N.A. | N.A.

Key Words: Resolution, brightness, depth of field, working distance

Definition:Numerical aperture provides an indication of the light gathering capacity and resolving power (at a fixed distance) of an objective

TECHNOLOGY:

The N.A. of an objective is printed on the microscope objective together with other objective specifications. It may vary from 0.04 for low power objectives to 1.3 or 1.4 for high power oil-immersion apochromatic objectives.

The N.A. of an objective is defined by:

N.A. = R.I. sin θ

Where R.I. is the refractive index of the medium, for example, air, water or oil, and θ is the half angle of the maximum cone of light that leaves, or enters, the objective.

Increasing the refractive index of the medium between the front lens of the objective and the sample results in a higher working N.A. Objectives are available for water (R.I. = 1.33), glycerine (R.I. 1.47) and immersion oil (R.I. 1.51) although some of these objectives, especially older ones, are generally not interchangeable without incurring imaging artefacts.

An objective with a high N.A. collects more light, provides a brighter image and is able to define finer details (i.e. has a greater resolving power) compared to an objective with a lower N.A. The relationship between N.A. and image intensity is given by:

N.A.⁴ / TM²

Where TM is total magnification.

APPLICATIONS:

Higher N.A. objectives are generally preferred for most imaging applications because they provide better resolution and brighter images. High N.A. objectives tend to have shorter working distances, however, and this may restrict the imaging of thicker specimens and / or use of micromanipulation equipment.

MICROSCOPE CONFIGURATION:

Nikon's CFI60 optical system provides objectives that are readily interchangeable allowing the most appropriate objectives to be selected for each application. This optical system also enables both higher N.A.s and longer working distances.

RECOMMENDED SYSTEM:

The Plan Apochromat 60x, N.A. 1.45 is the highest resolution lens available, while the Plan Fluor 40x, N.A. 1.3 is the brightest. The most flexible objectives currently available are the multi-immersion objectives (Plan 20x MI), which can be used dry, or with water, glycerine or oil. They are often used as a lower magnification companion lens for a high N.A. high-resolution immersion lens, especially in confocal imaging.

LINKS:

Properties of microscope objectives: [microscopyu] 

Introduction to microscope objectives: [microscopyu] 

CFI60 optical system: [microscopyu]