 110
(1.25) (88)
(2) (55)
150 0.8 92
(1) (73)
(1.25) (59)
(2) (37)

The FOV of each micrograph is recorded in the format [O,M] where O is the objective magnification and M is the magni-changer setting .  Mathematically, the FOV in microns is given by, FOV = 11,000/(O · M).

In the table above, several field of view values repeat for various magni-changer + objective settings.  For example, settings [5,2] and [10,1] both result in a FOV of 1100 microns.  The former setting is preferred because it has a higher resolving power (the numerical aperture of the objective lens far exceeds the numerical aperture of the magni-changer lens).  When such redundancies in FOV occur, the settings of lower resolving power are recorded in parentheses to indicate that these settings should be avoided.  Similarly, several high magnification settings are recorded in parentheses because the resulting magnifications exceed the maximum useful magnification of ~950x.