Microscope Glossary of Terms: A-Z

If you are new to the field of microscopy, a student or even a daily practitioner, it’s without a doubt extremely useful to have a glossary of microscope terms that you can always refer to.

It not just to help you with your work and your studies but a glossary is especially handy when you’re shopping for your first microscope and you’re weighing up the resolution, magnification and aperture of the models available because it pays to know just what these terms mean.


Abbe Condenser – A condenser originally designed for Zeiss microscopes by inventor Ernst Abbe. It differs from other condensers as it has two lenses one of which is an iris diaphragm that functions to control the diameter of the light beam. Like all condensers, it is mounted under the stage of the microscope allowing the light to be manipulated and to concentrate the light through the subject before entering the objective. This is more useful for high powered objectives with tiny diameters which require concentrated light.

Achromatic Objective Lens Achromatic lenses are used to improve the image quality by aligning refracted colors of light at the focal point. They are made from several different types of glass with different indexes of refraction reducing the occurrence of the colors that make up a light beam refracting and improving the resolution of the resulting image.

Analyzer – An analyzer is fitted to the head of a compound microscope, or the base of a stereo microscope and used in conjunction with a polarizer to produce polarized light. This is useful for chemists and crystallographers for studying the birefringent properties of their subjects. Polarized light microscopy is also picking up in the field of biology.

Aperture – Controls the amount of light that enters the focal point similar to the mechanism of the pupil of an eye. The numerical value of the aperture of a lens reflects its resolving power which is a vital factor when shopping for optical equipment.

Articulated Arm – Often used for industrial or educational purposes, the base of an articulated arm clamps onto a desk or table. You can attach a microscope body to the focusing holder of the arm. The purpose is to allow 3-dimensional movement with the microscope usually for studying large subjects such as textile work.


Barrel Focus – when the stage is fixed and the body tube of the microscope is moved to focus the objective aperture.

Base – The bottom of the microscope which the body attaches to. The base will have a stand and may have a clamp for securing a microscope to a desk.

Binocular Head – The head of a microscope with two eyepieces.  This term is typically used to describe high powered compound microscopes where only one objective lens is used. Stereo microscopes on the other hand, use an objective for each eyepiece.

Body – The upper part of the microscope including the stage and is often referred to alongside the eyepiece.

Body Tube Length – This refers to the distance between the objective and the very top of the body tube. This can be important as objective lenses are compatible with certain body tube lengths and a mismatch can cause spherical aberrations.

Bright Field Illumination – A method of exposing a transparent or translucent specimen to bright white light whilst on a light background. As opposed to darkfield illumination.


C-Mount – A C-mount is an adapter used with microscope cameras. They are specifically designed for use with certain brands and models of microscope in accordance with their focal length.

Calibration – Even two identical microscopes can have varying magnification factors. Calibration is a process to ensure the measurement of your eyepiece reticle is exact. Calibration is performed using a tool called a stage micrometre.

Chromatic Aberration – The refraction of light wavelengths induces a blurring of the image and color bleeding. An achromatic objective lens can usually fix this.

Coarse Focus – Course focus is achieved by using the knob on the side of the microscope to move the objective closer or further away from the subject. This is done to get into position before the fine focus knob is used to improve the resolution of the image.

Coaxial Focus – A method of focusing where the fine and coarse focus mechanisms operate on the same axis. Typically, the coarse focusing knob is larger and the fine knob is situated on the front side of the microscope.

Condenser – See Abbe Condenser

Cover Slip – Thin, square sheet of plastic or glass to be placed over the subject on a microscope slide.


Dark Field Illumination – A method resulting in the subject appearing light against a dark background through the use of light manipulation. As opposed to bright field illumination.

Depth of Field – A term used with stereo microscopy which refers to the distance an object can be located along the axis of the optical field and seen clearly.

Diaphragm – The diaphragm is a disc that has several holes around the rim. It is inserted under the stage and is designed for you to permit a certain amount of light onto your subject. This is especially handy with high powered microscope use.

Diopter Adjustment – The adjustment of the eyepiece ring which fine focuses the eye lens. This allows you to compensate for any vision deficiency one eye might have over the other in a stereo or binocular headpiece.

DIN-Norms – DIN (Deutsche Industrie Normen) is a standard by which lens manufacturers produce lenses which work with a 160mm tube length. They are not generally better or worse than non-DIN lenses but are interchangeable between DIN microscopes.


Eyepiece Diaphragm – An interior part of the eyepiece which provides the field of view and is where eyepiece reticles are placed.

Eyepiece Lens – The eyepiece provides the second stage of magnification after the objective lens. They can vary between 5x-30x magnification but are most commonly 10x.


