Contents
In short
A microscope's total magnification is the eyepiece power multiplied by the objective power, so a 10x eyepiece and a 40x objective give 400x. The useful limit is set by the objective lens, not the eyepiece: roughly 1,000 times the objective's numerical aperture, which for the 40x lens on a hobby microscope is about 650x. Anything above that enlarges the blur without adding detail, which is why "2000x" on a box under £100 tells you nothing. For cells you need 400x; for bacteria, 1,000x and good lenses; for insects and coins, 10x to 40x. The GCSE formula is different: magnification = image size divided by actual size.
Magnification is the first number on every microscope box and the least useful one for deciding what to buy. This guide explains how the figure is calculated, why there's a ceiling beyond which it means nothing, what you can expect to see at each power, and how the numbers work on stereo and digital microscopes, which play by different rules. There's also a short section on the GCSE calculation, because a lot of people land here looking for that.
How the number is worked out
A compound microscope magnifies in two stages. The objective lens, the one near the specimen on the rotating turret, makes an enlarged image inside the tube. The eyepiece then magnifies that image again for your eye. Total magnification is the two multiplied together.
Take the Rainbow 2L. Its eyepiece is 10x and its three objectives are 4x, 10x and 40x, so the three magnifications you can set are 40x, 100x and 400x. The Rainbow 50L PLUS has a 16x eyepiece and the same three objective powers, giving 64x, 160x and 640x, and it comes with a 2x Barlow lens that sits between the two and doubles each figure, which is where its 1280x headline comes from.
That's also how to read a quoted range. "40x to 400x" means the lowest and highest combinations available, not a smooth zoom; you step between them by turning the turret.
Here are the combinations on our two Rainbow models.
| Objective | Rainbow 2L, 10x eyepiece | Rainbow 50L PLUS, 16x eyepiece | 50L PLUS with the 2x Barlow lens |
|---|---|---|---|
| 4x | 40x | 64x | 128x |
| 10x | 100x | 160x | 320x |
| 40x | 400x | 640x | 1280x |
Useful magnification and empty magnification
The eyepiece can't add detail that the objective didn't capture. It can only make the objective's image bigger, and past a certain point that's like enlarging a photo until you see the pixels. Opticians call magnification beyond this point empty magnification, and the rule of thumb for where it starts is between 500 and 1,000 times the objective's numerical aperture (NA), a figure engraved on every objective lens that describes how much light it gathers.
On a hobby microscope the numbers run roughly like this: a 4x objective has an NA of about 0.10, a 10x about 0.25, and a 40x about 0.65. Multiply by 1,000 and you get useful ceilings of about 100x, 250x and 650x. So a 40x objective is giving you everything it has by about 650x. Putting a 25x eyepiece behind it to reach 1,000x, or a Barlow lens to reach 1,280x, enlarges the same information, and the image gets dimmer and softer as it grows. To go usefully past 650x you need a 100x oil immersion objective with an NA around 1.25, which is a laboratory instrument's lens and needs a drop of oil between the lens and the slide.
This is the whole story behind the 2000x microscope in the toy shop. It'll have a 40x plastic objective and a 50x eyepiece, and the image at 2000x is a dim grey fog. The number is arithmetic, not optics.
The underlying limit is the wavelength of light. No light microscope, however good, can separate two points closer than about 0.2 micrometres (a fifth of a thousandth of a millimetre). That's enough to see bacteria as shapes and a human cell's nucleus clearly, and not enough to see the inside of a bacterium or a virus, which is what electron microscopes are for.
