Scanning Electron Microscope Magnification

Magnification is a very simple concept, but it sometimes can create confusion because of its own definition. The aim of this blog is to clarify magnification in scanning electron microscopes (SEM) and focus on other parameters which can describe better how big an object is represented.

The first magnifying glasses date back to the Greeks, with Aristophanes describing the first attempt to look at small details as a leisure activity for kids. This was when the word magnification entered our language for the very first time.

Time has passed, and the interest of science for the micro and nano world has exponentially increased, creating the need for a quantification of magnification.

How to calculate microscope magnification

The modern definition of magnification is the ratio between two measurements, which implies that two objects are needed for a correct evaluation of the value.

The first object is obviously the sample. The second is a picture of it. But the thing is, although the sample will not change its size, the picture can be printed in an infinite number of different sizes. So allow me to do some maths:

This means that if I print a picture of an apple that fits on a standard printer sheet and I print it again to fit on a poster that will be used to cover a building, the magnification value will change dramatically.

A more scientific example can be applied to microscopy: when storing a digital image of the sample, resizing the image causes the magnification number to become ostensibly wrong.

Magnification is thus a relative number and it is of no practical use in the scientific field.

What scientists use is a couple of parameters that describe the actual imaged area (field of view – the area that the microscope points at) and how sharp this image is (resolution). The formula of magnification also changes accordingly:

As you can see, the formula still remains a vague description and does not consider the resolution. This means that scaling the same image to a bigger screen will cause the magnification number to change.

What is the field of view of a microscope?

The field of view defines the size of the feature to be imaged. This value typically ranges between some millimeters (a bug) to few microns (the hair of a bug) and a couple of nanometers (the molecular macrostructure of the exoskeleton). With modern instruments, objects in the range of few hundred picometers can be imaged – and that is the average size of an atom.

What is the required field of view to image your samples?

Once again, this is quite a tricky question, but it can easily be answered with an example. In a picture with your best friends, normally a face covers 5-10% of the surface of the space. This is already enough for you to recognize the persons in the image. But if you have a close up of a face, small details such as hairs, spots on the skin and the color of the eyes can be observed.

This means that if you, for example, have particles with an average size of 1 micron and you want to count them, it is ok to have 20 particles per image, rather than wasting time by imaging one particle at a time. Also taking into account empty space between particles, a field of view of 25-30 microns is enough for such sample.

On the other hand, if your interest lies in the structure of a particle, a close up is needed and the observed area must be closer to 2-3 microns, if not smaller.

Desktop scanning electron microscopes are becoming more and more popular due to the potential that they offer for a price that is now comparable to that of a high-end light microscope. The resolution is higher and the integration of other analysis tools to measure features such as surface roughness and elemental composition makes them the most versatile instrument for imaging.

Fig.1 Images of particles. a) a close-up of a particle shows the surface topography (FOV = 92.7 μm) b) a larger field of view allows you to image more particles (FOV = 1010 μm).