Numerical Aperture - N.A.

In microscopy Numerical Aperture measures the ability of an objective to gather light. The higher the numerical aperture (NA), the more light can be collected by the lens in the microscope objective. This results in a better resolution and a higher intensity image. A high numerical aperture also allows for greater field of depth and a better overall image.

Numerical Aperture in a high quality 100x Oil Objective is typicaly 1.25.


In Fluorite's that number rises to 1.30.


In Apochromat Objectives the numberical aperture is 1.40

However, don't be fooled. Just because the numerical aperture of a given microscope objective is 1.40, that does not mean it is the same quality as a high quality objective like Nikon or Olympus.

Nikon objectives are some of the highest quality objectives that you will find in the world. Their CFI60 objectives use a 200mm tube length and a wider body (25mm thread pattern) to achieve high numerical apertures and longer working distances. (*The working distance is the distance between the tip of the objective and the specimen.)

Optical Abberations in Microscopy with regards to Objectives.

What is an aberration?

In microscopy we often hear that engineering has been used to correct for "aberrations". To understand this, we must first understand what an aberration is as it relates to optics.

An optical aberration is any disturbance of the rays of a pencil of light such that they can no longer be brought to a sharp focus or form a clear image at the tip of that cone shaped pencil of light. The finer the tip of the cone or pencil, the clearer and sharper the image.

ACHROMAT OBJECTIVES have best optical performance in the center of the view field, and correct effectively for many kinds of aberrations. C (red) and F (blue) chromatic aberrations are removed. As the image approaches the outer diameter the field becomes less flat and the image becomes less than perfect. Achromat objectives are low quality and are typically used by primary schools, where image quality comes second to teaching students how to use microscopes for the first time.

PLAN OBJECTIVES produce flat, sharp images throughout the filed of view from center to edges. Plan objectives are acceptable for clinical use in high quality brands like Nikon and Olympus.

InAPOCHROMAT OBJECTIVES not only C (red) and F (blue) chromatic aberrations, but also G (violet) aberrations are corrected in these objectives to cover the full spectrum of visible light. These top-of-the-line lenses combine a large numerical aperture and ideal correction of aberrations all the way to the edges of the field view.

FLUORITE OBJECTIVES can be used for brightfield, Nomarski DIC, or epi-fluorescence microscopy. These lenses have a particularly high ultraviolet transmittance rate and low autofluorescence, enabling high contrast fluorescence images to be obtained.

Many different engineering methods are used to correct for aberrations of light all across the visible spectrum. In microscopy, depending on how much money you're willing to spend and how precise of an image you need to get in most cases there's an objective available to you that will get the job done. Optical leaders like NIKON, OLYMPUS, and LEICA have put in many years or research and development to be at the forefront of microscopy optics.

Microscope Objectives