What Is The Eyepiece Power Of Your Microscope?

The eyepiece, or ocular lens, is the part of the microscope that magnifies the image produced by the microscope's objective so that it can be seen by the human eye. When you want to know how much the eyepiece of a microscope magnifies a specimen, you can check each eyepiece for a number followed by an x. For example, if you see 10x written on the eyepiece, it provides 10x magnification. Most eyepieces are 10x or 16x, which provides plenty of power for optical microscopes. However, 10x is not the total power of the microscope. To know the total power of a microscope, multiply the eyepiece power by the objective power.

How to Calculate Total Magnification

To find the total magnification, multiply the power of the objective lens by the power of the eyepiece. The total magnification of a microscope equals the magnification of the eyepiece times the magnification of the objective.

1. Identify the magnification of the objective lens (e.g., 4x, 10x, 40x, or 100x).
2. Identify the magnification of the eyepiece (usually 10x, but may vary).
3. Multiply the two values together to get the total magnification.

Example: a 40x objective lens × 10x eyepiece = 400x total magnification. For a biological microscope with 10x eyepieces and a 40x objective, the total magnification is 400x.

Eyepiece Magnification Range and Empty Magnification

At one time, eyepieces were available in a wide spectrum of magnifications ranging from 6.3x to 25x and sometimes even higher for special applications. These eyepieces are very useful for observation and photomicrography with low-power objectives. Unfortunately, with higher power objectives, the problem of empty magnification becomes important when using very high magnification eyepieces, and these should be avoided. Today most manufacturers restrict their eyepiece offerings to those in the 10x to 20x range.

• Empty magnification occurs when the image continues to be enlarged, but no additional detail is resolved.
• The maximum useful magnification of an image is usually set at 1,000 times the numerical aperture (1000 × NA).
• The minimum magnification necessary for the detail present in an image to be resolved is usually set as 500 times the numerical aperture (500 × NA).
• Magnifications higher than the maximum useful value will yield no further useful information or finer resolution of image detail, and will usually lead to image degradation.
• For example, pairing 20x eyepieces with a 100x objective would not provide good resolution and would result in empty magnification: 1.25 NA × 1000 = 1250 magnification maximum, but 100x objective × 20x eyepieces = 2000, which is above the maximum magnification.

Eyepiece Field Number and Field of View

The diameter of the viewfield in an eyepiece is expressed as a field-of-view number or field number (FN). The field number (FN) is the diameter of the range that can be seen through the eyepiece, written on the side of the eyepiece usually followed by mm for millimeters. The final area depends on the objective lenses as well, but the field number lets you know the capability of the eyepiece itself.

Viewfield Diameter Formula

Viewfield diameter = FN / (M(o) × M(t)), where FN is the field number in millimeters, M(o) is the objective magnification, and M(t) is the tube lens magnification factor (if any).

Field of View Calculation

The FOV = field number / magnification of the objective. For example: 18mm field number with a 10x objective → 18/10 = 1.8mm = 1800µm FOV. A 20mm field number with a 40x objective → 20/40 = 0.5mm = 500µm FOV.

Higher Field Number

The higher the number, the wider the field of view for that eyepiece. If you need a wider field of view, you can switch it out for a wider eyepiece with a higher field number.

Types of Eyepieces

There are two major types of eyepieces grouped according to lens and diaphragm arrangement: negative eyepieces (or Huygenian eyepieces) with an internal diaphragm and positive eyepieces (or Ramsden eyepieces) that have a diaphragm below the lenses of the eyepiece.

• Huygenian eyepiece: The simplest negative eyepiece design, found on most teaching and laboratory microscopes fitted with achromatic objectives. If an unknown eyepiece has only the magnification inscribed on the housing, it is most likely a Huygenian eyepiece and is best suited for use with achromatic objectives of 5x to 40x magnification.
• Ramsden eyepiece: A positive eyepiece with a diaphragm below its lenses. The front focal plane of this eyepiece lies just below the field lens, at the level of the eyepiece diaphragm, making this eyepiece readily adaptable for mounting reticles.
• Kellner eyepiece: A modified version of the Ramsden eyepiece. These improved eyepieces contain a doublet of eye-lens elements cemented together and feature a higher eyepoint than either the Ramsden or Huygenian eyepiece, as well as a much larger field of view.
• Compensating eyepieces: Used to fix chromatic difference of magnification in the intermediate image, especially when combined with high magnification achromatic objectives or fluorite or apochromatic objectives. They must be used at all magnifications with fluorite, apochromatic, and all variations of plan objectives.
• Widefield eyepieces: Modern manufacturers produce widefield eyepieces that increase the viewable area of the specimen by as much as 40 percent.

Eyepiece Inscriptions and Designations

Inscriptions on the side of the eyepiece describe its characteristics and functions. The eyepiece magnification is indicated on the housing, such as 10x. The inscription A/24 indicates the field number is 24, which refers to the diameter (in millimeters) of the fixed diaphragm in the eyepiece.

• WF — widefield
• UWF or UW — ultra-widefield
• SW and SWF — super widefield
• HE or H — high eyepoint, enables microscopists to wear glasses while viewing samples
• CF — eyepieces intended for use with CF corrected objectives
• K, C, or Comp — compensating eyepieces, often also inscribed with the magnification
• Plan-Comp — eyepieces used with flatfield objectives

High Eyepoint Eyepieces and Eye Relief

Light rays emanating from the eyepiece intersect at the exit pupil or eyepoint, often referred to as the Ramsden disk, where the pupil of the microscopist's eye should be placed in order to see the entire field of view (usually 8–10 mm from the eye lens). By increasing the magnification of the eyepiece, the eyepoint is drawn closer to the upper surface of the eye lens, making it much more difficult for the microscopist to use, especially if they are wearing eyeglasses.

To compensate for this issue, manufacturers have designed high eyepoint eyepieces that feature eyepoint distances approaching 20–25 mm above the surface of the eye lens. These improved eyepieces have larger diameter eye lenses that contain more optical elements and usually feature improved flatness of field. Eye relief is the ideal distance from the outside surface of the eyepiece to your eye, so your image is maximized in size. If you wear glasses and you don't want to remove your glasses when viewing through the microscope, it's helpful to have a microscope eyepiece with long eye relief. High eyepoint eyepieces are especially useful for microscopists who wear eyeglasses to correct for near or far sightedness, but they do not correct for several other visual defects, such as astigmatism. Today, high eyepoint eyepieces are very popular, even with people who do not wear eyeglasses, because the large eye clearance reduces fatigue and makes viewing images through the microscope much more comfortable.

How to Choose the Right Eyepiece

The recommendation is to carefully choose the objective first, then purchase an eyepiece designed to work with the objective. When choosing eyepieces, it is relatively easy to differentiate between simple and more highly compensated eyepieces. Simple eyepieces such as the Ramsden and Huygenian (and their more highly corrected counterparts) will have a blue ring around the edge of the eyepiece diaphragm when viewed through the microscope or held up to a light source. In contrast, more highly corrected compensating eyepieces will have a yellow-red-orange ring around the diaphragm under the same circumstances. Because eyepiece-objective correction techniques vary from manufacturer to manufacturer, it is important to use only the eyepieces recommended by a specific manufacturer for use with their objectives.