Written and maintained by CASRAI Editorial Board
Last updated
This page describes the optical design of a laboratory instrument and general specifications. A compound microscope forms an image in two stages: an objective lens collects light from the specimen and builds a magnified intermediate image, and an eyepiece (or camera) magnifies that image again. This page follows the light path through a modern infinity-corrected instrument, explains why objective magnification is a ratio of focal lengths, and tabulates the numerical aperture (NA) values that different objective classes reach.
Who uses it: research and teaching laboratories, core imaging facilities, and technicians who examine slides, cultures and mounted specimens by transmitted light.
What a compound microscope is
A compound microscope uses two lens systems in series. The objective lens sits close to the specimen and produces a real, magnified intermediate image; the eyepiece magnifies that intermediate image for the eye. Its purpose is to resolve specimen detail that cannot be seen unaided. For a wider survey of microscope types, see what a microscope is and how research types differ.
How the light path is built
In an infinity-corrected design, which is how current research microscopes are built, the path runs in this order:
- Specimen. It sits near the front focal plane of the objective.
- Objective. It outputs the light from each specimen point as parallel bundles, described by Nikon’s MicroscopyU as “a flux of parallel light wavetrains imaged at infinity”.
- Infinity space. This is the region of parallel light between the objective and the tube lens. The term does not mean an infinite space exists inside the microscope.
- Tube lens. It focuses the parallel light to form the image at the intermediate image plane.
- Eyepiece. It magnifies the intermediate image.
Why objective magnification is a focal-length ratio
In an infinity system, objective magnification is calculated by dividing the reference focal length (the tube lens focal length) by the focal length of the objective. A shorter objective focal length therefore gives a higher magnification for the same tube lens.
Types and variants
Tube lens focal lengths differ by manufacturer
The reference focal length is not standard across makers, so objectives are not freely interchangeable between brands.
| Manufacturer | Tube lens focal length | Key difference |
|---|---|---|
| Leica | 200 mm | Same reference length as Nikon |
| Nikon | 200 mm | Same reference length as Leica |
| Olympus | 180 mm | Shorter than Nikon and Leica |
| Zeiss | 165 mm | Shortest of the four |
Source: Nikon MicroscopyU, “Infinity Optical Systems”, Table 1. The same page states that tube lengths between 200 and 250 millimeters are considered optimal.
Numerical aperture: the specification that matters most
Numerical aperture is defined as n × sin(a), where n is the refractive index of the medium between the objective front lens and the specimen and a is one-half of the objective’s angular aperture. Because NA includes the refractive index, the imaging medium (air or immersion oil) sets a ceiling on it. Nikon states that it is difficult to achieve NA values above 0.95 with dry objectives, and that most oil immersion objectives have a maximum NA of 1.4.
| Objective | Plan Achromat | Plan Fluorite | Plan Apochromat |
|---|---|---|---|
| 4× | 0.10 | 0.13 | 0.20 |
| 10× | 0.25 | 0.30 | 0.45 |
| 20× | 0.40 | 0.50 | 0.75 |
| 40× (dry) | 0.65 | 0.75 | 0.95 |
| 60× (dry) | 0.75 | 0.85 | 0.95 |
| 100× (oil) | 1.25 | 1.30 | 1.40 |
Source: Nikon MicroscopyU, “Numerical Aperture”, Table 1 (typical values). For how NA, correction class and immersion drive objective choice, and how NA relates to resolution, read microscope objective selection and the optical microscopy resolution limit.
How to choose
Two points follow directly from the sourced material above. First, check the tube lens focal length of your microscope’s manufacturer before pairing an objective from another maker, because the reference lengths are 200, 180 and 165 mm depending on the brand. Second, compare objectives by NA as well as by the magnification printed on the barrel; at the same magnification, the table shows NA rising with the correction class (Plan Achromat to Plan Fluorite to Plan Apochromat).
Common mistakes
- Treating magnification as the only specification. Two 40× dry objectives in the table above differ in NA from 0.65 to 0.95.
- Mixing makers’ objectives and tube lenses. A 165, 180 or 200 mm reference length changes the magnification calculation, so an objective is matched to its maker’s design.
- Expecting a dry objective to reach oil-immersion NA. Dry objectives are difficult to push above 0.95, while oil objectives reach about 1.4.
- Reading “infinity” literally. Infinity space means parallel light between the objective and tube lens, not extra physical room.
FAQ
What are the optical parts of a compound microscope?
The objective lens, the tube lens (in infinity-corrected designs), and the eyepiece. The objective and tube lens form the intermediate image; the eyepiece magnifies it.
What is infinity space?
It is the region of parallel light between the objective and the tube lens in an infinity-corrected microscope. It does not mean an infinite space is available inside the instrument.
What does numerical aperture mean?
NA = n × sin(a), where n is the refractive index of the medium between objective and specimen and a is half the angular aperture. Dry objectives are hard to push above 0.95; oil immersion objectives are typically at most 1.4.
Can I use any manufacturer’s objective on any microscope?
Not reliably. Tube lens focal lengths are 200 mm (Leica, Nikon), 180 mm (Olympus) and 165 mm (Zeiss), so objectives are designed around their maker’s reference length.
Sources
- Nikon MicroscopyU, “Numerical Aperture”: https://www.microscopyu.com/microscopy-basics/numerical-aperture (checked 2026-10-29).
- Nikon MicroscopyU, “Infinity Optical Systems”: https://www.microscopyu.com/microscopy-basics/infinity-optical-systems (checked 2026-10-29).
