Is a higher or lower numerical aperture better?

Numerical Aperture: Higher vs. Lower

The numerical aperture (NA) of an optical system, such as a microscope objective or a camera lens, is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. It is defined as NA = n sin(θ), where n is the refractive index of the medium in which the lens is working (typically air, for which n=1), and θ is the half-angle of the maximum cone of light that can enter or exit the lens.

Higher Numerical Aperture

A higher numerical aperture indicates a wider range of angles, which means the lens can gather more light and resolve finer details. This is particularly important in applications like microscopy, where capturing detailed images at a high resolution is crucial. A higher NA also results in a shallower depth of field, which can be both an advantage and a disadvantage depending on the application.

Lower Numerical Aperture

A lower numerical aperture, on the other hand, means the lens gathers light over a narrower range of angles. This results in less light being collected, which can lead to lower resolution images. However, a lower NA also means a greater depth of field, making it easier to keep more of the subject in focus at once, which can be beneficial in certain contexts, such as landscape photography.

Conclusion

Whether a higher or lower numerical aperture is better depends on the specific requirements of the application. For tasks requiring high resolution and fine detail, such as in microscopy, a higher NA is preferable. For applications where a greater depth of field is beneficial, a lower NA may be more suitable. Ultimately, the choice of numerical aperture should be guided by the balance between resolution and depth of field that best meets the needs of the application.