What you can see at each power
| Magnification | Microscope type | What it shows |
|---|---|---|
| 10x to 20x | Stereo or digital | A whole insect, a coin, a flower head, a circuit board, a fossil. Depth and texture. |
| 30x to 40x | Stereo, digital, or compound at lowest power | An insect's head and legs, the detail on a coin, a solder joint, a whole prepared slide specimen at a glance. |
| 100x | Compound | Plant cells in rows, a pond creature swimming, a hair's structure, pollen grains, a mould's threads. |
| 400x | Compound | Individual cells with their nuclei, blood cells, chloroplasts in a leaf, yeast budding. The working magnification for school and home biology. |
| 640x to 1000x | Compound, good lenses | Bacteria as rods and dots, detail inside a cell. Needs careful lighting and, at 1000x, an oil immersion objective. |
Stereo microscope magnification
Stereo microscopes work at low power on purpose. The Levenhuk 2ST, for example, has two 10x eyepieces and a 4x objective for a fixed 40x. The reasons the numbers stay low are that the object is solid and lit from above, so there's a limit to how much light comes back, and that the point of a stereo microscope is depth and working distance, both of which fall away as magnification rises. Zoom stereo microscopes typically run from about 7x to 45x. If a stereo microscope claims 200x, be suspicious of the image.
Digital microscopes and digital zoom
Digital microscopes add a third kind of number. There's the optical magnification of the lens, there's digital zoom (the camera cropping and enlarging its own image, which adds no detail), and there's sometimes "screen magnification", meaning how big the object appears on the display compared with real life, which depends on the size of the screen. A model quoted as 20x to 500x may be a lens that covers 20x to 125x optically with 4x digital zoom on top, and the manufacturer isn't wrong to add them up, but you should know which part is which. The DTX 500, for instance, quotes 20x to 500x with 4x digital zoom. The way to judge a digital microscope is the sensor (5 megapixels is good, 0.3 is basic) and the sharpness at the lowest power. Our digital microscope guide covers this.
How to calculate magnification for GCSE and A-level
Exam questions use a different formula, because they're asking how much bigger a drawing or photo is than the real object, rather than what the microscope was set to.
Magnification = size of the image ÷ actual size of the object.
Both sizes have to be in the same units. A worked example: a cell measures 20 mm across in your drawing and its actual width is 0.05 mm (50 micrometres). Convert so the units match, then divide: 20 ÷ 0.05 = 400, so the magnification is ×400. The same formula rearranges to find actual size (image size ÷ magnification) or image size (actual size × magnification). The conversions you need are 1 mm = 1,000 micrometres (µm) and 1 µm = 1,000 nanometres (nm).
And for the other question that comes up in the same paper, the total magnification of the microscope is still eyepiece × objective, as above.
What this means when you buy
Ignore the top number and look at the objective lenses. Three glass objectives of 4x, 10x and 40x with a 10x or 16x eyepiece is the right specification for a first compound microscope, and it will show you everything up to and including cells and their nuclei. Spend extra on achromatic lenses, a metal body and lighting from above and below before you spend it on magnification. For whole objects, 10x to 40x is the range you'll use, whatever the box says. Our types guide turns this into a buying decision.
FAQs
What magnification do I need to see cells?
100x shows plant cells in rows; 400x shows individual cells with their nuclei clearly. Every compound microscope we sell reaches 400x.
What magnification do I need to see bacteria?
Around 1,000x with good lenses, and even then they appear as dots and rods. At 400x on a hobby microscope, yoghurt bacteria are just visible as specks. Seeing structure inside a bacterium needs an electron microscope.
What is the highest magnification of a compound light microscope?
About 1,000x to 1,500x usefully, using a 100x oil immersion objective. Higher figures on consumer microscopes are empty magnification.
Is a 1000x digital microscope real?
The figure usually combines optical magnification and digital zoom. The optical part on a consumer digital microscope is typically under 200x. Judge it by the sensor and the image, not the number.
Is a higher magnification microscope better?
Not past about 400x to 640x for a hobby instrument. Lens quality, lighting and a stable body decide what you can actually see. A sharp 400x beats a foggy 1200x every time.
Specifications quoted are the manufacturers' figures, checked October 2026. The microscopes mentioned are dispatched from our warehouse in Northern Ireland with a 14-day return period.
