The Quick Answer
To find the total magnification of a compound microscope, multiply the eyepiece magnification by the magnification of the objective you are using. With a 10x eyepiece and a 40x objective, the total is 10 × 40 = 400x. If the microscope has any extra magnification factor in the light path, multiply by that as well.
The steps below show where to find each number on a real microscope. For the theory behind the formula and the biology image-size formula, see our main guide to calculating total magnification.
Step 1: Read the Eyepiece
The eyepiece (ocular) is the lens you look into. Its magnification is usually engraved or printed on the top or side of the barrel.
Typical markings look like this:
- 10x or x10: 10 times magnification.
- WF10x/18: a wide-field 10x eyepiece with an 18 mm field number.
- 15x/12 or WF15x: 15 times magnification.
The number followed by “x” is the magnification. The number after the slash, if any, is the field number, which is used for field of view calculations, not for magnification.
On a binocular microscope, both eyepieces should have the same magnification. Use that single value, not the two added together.
Step 2: Read the Objective in Use
The objectives are the lenses on the rotating nosepiece just above the stage. Only one is in use at a time: the one pointing straight down at the slide, usually clicked into place.
Objective barrels often carry several numbers, for example:
- 4/0.10
- 10/0.25
- 40/0.65 160/0.17
- 100/1.25 Oil
The first number is the magnification. The number after the first slash is the numerical aperture, which affects detail rather than magnification (see our numerical aperture guide). A second pair such as “160/0.17” gives the tube length and coverslip thickness the objective is designed for. An “∞” symbol in place of 160 means it is an infinity-corrected objective.
Many objectives also have a colored ring. Color codes for magnification are often similar across makers, but they are not reliable enough to replace reading the number.
Step 3: Check for Any Extra Magnification
Most student and hobby microscopes have no extra factor, and you can skip this step. Some microscopes do, and missing it gives the wrong answer. Look for:
- A factor marked on the binocular or trinocular head, such as “1.25x” or “1.5x”.
- A magnification changer dial or slider on the body, on some research microscopes.
- Auxiliary lenses or a zoom knob on stereo microscopes (see below).
If you find one, include it in the multiplication.
Step 4: Multiply
Total magnification = eyepiece × objective × any extra factor
Worked Examples
| Eyepiece | Objective | Extra factor | Total |
|---|---|---|---|
| 10x | 4x | none | 40x |
| 10x | 10x | none | 100x |
| 10x | 40x | none | 400x |
| 10x | 100x | none | 1000x |
| 15x | 40x | none | 600x |
| 10x | 40x | 1.25x head | 500x |
| 20x | 100x | none | 2000x |
The last row is a real combination on some microscopes, but it is well beyond what a typical 100x objective can resolve, so it is unlikely to show more detail than 1000x. Our article on empty magnification explains why.
Stereo (Dissecting) Microscopes
Stereo microscopes work a little differently. Many have a zoom knob marked with a range such as 0.7x to 4.5x, and some accept an auxiliary lens screwed under the body, such as 0.5x or 2x.
Total magnification = eyepiece × zoom setting × auxiliary lens
Example: 10x eyepieces, zoom set to 3x, and a 0.5x auxiliary lens give 10 × 3 × 0.5 = 15x. Without the auxiliary lens, the same setting gives 30x.
Fixed-power stereo microscopes often have a turret with two objectives, such as 1x and 3x, or 2x and 4x. Multiply the eyepiece by whichever is in use.
What If a Lens Isn’t Marked?
Some budget or very old microscopes have unmarked lenses. A few options:
- Check the manual or product listing for the eyepiece and objective magnifications.
- Look under the eyepiece rim or on the lower barrel, where markings are sometimes hidden.
- Measure the field of view with a clear ruler or stage micrometer and compare it with what you would expect for common setups. This is a rough check rather than an exact method.
- Use a stage micrometer and calibrated scale if you mainly need real sizes. Measured sizes are often more useful than a magnification number; see how to measure specimens with a reticle and stage micrometer.
A Number to Check Before Trusting the Result
Total magnification tells you how much bigger the image is, not how much detail it contains. As a rough rule, total magnification is most useful up to about 1000 times the objective’s numerical aperture. With a 40x/0.65 objective, that is about 650x; with a 100x/1.25 oil objective, about 1250x.
If your calculated total is well above that, the extra magnification probably isn’t adding detail. For the full picture, see magnification vs resolution.