Compound Microscope
The standard laboratory and educational microscope — uses two separate lens stages (objective and eyepiece) working together to achieve high magnification of thin, typically transmitted-light specimens mounted on glass slides. The "compound" name refers specifically to this two-stage magnification system, distinguishing it from a single-lens magnifier.
Magnification
40× – 1000×+ (objective × eyepiece)
Illumination
Transmitted, from below specimen
Specimen type
Thin sections, smears, slides
Image
2D, flat-field
Overview
- Light from an illuminator beneath the stage passes up through a condenser lens (which focuses and controls the light cone), through the thin specimen, into the objective lens, which forms a real, magnified intermediate image inside the tube — then the eyepiece (ocular) magnifies that intermediate image a second time for the eye or camera, giving the system its "compound" (two-stage) magnification
- Total magnification is the product of objective power and eyepiece power (e.g., a 40× objective with a 10× eyepiece yields 400× total), with most compound microscopes offering a rotating nosepiece holding several objectives (commonly 4×, 10×, 40×, and 100× oil-immersion) to switch magnification quickly
- Because the light must pass through the specimen, samples must be thin enough (or sectioned/smeared thin enough) to be translucent — thick or opaque specimens simply block the light and can't be usefully imaged this way, which is the core limitation that stereo microscopes exist to address
- Resolution at high magnification is fundamentally limited by the wavelength of visible light and the objective's numerical aperture, not simply by how much the eyepiece magnifies — pushing magnification past what the objective's resolution supports only produces a larger, blurrier "empty magnification" image with no additional real detail
Key Features
Two-stage magnification
Objective and eyepiece each magnify in sequence, multiplying together to reach the high total magnifications needed to resolve cellular and sub-cellular detail.
Rotating nosepiece
Holds multiple objectives of different power, allowing quick switching between low-power scanning and high-power detailed observation of the same slide.
Oil-immersion capability
The highest-power objective (typically 100×) is often designed for oil immersion, using index-matching oil between lens and slide to achieve numerical apertures beyond what air alone permits.
Design and Construction
Optical train
- Illuminator (LED or halogen) → field lens → condenser (focuses light onto the specimen plane, often with adjustable aperture diaphragm to control contrast/depth of field) → specimen → objective → intermediate image plane → eyepiece
- Objectives are typically multi-element, highly corrected lens groups (achromat, plan-achromat, or apochromat grade) to flatten the field and correct color and spherical aberration across the viewed area
Numerical aperture & resolution
- Numerical aperture (NA), not magnification, is the parameter that actually governs resolving power — higher-NA objectives resolve finer detail and gather more light, at the cost of shorter working distance and shallower depth of field
- Oil-immersion objectives achieve NA values above 1.0 (impossible in air, which caps NA near 1.0) by eliminating the refractive-index mismatch between the glass slide/coverslip and the objective's front lens
Applications
Clinical & Diagnostic Labs
Blood smears, biopsies, pathology
The standard instrument for examining stained tissue sections and cell samples in clinical and diagnostic laboratory settings.
Education
Biology & life science coursework
The most common microscope type in educational settings, from secondary school through university biology labs.
Microbiology Research
Bacteria, cell cultures & tissue study
Essential for examining microorganisms and thin-sectioned research specimens at cellular and sub-cellular resolution.
Why choose a Compound Microscope
Highest magnification for thin specimens
The standard choice wherever cellular or sub-cellular detail on a thin, translucent sample needs to be resolved.
Flexible power range
A rotating nosepiece with multiple objectives covers a wide magnification range on a single instrument.
Frequently asked questions
Here are some common questions about achromatic lens.
The eyepiece only magnifies the intermediate image the objective has already formed — it cannot add detail that the objective's numerical aperture and the wavelength of light didn't already resolve into that intermediate image. Beyond a certain point (roughly 1000× the objective's numerical aperture, a commonly cited practical ceiling), further eyepiece magnification just enlarges the same blurred information with no new detail revealed — a phenomenon called "empty magnification." Real improvements in resolved detail require a higher numerical aperture objective (such as switching to oil immersion) rather than a stronger eyepiece.
