The short answer
4x, 10x and 40x are the magnifications of the three most common objective lenses on a compound microscope. Each is used for a different stage of looking at a slide:
| Objective | Common name | Total with 10x eyepiece | Used for |
|---|---|---|---|
| 4x | Scanning objective | 40x | Finding the specimen and getting an overview |
| 10x | Low-power objective | 100x | Looking at tissue layout, larger cells, small organisms |
| 40x | High-power (high-dry) objective | 400x | Individual cells and larger structures inside them |
Higher numbers mean more magnification, but also a smaller visible area, less room between the lens and the slide, and more sensitivity to focus and lighting.
Is 40x the same as 400x?
Not on its own. The “40x” printed on the objective is the objective’s magnification only. Total magnification is the objective multiplied by the eyepiece. With the usual 10x eyepiece, a 40x objective gives 400x; we explain the arithmetic in why total magnification is 400x with a 40x objective. With a 15x eyepiece, it would give 600x.
The 4x objective: scanning
The 4x has the widest field of view, often around 4 to 5 mm across with a standard eyepiece, and the most room between lens and slide. Use it to find the specimen, get it centered, and see its overall shape, such as a whole cross-section of a stem or an entire small insect mount.
Detail is limited, though. You’ll see where the cells are, but not much inside them.
The 10x objective: low power
The 10x shows an area roughly 2 mm across. It’s the workhorse for many specimens: you can see the arrangement of cells in tissue, larger single-celled organisms in pond water, pollen grains and similar. It still has a comfortable gap between lens and slide, typically several millimeters, so focusing is forgiving.
The 40x objective: high power
The 40x shows an area only about half a millimeter across. It reveals individual cells clearly, along with larger structures inside them such as nuclei and, in plant cells, chloroplasts.
It’s also much less forgiving:
- The gap between lens and slide is often under a millimeter, so it’s easy to hit the slide with careless focusing.
- Depth of field is very shallow, so you’ll keep using the fine focus to move through the specimen.
- It needs more light and a properly set condenser and diaphragm.
Many 40x objectives are spring-loaded, so the front retracts if it touches the slide. That’s a backup, not a substitute for care.
What changes as you move up
Typical values for standard achromat objectives with an 18 mm field number eyepiece are shown below. Exact figures vary by maker.
| 4x | 10x | 40x | |
|---|---|---|---|
| Field of view | About 4.5 mm | About 1.8 mm | About 0.45 mm |
| Working distance | Often 15 mm or more | Roughly 5 to 8 mm | Often around 0.5 mm |
| Numerical aperture (typical) | 0.10 | 0.25 | 0.65 |
| Smallest detail resolvable (approx., theoretical) | About 3 µm | About 1.3 µm | About 0.5 µm |
| Depth of field | Deep | Moderate | Very shallow |
| Image brightness | Bright | Medium | Dimmer; needs more light |
The numerical aperture row explains why the 40x shows so much more than just a bigger picture. It collects light over a much wider angle, which lets it resolve finer detail. For more on what these figures mean, see our guide to objective lenses, and for the gap between lens and slide, objective working distance.
Which objective to start with, and how to move up
Always start with the lowest-power objective, usually the 4x:
- Rotate the 4x into place and focus with the coarse knob.
- Center the part you want to examine. Whatever is at the edge at 4x will be out of view at higher power.
- Rotate to the 10x. On a parfocal microscope the image should be nearly in focus, so use only the fine focus.
- Center again, rotate to the 40x, and use only the fine focus. Avoid the coarse knob at this power.
- Open the condenser’s iris diaphragm a little and adjust the light at each step. See how to set the iris diaphragm.
At 400x, a slide moved by hand under stage clips tends to jump across the field. A mechanical stage makes this much easier.
Where the 100x fits in
Many microscopes have a fourth objective, the 100x oil immersion lens, giving 1,000x with a 10x eyepiece. It’s used for very small detail such as bacteria and blood cell structure. It needs a drop of immersion oil between the lens and the slide, and it has an extremely short working distance, often a fraction of a millimeter. Move to it only from a well-focused 40x image, and clean the oil off afterwards. Our guide to using a 100x oil immersion objective covers the steps.
To see how the objectives fit in with the rest of the instrument, see our guide to the parts of a compound microscope.