Microscope Magnification
Multiply the objective lens by the eyepiece to get the total magnification of a compound microscope.
40× objective × 10× eyepiece = 400× total
Total magnification on a compound microscope is the objective lens magnification multiplied by the eyepiece (ocular) magnification. With the common high-power 40× objective and a standard 10× eyepiece, the total is 40× × 10× = 400×. Swap in the 100× oil-immersion objective and the same eyepiece gives 1000×.
How total magnification works
A compound light microscope magnifies in two stages. The objective lens — mounted on the rotating turret just above the specimen — produces a first magnified image, and the eyepiece (the ocular lens you look through) magnifies that image again. Because the two lenses act in series, their powers multiply rather than add. The eyepiece is almost always 10×, so the total scales directly with whichever objective you rotate into place: 4×, 10×, 40×, or 100×.
Both lenses act in series, so their magnifications multiply.
Worked example: 40× objective, 10× eyepiece
Find the total magnification when viewing a cheek-cell slide under high power:
- 1 Read the objective lens. The high-power dry objective is engraved 40×.
- 2 Read the eyepiece. The ocular lens is marked 10×, the standard value.
- 3 Multiply the two. 40× × 10× — the lenses act in series, so multiply rather than add.
- 4 Read off the total. 40 × 10 = 400, so the specimen is magnified 400× overall.
Total magnification by objective (with a 10× eyepiece)
Standard objective powers on a compound microscope, each paired with the usual 10× ocular.
| Objective | Eyepiece | Total | Typical use |
|---|---|---|---|
| 4× | 10× | 40× | Scanning — locate the specimen |
| 10× | 10× | 100× | Low power — survey the field |
| 40× | 10× | 400× | High power (dry) — cells in detail |
| 100× | 10× | 1000× | Oil immersion — bacteria, fine structure |
Magnification, resolution, and the shrinking field
Magnification is not resolution. Magnification only enlarges the image; resolution is the smallest distance at which two points stay separate. Past the useful limit, more magnification just produces a bigger, blurrier picture — “empty magnification.” The objective’s numerical aperture, not its power, sets how much real detail you can resolve.
Higher power narrows the field of view. As total magnification rises, the visible circle shrinks in proportion: the field-of-view diameter at high power equals the low-power diameter times the ratio of the magnifications. Going from 100× to 400× (a 4× jump) shrinks the field to a quarter of its width, so you see less of the slide and must re-centre your specimen before switching up.
The 100× objective needs oil. The 100× oil-immersion lens is used with a drop of immersion oil between the lens and the coverslip. The oil’s refractive index matches the glass, bending more light into the lens, raising the numerical aperture and giving the sharp, high-resolution image needed at 1000×.