What is a microscope image?

The Short Answer

A microscope image is the enlarged view of a specimen produced by a microscope — either the image you see through the eyepieces, or a recorded picture of it. A recorded microscope image is called a micrograph. It is a representation of the specimen, shaped by the type of microscope, the lighting, any stains and the camera, rather than a simple enlarged photograph of what is “really there”.

Micrograph, Photomicrograph and Related Terms

• Micrograph: any image taken through a microscope.
• Photomicrograph (or light micrograph): a micrograph taken through a light microscope.
• Electron micrograph: a micrograph produced by an electron microscope, often labelled SEM or TEM.
• Microphotograph: confusingly, this traditionally means a very small photograph — such as microfilm — not a photograph taken through a microscope. Many people now use it loosely as a synonym for photomicrograph, but in careful usage they differ.

How a Light Microscope Forms an Image

In a compound microscope, image formation happens in two stages:

1. The objective lens collects light that has passed through the specimen and forms a magnified, real image inside the microscope tube. “Real” means it could be projected onto a screen or camera sensor placed at that point.
2. The eyepiece acts like a magnifying glass on that intermediate image, producing an enlarged virtual image that your eye sees as if it were some distance away.

(For more on the distinction, see real vs virtual images in a microscope.) This is why a camera can capture an image from the camera port without an eyepiece: it records the real image directly, sometimes through a relay lens.

A side effect of this optical arrangement is that the image in a compound microscope is upside down and reversed left to right (here’s why). Move the slide to the left and the specimen appears to move to the right. Stereo microscopes include prisms that correct this, so the view is upright and correctly oriented — one reason they are easier to work under. See stereo vs compound microscopes for more on that difference.

Magnification Is Not the Same as Detail

Two terms describe any microscope image:

• Magnification is how much larger the image appears than the object.
• Resolution is the smallest distance between two points that can still be seen as separate.

A microscope can make an image bigger without making it more detailed. In light microscopes, resolution is limited by the wavelength of light and the numerical aperture of the objective, to about 0.2 micrometres at best. Magnifying beyond what the optics can resolve produces “empty magnification” — larger but blurrier. That is why a sharp image at 400x can be more informative than a blurry one at 1000x, and why claims of 2000x or more on student microscopes should be treated with caution. Our microscope buying guide explains how to spot honest magnification.

Why Microscope Images Look So Different

The same specimen can look completely different depending on the technique:

• Brightfield: the standard view. Specimens look darker or coloured against a bright background. Unstained cells can be very faint.
• Darkfield: specimens glow brightly against a black background, because only light scattered by the specimen reaches the objective.
• Phase contrast: transparent cells show up as darker or lighter shapes, often with a characteristic bright “halo” around their edges.
• Differential interference contrast (DIC): gives a shaded, embossed, almost 3D look.
• Fluorescence: specific structures glow in particular colours against a dark background, because they have been labelled with fluorescent dyes or proteins.
• Polarised light: crystals and some fibres appear bright and often vividly coloured against a dark background.
• Stereo microscope: looks much like a close-up photograph of a solid object, with natural surface colours.

Electron microscope images look different again:

• SEM images show surfaces in striking relief — insect eyes, pollen spikes, the surface of a fractured metal — with a great depth of field. They are naturally grayscale.
• TEM images show thin slices in shades of gray, revealing internal structures such as membranes, organelles or crystal lattices.

Any colour in an electron micrograph has been added afterwards (“false colour”), usually to highlight particular features. We explain why in the principle of SEM and TEM.

How to Read a Microscope Image

A few habits make microscope images much easier to interpret:

Look for the scale bar. A scale bar (for example, a line marked “10 µm”) tells you the real size of features. Stated magnifications such as “400x” stop being accurate as soon as an image is printed or shown on a different screen, so a scale bar is far more reliable.

Know what the colours mean. In light micrographs, colours often come from stains. In a common tissue stain called H&E, for example, nuclei typically appear blue-purple and much of the surrounding material appears pink. The colour tells you about chemistry, not the natural appearance. In fluorescence images, each colour usually corresponds to a particular label.

Remember it is one focal plane. At high magnification, a light microscope shows only a very thin slice of the specimen in focus. A cell that looks like a flat disc may be a sphere. Changing focus, or combining images by focus stacking, reveals more.

Watch for artefacts. Common ones include:

• air bubbles under the coverslip, which appear as dark-ringed circles
• dust on the eyepiece or camera, which stays fixed when you move the slide
• folds, tears or shrinkage from preparing the specimen
• uneven illumination that makes one side of the image brighter
• crystals from stains or dried mounting medium

Check the technique and stain in the caption. Without that, it is easy to misread what an image shows.

Digital Microscope Images

Most microscope images today are digital. The camera’s sensor turns the optical image into a grid of pixels. That brings some practical points:

• Calibration links pixels to real distances, so you can measure features.
• File format matters: RAW or TIFF preserve detail for analysis; JPEG is fine for sharing.
• Processing should be applied evenly to the whole image, and the original should be kept unaltered.
• Combining images — focus stacking, stitching or 3D reconstruction — can create images that no single exposure could capture.

Our guide to digital imaging and image analysis for microscopy covers capture, calibration and analysis software in detail, and what microscopes make 3D images explains how depth can be recovered from microscope data.