# Getting good results with the 100x oil-immersion objective

> The 100x objective shows the finest detail your microscope can resolve, but only with the right oil, the right order of steps and careful cleaning afterwards.

Canonical: https://unilab-cno.pages.dev/blog/oil-immersion-100x-objective/
Last updated: 2026-09-05

Technique · By the Unilab Microscopes team

Bacteria, blood cell inclusions and fine nuclear detail are usually examined with the 100x objective. It is also the objective most often damaged or left dirty. Used correctly, it is straightforward.

## Why it needs oil

When light leaves a glass slide and passes into air, it bends. At high magnification much of that light misses the objective, and the image loses brightness and detail. Immersion oil has almost the same refractive index as glass, so with a drop of oil between slide and lens the light travels as if through one continuous piece of glass. This is what lets a 100x objective reach a numerical aperture of around 1.25, and resolve detail that a dry objective cannot.

Use proper microscope immersion oil. Cedar oil, glycerine or water will not give the same result and can harm the lens.

## Step by step

1. **Find and focus the area with a lower objective.** Start at 10x, then 40x. Centre the exact area you want to examine.
2. **Swing the objective halfway.** Rotate the nosepiece so no objective is over the slide.
3. **Place one small drop of oil** on the slide, over the illuminated spot. Avoid air bubbles.
4. **Rotate the 100x objective into place.** It will dip into the oil. Do not touch the coarse focus.
5. **Focus with the fine focus only.** The image should be close to focus already. A small adjustment is enough.
6. **Open the condenser iris and raise the condenser** close to the slide, and increase the brightness. High magnification needs more light.

## Common mistakes

- **Moving back to the 40x through the oil.** The 40x is a dry objective. If it touches the oil, clean it immediately. Many 40x and 100x objectives are spring-loaded to protect the slide and the lens, but they are not designed to sit in oil.
- **Using the coarse focus at 100x.** The working distance is a fraction of a millimetre. Coarse focusing can drive the lens into the slide.
- **Too much oil.** One drop is enough. Excess oil runs onto the stage and into the mechanism.

## Cleaning afterwards

Wipe the oil off the 100x objective with clean lens tissue as soon as you finish, not at the end of the day. Dried oil becomes sticky and attracts dust. Wipe the slide too, and check that no oil has reached the stage or the condenser. A small amount of lens cleaning solution on fresh tissue removes any film that remains.

Done this way, a 100x objective stays clear and sharp for the life of the microscope.

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