Lens Errors

Maize. Vascular tissue.
Maize. Vascular tissue.
Maize. Vascular tissue.

Modern microscope optics correct a range of different lens errors or aberrations. Here is a short description of some common lens errors.

It is possible to construct a simple compound microscope using only two lenses: one highly magnifying objective lens and one low magnification ocular (eyepiece) lens. Why then, are modern research and even course microscopes so complex? After all, some modern objectives can contain up to 10 or more individual lens elements.

The simple 2-lens system, although cheap to construct, does have certain drawbacks. The image quality is low and also the color representation is not optimal. Modern microscopes compensate a whole range of limitations and lens errors inherent in simpler systems.

The objective has the most influence in determining the image quality of the microscope. An objective should deliver an image with a high resolution, a high contrast and a low lens error. Naturally it is difficult to achieve all of these goals simultaneously. Several lens elements are necessary to compensate a range of lens errors. This, however, impacts negatively on the image contrast. The reduced contrast must be compensated with an appropriate lens coating. All of these corrective measures naturally increase the cost of the objective.

Numerical Aperture: One of the key values that characterizes the performance of an objective is its numerical aperture. This value is essentially a direct measure of the resolving power of the objective. The higher the numerical aperture, the finer is the visible detail. Objectives with a high numerical aperture are also capable of collecting more light, the image is brighter. Objectives have their numerical aperture engraved on the outside.

Chromatic Aberration: It is in the physical nature of light, that the light waves towards the red end of the spectrum are not refracted as much as the waves towards the blue end. As the white light of the lamp passes through a lens it is split up into different colors. The focal point of the different colors is not the same. This phenomenon is called chromatic aberration. Modern objectives attempt to correct this lens error by coupling of several lens elements. Achromatic objectives are very popular and commonly used in schools. These objectives are optimized to correct two colors of the spectrum. A small amount of chromatic aberration is still visible. The more expensive apochomatic objectives are optimized for three colors. They show no visible chromatic aberration and are frequently employed for photographic documentation.

Spherical aberration: The objectives must also compensate for spherical aberration. Light rays that hit a lens towards the side are more strongly refracted than light rays that hit the lens closer to its optical axis. This effect is also dependent on the wavelength of the light ray. This lens error can also be minimized by a combination of different lens elements.

Field curvature:Cheaper objectives do not produce a flat plane of focus. When the center of the image is in focus, the sides of the image are not in focus, and vice versa. This aberration is due to the fact that lenses have curved surfaces. This is generally not a problem for routine visual observation. It does, however, become very annoying when taking photographs. Flat field objectives correct the field curvature. These objectives are designated with the word „plan“, such as plan achromats, plan apochromats or plan fluorites. [image demonstrating field curvature]. Flat-field objectives, and especially plan apochromats are expensive and an unnecessary luxury for instructional course work.