The Eyepiece (Ocular Lens): Function, Diopter and Pointers

What the eyepiece does

The eyepiece, also called the ocular lens, is the lens at the top of the microscope that you look into. Its job is to take the magnified image formed by the objective and enlarge it further so your eye can see it comfortably.

Inside the microscope, the objective projects a real, magnified image into the tube, just below the eyepiece. The eyepiece works much like a magnifying glass held over that image. Most eyepieces magnify by 10x, so total magnification is simply the objective’s power multiplied by 10. A 40x objective under a 10x eyepiece gives 400x.

The eyepiece is the last optical part light passes through before reaching your eye. If you want to see how it fits into the rest of the instrument, see our guide to the parts of a compound microscope. For a quick definition of the term itself, see what “ocular lens” means.

Objective vs ocular lens: how the work is divided

The two lenses both magnify, but they play very different roles.

The key point is the “sets resolution” row. The objective decides how much fine detail exists in the image. The eyepiece can only make that detail bigger. If two tiny structures blur together in the objective’s image, no eyepiece will separate them. That’s why upgrading objectives usually does more for image quality than upgrading eyepieces. More on objectives in our objective lens guide.

Reading eyepiece markings

Common markings include:

• 10x, 15x, 20x: the eyepiece magnification.
• WF: wide field. The eyepiece shows a wider area than an older narrow-field design of the same power.
• A number after a slash, such as WF10x/18 or 10x/22: the field number (FN), in millimeters. This is the diameter of the eyepiece’s internal field stop. Divide it by the objective magnification to estimate the diameter of specimen you can see. With an FN of 18 and a 10x objective, the field is about 1.8 mm across. More on this in our guide to microscope field of view.
• An eyeglass symbol, or “H” or “HE”: high eyepoint. The eye can sit farther from the lens, which helps glasses wearers.
• A small adjustment ring with a scale: a diopter-adjustable eyepiece.

Eyepiece designs you’ll come across

Eyepieces contain at least two lens groups: an eye lens at the top and a field lens lower down, with a fixed field stop (a ring that defines the edge of the visible circle) in between or below. Designs you might see named include:

• Huygens: an old, simple design found on older and very cheap microscopes. Adequate at modest power, with a narrow field and short eye relief.
• Ramsden and Kellner types: improved designs with better edge correction.
• Wide-field (WF): the common modern type, usually with more elements for a larger, flatter field.
• Compensating eyepieces: designed for certain older finite-system objectives whose color correction was split between objective and eyepiece. Mixing these with the wrong objectives can add color fringes.

Eye relief, exit pupil and wearing glasses

If you hold your eye a short distance above an eyepiece, you’ll find a spot where the whole field appears at once. That spot is the exit pupil (also called the eyepoint), and the distance from the lens to it is the eye relief.

Standard eyepieces often have quite short eye relief, so glasses wearers may not be able to see the whole field with their glasses on. Two options:

• Use high-eyepoint eyepieces, which allow viewing with glasses.
• Remove your glasses and use the focus and diopter adjustments instead. This works well for nearsightedness and farsightedness. It doesn’t correct astigmatism, so people with significant astigmatism often prefer to keep their glasses on with high-eyepoint eyepieces.

Rubber eyecups can help block stray light and position your eye consistently, but fold them down if you wear glasses.

The diopter adjustment

Few people’s eyes are exactly matched. On a binocular microscope, if you focus for one eye, the other may see a slightly soft image, which leads to eye strain and headaches. The diopter adjustment fixes this.

Usually one eyepiece tube (often the left) has an adjustable ring, while the other is fixed. Some microscopes have adjustable rings on both eyepieces. The ring typically covers a range of about plus or minus 5 diopters, though this varies by model.

How to set the diopter

1. Start at zero. Turn any diopter ring to its zero or center mark.
2. Set the eye spacing (see interpupillary distance below) so you see one circular field.
3. Focus with the fixed eyepiece. Close the eye on the adjustable side. Looking through the fixed eyepiece, bring a specimen into sharp focus with the normal focus knobs. Use a flat, detailed specimen at medium power, such as 10x or 40x.
4. Adjust the other eye with the ring only. Close the first eye and open the other. Without touching the focus knobs, turn the diopter ring until the image is sharp.
5. Check with both eyes. The image should merge into one sharp, comfortable view.

If both eyepieces are adjustable, set one to zero and treat it as the fixed one. If your microscope has a reticle or pointer in one eyepiece, focus that eyepiece’s ring on the reticle first, then focus the specimen.

Note your diopter setting. If you share a microscope, each person can reset it quickly.

Interpupillary distance

Binocular heads let you change the spacing between the eyepieces to match the distance between your pupils, usually by pushing the tubes together or apart or by tilting them on a hinge. Adjust until the two circles merge into one. Many heads have a scale so you can return to your setting.

Pointers and reticles

A pointer is a thin needle or wire mounted inside an eyepiece. It appears as a sharp black line over the specimen, which makes it useful for teaching, for showing someone exactly which cell you mean, or as a reference mark.

The pointer appears sharp because it sits at the eyepiece’s field stop. That’s the same plane where the objective forms its intermediate image, so the pointer and specimen come into focus together. Rotating the eyepiece swings the pointer around the field.

The same plane is used for reticles (graticules): small glass discs etched with scales or grids for measuring specimens. Reticles need calibrating against a stage micrometer for each objective; see how to measure specimens with an eyepiece reticle.

To install a pointer or reticle, unscrew the bottom of a compatible eyepiece, drop the part onto the field stop shelf with any engraving facing the correct way per the instructions, and secure the retaining ring. Work in a dust-free space and avoid touching the glass.

Eyepiece tube sizes and swapping eyepieces

Eyepieces slide into the eyepiece tube, so a replacement must match the tube’s inner diameter. Common sizes include 23.2 mm (very common on student and many lab microscopes), 30 mm and 30.5 mm. Measure the barrel of your existing eyepiece before buying.

Keep both eyepieces on a binocular head matched in power and type.

Higher-power eyepieces and empty magnification

It’s tempting to fit a 20x or 25x eyepiece to get more magnification. Up to a point this helps, but beyond about 1,000 times the objective’s NA, extra magnification usually just makes the image bigger and blurrier without adding detail. This is called empty magnification. For a 40x/0.65 objective, that limit works out to roughly 650x total, so a 15x eyepiece (600x) is about the useful maximum. Higher-power eyepieces also tend to have narrower fields and shorter eye relief.

Keeping eyepieces clean

Eyepieces collect eyelash oil and dust more than any other part. If you see specks that rotate when you turn the eyepiece, the dirt is on the eyepiece.

• Blow off loose dust first with a hand blower.
• Wipe gently with lens tissue or a clean microfiber cloth, using a small amount of lens-cleaning fluid if needed.
• Avoid household glass cleaners and rough tissues.
• Leave eyepieces in place, or cap the tubes, so dust can’t fall onto the prisms below.