The numerical aperture of a microscope, usually marked as “NA” on the objective lens with values such as 0.65, 0.85 and 1.25, is defined by the following formula:
NA=n×sinθ
n：Refractive index of the medium between the sample and the optical component (n≈1 for air, n≈1.33 for water, n≈1.515 for cedar wood oil)
θ：Half-aperture angle (half of the aperture angle), referring to the angle between the outermost ray emitted from the object point on the optical axis that can enter the optical system and the optical axis.
A larger numerical aperture (NA) brings more significant advantages to the overall performance of an optical system. First, it enables a stronger light-collecting capability, capturing light rays over a wider angular range, which makes the imaging brighter and effectively improves the signal-to-noise ratio of the image. Clear images can thus be obtained even in low-light conditions. Second, NA directly determines the resolution; a larger value allows for the differentiation of smaller fine structures, revealing more delicate details of the sample and avoiding ineffective magnification. In microscopes and other optical devices, a high NA achieves a higher effective magnification and more precise focusing. For optical fibers, a large NA means a wider light-receiving range and more efficient optical signal coupling. However, a high NA is accompanied by a shallow depth of field. Overall, NA is a core indicator for measuring the resolution and light-collecting capability of an optical system.
Nevertheless, a larger numerical aperture is not always preferable. A higher NA results in higher resolution, greater light intake and sharper imaging, but it leads to a shallow depth of field—only a very thin layer of the sample will be in sharp focus, and slight deviations from the focal plane will cause blurring. Meanwhile, a higher NA demands stricter precision in processing and assembly, leading to higher costs, and aberration control also becomes more challenging. Therefore, the optimal choice is a suitable and sufficient NA value that meets the requirements for resolution and brightness.
