The objective lenses on most compound microscopes typically come in 4x, 10x, 40x, and 100x magnifications. Some microscopes may also have a 60x or 150x objective.
Low Power Objective Lenses (4x and 10x)
The lowest power objectives are the 4x and 10x lenses.
The 4x scanning objective has a numerical aperture of 0.10 and a focal length of 16mm. It has a field of view of 5mm across.
The 10x objective lens typically has a numerical aperture of 0.25 and a focal length of 4mm. The field of view at 10x is around 2mm across.
These lower magnification objectives provide a wide field of view which is useful for scanning slides and locating specimens. Resolution and fine detail are low, but the overview of the sample is clear.
Medium Power Objective Lenses (40x)
The 40x objective is considered a medium power lens. It has a numerical aperture of 0.65 and a focal length of 0.65mm.
The field of view at 40x is 0.5mm across – much smaller than the lower power lenses but still enough to see most cell details. 40x is commonly used for studying cell structures.
High Power Oil Immersion Objective Lenses (100x)
The 100x oil immersion objective lens provides the highest magnification in most microscopes.
It has a very high 1.25 numerical aperture and a short focal length of 0.2mm. At 100x the field of view is only 0.2mm wide.
To achieve the full NA of 1.25, immersion oil is placed between the slide and the 100x objective. This prevents refraction and loss of resolution from the air-glass interface.
The high NA paired with oil immersion allows the 100x objective to see fine subcellular details like organelles and chromosomes. It is used to study cellular structure and mitosis.
Special Grade Objective Lenses
Some microscopes may also have specialized oil immersion objectives like 60x or 150x. Like the 100x lens, these objectives require immersion oil to reach their maximum NA and magnifying power. Immersion oils are specially formulated to have specific refractive indexes matched to the lens.
Plan apochromat objectives are high-quality optimized lenses that reduce optical aberrations and provide flat, sharp images. Fluorite lens elements allow a wider spectrum of light throughput. These advanced objectives paired with oil immersion provide the clearest high-magnification views.
| Objective Magnification | Numerical Aperture | Focal Length | Field of View | Use |
|---|---|---|---|---|
| 4x | 0.10 | 16mm | 5mm | Scanning slide, low power overview |
| 10x | 0.25 | 4mm | 2mm | Low power scanning and overview |
| 40x | 0.65 | 0.65mm | 0.5mm | Medium power, cell structure |
| 60x | 0.80 | 0.4mm | 0.3mm | Medium high power, fine cellular details |
| 100x Oil | 1.25 | 0.2mm | 0.2mm | High power with oil, subcellular details |
| 150x Oil | 1.25 | 0.13mm | 0.15mm | Very high power with oil, subcellular fine structure |
Total Magnification Equation
The total magnification of the virtual image seen when using a compound microscope is determined by:
Total Magnification = Ocular Lens Magnification x Objective Lens Magnification
Most compound microscopes have ocular lenses with 10x magnification.
So the total magnifications when using a 10x ocular (eye-piece) lens for different objectives are:
- 4x objective = 10x x 4x = 40x total magnification
- 10x objective = 10x x 10x = 100x total magnification
- 40x objective = 10x x 40x = 400x total magnification
- 100x oil immersion objective = 10x x 100x = 1000x total magnification
Higher magnification oculars like 15x or 20x are sometimes used, which further increase the overall magnification.
How to Choose the Correct Objective Lens?
The optimal magnification to use depends on the specimen and details you want to observe.
Lower magnification gives a wide field of view to find and identify organisms. Medium magnifications like 400x are ideal for studying cell structure. 1000x is used for finer details like organelles.
Start with the lowest power 4x or 10x objective to locate the specimen. Then increase magnification to observe specific structures. Use the 100x oil immersion lens for the finest details.
Proper illumination and slide preparation are also critical for resolving specimen details at high magnifications.