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| Numerical Aperture of Objectives |
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The numerical aperture of objectives increases with the magnification, up to about the 40x objective. For example, the Plan-Neofluar objectives with the magnifications 5x, 10x and 20x have the numerical apertures 0.15, 0.30 and 0.50. The theoretical limit in air is a numerical aperture of 1.0. This would be identical to a full 180° aperture angle (2α) of the objective. In practice, it is possible to obtain a numerical aperture of 0.95, which corresponds to an angle (2 α) of more than 140°. Very high apertures at low magnifications are difficult to implement because of the large object fields and lens diameters.
The table shows the values for the resolution resulting from the calculation for some objectives. The distance d0 is referred to the specimen and, when multiplied by the magnification, results in the point distance D0 in the intermediate image (for green light &lamda; = 550 nm). Finally, the number n represents the number of resolved pixels if they are “lined up” along the field diameter of 20 mm (20mm / D0).
| Objektiv/NA | d0 (µm) | D0 (µm) | n |
| 5x/0,15 | 2,2 | 11,2 | 1786 |
| 10x/0,30 | 1,1 | 11,2 | 1786 |
| 20x/0,50 | 0,7 | 13,4 | 1493 |
| 40x/0,75 | 0,45 | 17,9 | 1117 |
| 40x/1,30 Oil | 0,26 | 10,3 | 1942 |
| 63x/1,40 Oil | 0,24 | 15,1 | 1325 |
| 100x/1,30 Oil | 0,26 | 25,8 | 775 |
The pictures show the difference in the resolving power between the objectives 40x/0.75 (on the left) and 40x/1.30 Oil (on the right). Scarcely any difference can be recognized in the printed pictures taken with a low magnification (at the bottom), while the image sections taken with a 8x magnification below) exhibit marked differences. | |
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