Field of View (FOV) – The area which you can see when looking into the eyepiece. This decreases as the microscope increases in power. It can be measured using a stage micrometre.

Filter – Filters are placed over the illuminator or in a specialized filter slot. There are many different filters that alter the resulting image. The variety of microscope filters are used for different reasons such as a neutral density filter is used to reduce the light that passes into the subject.

Fine Focus – Is used following coarse focus to improve the resolution of the image.



Huygenian Eyepiece – This is a specific type of eyepiece which is mostly used with achromatic objective lenses.


Illuminator – The source of light for the microscope which is mounted either above or below the stage depending on what type of microscope you have. LED are the most commonly used in modern times but there is also tungsten, fluorescent and halogen. Both halogen and tungsten give off heat and can interfere with live subjects.

Immersion Oil – Used with immersion oil objective lens to increase the resolution and clarity of the image in magnifications greater than 100x. The immersion oil is dropped onto the coverslip and the objective is gently dipped into the oil to create a bridge between the lens and the coverslip.

Interpupiliary Adjustment – This allows you to adjust the space between the eyepieces on stereo or binocular heads. This makes the microscope much more user friendly, especially to children who will have a smaller interpupiliary distance than an adult.



Koehler Illumination – A method of ensuring optimal contrasting and resolution of the subject by adjusting and focusing the light and then spreading it evenly over the FOV.



Magnification – Magnification is the process of enlarging the image through the use of the microscope’s lens system. The total magnification can be calculated by multiplying the magnification of the eyepiece lens by the objective lens magnification.

Micrometer Disc – A glass or plastic disc with a grid for measuring. It is placed in the eyepiece and superimposed on the sample to get a measurement in mm, nm or μm.

Monocular Head – A microscope head with only one eyepiece as opposed to a binocular head with two eyepieces.


Nosepiece – See Turret

Numerical Aperture (NA) – A value which describes the light gathering ability and resolution of an objective lens. A higher NA means a greater resolving power. It is calculated using a formula which takes into account the range of angles a cone of light can travel through the lens system.


Objective Lens – Commonly referred to simply as the objective, the objective lens is situated closest to the object that you are studying. An objective lens of a compound microscope is built into the body and can’t be changed for another lens. Stereomicroscopes have a separate objective per eyepiece lens.


Parcentered – A fault which occurs when the objective is changed but the image is not still centered.

Parfocal – A parfocal microscope is one where you won’t have to re-focus so much when switching from one objective to another. You’ll only really need to do some fine focusing without having to use the coarse focus dial.

Phase Contrast – This is a technique that allows living cells to be studied without being fixed. It works by the use of a device to read the difference in thickness of the subject. Differences in thickness affect the phasing of light resulting in a high contrast image.

Polarizer – Used with an analyzer to produce a polarized image. See Analyzer.



Rack and Pinion – Mechanism which moves the barrel closer and further away from the stage, or which moves the stage itself during coarse focusing.

Refractive Index – A ratio which indicates the degree to which light bends when passing through a material. It is based on the speed of light in a vacuum compared to its speed when passing through another material.

Resolution – The ability of a lens to differentiate between objects in the image and display fine details clearly. A very low resolution would result in a blurred image. This is an extremely important factor when purchasing a microscope or lens depending on the application you intend your microscope to be used for and the magnification you require.


Slide – A rectangular plate composed of glass or plastic on which the subject is placed for study. Slides can be purchased already prepared for those who are not yet used to preparing microscope slides. They can also be made with a dimple so that it can hold drops of liquid to be studied.

Spherical Aberration – This occurs when light rays fail to converge to a single point resulting in the image losing definition.

Subject/Specimen – The object you are viewing through the microscope.

Stage – The portion of the microscope which sits between the illuminator and the objective and on which the slide is placed. Some stages have arms to hold the slide in place, others are larger as with those seen in dissection microscopes.


T-Mount – An adapter usually to connect SLR cameras to a microscope.

Trinocular Head – The head has two eyepieces and another port to fit a microscope camera.

Turret – The turret is the part which revolves with the objective lenses attached, allowing you to switch between them.




Wide Field Eyepiece – These are eyepiece lenses which have the greatest diameter and allow a wide FOV.

Working Distance – This value describes the distance between the tip of the objective lens and the slide when on the stage. Larger subjects will require microscopes with larger working distances at the sacrifice of magnification and numerical aperture.




Zoom – This is the ability to increase magnification on the subject whilst keeping it in focus. This is a feature that some lenses offer and is usually expected of USB microscopes.