numerical aperture

optics

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July 1, 2026

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Numerical aperture (NA) is a measure of the light-gathering ability of an optical system, particularly in microscopes. It is calculated by multiplying the sine of half the angle subtended by the aperture at an object point by the refractive index of the medium between the object and the objective lens. A higher numerical aperture indicates a greater ability to collect diffracted light, which directly correlates to the microscope's power to resolve fine details in an object. The NA is crucial for determining the smallest details that can be observed; for instance, an NA of 0.55 is needed to resolve details spaced 1 micrometer apart. While the theoretical maximum NA in air is 1.0, practical limits are around 0.95 for dry objectives. Using immersion objectives can increase the NA to 1.4, allowing for the observation of even finer structures.12

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The theory of image formation
He designated the term numerical aperture (N.A.) as the measure of the objective’s ability to collect diffracted light and thus also of its power to resolve detail. On this basis it is obvious that the greater the magnification of the objective, the greater the required N.A. of the objective.…
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He designated the term numerical aperture (N.A.) as the measure of the objective’s ability to collect diffracted light and thus also of its power to resolve detail. On this basis it is obvious that the greater the magnification of the objective, the greater the required N.A. of the objective.…
Read More