Optical Microscopy of Meteoritic Metal

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FIELD OF VIEW

Micrographs are often published with an accompanying footnote that reports the "total magnification".  This approach leads to confusion because the "total magnification" changes every time the micrograph is reduced or enlarged.  A more practical approach is to report the dimensions of the field of view (FOV), as this quantity is constant regardless of the of image's final print size.

My micrographs were obtained using a Canon T3i coupled to the Polyvar microscopy by way of Wild-Herzburg eyepiece (5x) inserted into a Zeiss camera adapter (0.25x).  This combination of optics yields a magnification of 1.25x for the camera coupling system.

The field of view of the microscope + camera system depends on four factors:

1) The objective magnification (O) (variable at 2.5, 5, 10, 20, 50, 100, and 150x).

2) The Polyvar magni-changer setting (M) (variable at 0.8, 1, 1.25, and 2x, but normally set to 1x).

3) The camera adapter magnification (A) (fixed at 1.25x).

4) The size of the camera CCD sensor (S).

 

The field of view is given by the relationship, FOV = S/(OMA).

 

The Canon T3i utilizes an APS-C sensor with a horizontal dimension of 22.3mm and a vertical dimension of 14.9mm.  The resulting FOV values for various microscope settings are reported below; these values have been confirmed using a stage micrometer.  For each micrograph, the objective and magni-changer values are recorded in the format [O,M].  The corresponding FOV values can be read directly from the table below.

 

In many cases, I have added a micrometer bar to the micrograph. This is easily accomplished in ImageJ using the macro function.

 

Objective

Power (O)

Magni-changer

setting (M)

Horizontal FOV

(microns)

Vertical FOV

(microns)

2.5 0.8 8920 5960
5 0.8 4460 2980
10 0.8 2230 1490
20 0.8 1115 745
50 0.8 446 298
100 0.8 223 149
150 0.8 149 99
2.5 1 7136 4768
5 1 3568 2384
10 1 1784 1192
20 1 892 596
50 1 357 238
100 1 178 119
150 1 119 79
2.5 1.25    
5 1.25 2854 1907
10 1.25 1427 954
20 1.25 714 477
50 1.25 285 191
100 1.25 143 95
150 1.25 95 64
2.5 2 3568 2384
5 2 1784 1192
10 2 892 596
20 2 446 298
50 2 178 119
100 2 89 60
150 2 59 40

RANGE OF USEFUL MAGNIFICATIONS

Every microscope has a range of useful magnifications, and higher magnifications produce image enlargement without a corresponding increase in spatial resolution (sometimes called empty magnification).  The objective optics is the most critical factor in defining this range, and the maximum useful magnification is approximately 1000 times the objective's numerical aperture (N.A.). The Polyvar objectives are specified as:

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 maximum useful magnification of ~950x.  Higher magnifications are useful for enlarging features of interest but do not contribute toward resolving power.