The short answer
The ocular lens is the eyepiece of a microscope: the lens at the top that you put your eye to. It magnifies the image produced by the objective lens, usually by 10x, so you can see it comfortably. “Ocular lens,” “ocular” and “eyepiece” all mean the same part.
Where the word “ocular” comes from
“Ocular” comes from the Latin oculus, meaning “eye.” As an adjective it means “relating to the eye.” In optics, the noun “ocular” came to mean the lens or lens group nearest the eye in an instrument such as a microscope or telescope. So “ocular lens” simply means “eye lens,” the counterpart of the objective lens, which sits nearest the object.
What the ocular lens does
In a compound microscope, the objective forms a magnified image inside the microscope tube. The ocular lens then acts like a magnifying glass on that image, enlarging it further and presenting it to your eye.
To find total magnification, multiply the two:
- 10x ocular × 4x objective = 40x
- 10x ocular × 10x objective = 100x
- 10x ocular × 40x objective = 400x
- 10x ocular × 100x objective = 1,000x
Inside, an ocular usually contains at least two lens groups, an eye lens at the top and a field lens lower down, plus a fixed ring called the field stop that sets the edge of the circle you see. Binocular microscopes have two oculars, often with a diopter ring to correct for differences between your eyes. Our full guide to the eyepiece explains how to set it.
Ocular lens vs objective lens
| Ocular lens | Objective lens | |
|---|---|---|
| Where it is | Top of the microscope, at your eye | Bottom of the tube, just above the slide |
| How many | One or two (monocular or binocular) | Usually three or four on a nosepiece |
| Typical power | 10x | 4x, 10x, 40x, 100x |
| Main job | Enlarges the objective’s image for viewing | Forms the first image and sets how much detail is visible |
The important difference is that the ocular lens cannot add detail. It only enlarges what the objective has already resolved. Fitting a stronger ocular makes the image bigger, but past a certain point it won’t make it any sharper (this is called empty magnification). That’s why the objective is considered the more important lens; see our guide to objective lenses.
What the numbers on an ocular mean
You’ll often see an engraving like WF10x/18:
- WF: wide field.
- 10x: the ocular’s magnification.
- 18: the field number in millimeters. Divide it by the objective’s magnification to estimate how wide an area of the specimen you can see. With a 10x objective, that’s about 1.8 mm.
An eyeglass symbol means a high-eyepoint ocular, which is easier to use with glasses on.
Other “ocular” terms you’ll see
- Ocular tube: the tube the ocular slides into.
- Ocular micrometer (reticle or graticule): a small glass disc with a scale, placed inside the ocular to measure specimens.
- Ocular pointer: a fine needle inside the ocular for pointing at features.
- Monocular / binocular: one ocular or two.
For how the ocular fits in with every other part of the instrument, see our guide to the parts of a compound microscope.