Objective Lenses
The most important lens on the instrument, and how to read its markings.
The objective is the lens that decides what a microscope can resolve. The eyepiece only enlarges what the objective already delivered; it cannot add detail that was never collected.
Reading the engraving
An objective marked 40× / 0.65 and ∞ / 0.17 is telling you four things: 40× magnification, numerical aperture 0.65, corrected for an infinity-corrected tube, and designed for a 0.17 mm cover glass.
Numerical aperture
Numerical aperture (NA) measures the width of the cone of light the lens can accept. It is the single number that determines resolution:
d = λ / (2 × NA)
With green light near 550 nm and an NA of 0.65, the smallest resolvable separation is about 0.42 micrometres. Raising magnification without raising NA cannot improve that.
Standard set
| Objective | Typical NA | Working distance | Used for |
|---|---|---|---|
| 4× scanning | 0.10 | ~ 20 mm | Finding the specimen |
| 10× low power | 0.25 | ~ 7 mm | General survey |
| 40× high dry | 0.65 | ~ 0.6 mm | Cell detail |
| 100× oil | 1.25 | ~ 0.15 mm | Bacteria, fine structure |
Why oil
NA cannot exceed the refractive index of the medium between specimen and lens. Air caps it near 0.95. Immersion oil has a refractive index around 1.515, matching the glass, so steeply angled rays that would refract away in air are captured instead. That is the whole reason a 100× objective needs oil — without it the image is dim and poorly resolved, not merely smaller.
Working distance and parfocality
Working distance shrinks sharply as magnification rises, which is why coarse focus is forbidden at high power: the lens reaches the slide before the image reaches focus. Most sets are parfocal, staying roughly in focus when you change objective, and parcentred, keeping the same field centred.