Optical Microscopy of Meteoritic Metal
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/(O·M·A).
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.
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:
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.