Considerations when selecting a loupe. |
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The size of the lens has no influence on the size of the field of view.
The size of the field of view depends on the magnification. The stronger the magnification the smaller the field of view. Also the depth of view will get smaller. 10X is a standard for diamond grading. With a loupe you can go up to 15X, 20X or even 28X magnification, then you will reach a technical limit as the focus point is getting to close to the lens.
Loupes with aplanatic and chromatic corrections are called triplets as they are composed of a three lens system. A good loupe requires colorless glass. This is important when color grading diamonds. Place different brands of loups on a white paper and compare the color of the lens. The chromatic correction avoids yellow and blue shadows at the limit of the field of view. The aplanatic correction avoids the spherical deformation of what you see. A straight grid of lines should stay straight.
How to hold a loupe properly.
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