Magnification versus Resolution – which is more important?

We often get asked about magnification in Tabletop SEM.  Magnification is a relative term since the actual magnified size of an image depends on how it is displayed.  This is why the micrographs captured by Scanning Electron Microscopes almost always contain a “scale bar” so that no matter how the image is displayed, one could calculate the true magnification for that instance.

Unlike light or optical microscopy where the resolution is controlled by the wavelength of light and the quality of the optical lenses, in Electron Microscopy the resolution depends on a number of factors.  Suffice to say though, an electron microscope cannot resolve a feature less than its ultimate resolution.  Typically, resolution of the electron microscope is directly proportional to its cost.  To see smaller and smaller objects, one has to spend more, often exponentially more.

Many within the microscopy industry maintain that it is misleading to the viewer to have the magnification printed on the saved image (ie, 1000X for instance) since if that small captured image is then projected onto a large screen during a presentation, the actual magnification is quite a bit larger.  All digitally captured images reach a point of “hollow” or empty magnification where further magnification reveals not additional detail.  THat magnification limit is determined by the resolution of the microscope.

Manufacturers often list magnification capability in their literature.  Ask if that is that the “live” magnification of a real-time image or is it the theoretical magnification of a captured image displayed on a particular device at an established width by height?  Unfortunately there is no standardization on this and this can lead to misinterpretation of the actual capability.

Take a theoretical example – with a micrograph having a 2 micron FOV (field of view) and captured at 5120 x 3840 pixels and then displayed on a 50″ monitor that typically has around 90-100 pixel per inch density.  If displayed pixel per pixel such that their is no pixelation, the magnification would be over 500,000X — but you cannot create more detail.

What is more important for electron microscopes than their claimed magnification, is the Resolution of the microscope and what is called the onset of “hollow magnification” where no further detail is revealed by increased magnification.

Therefore, when evaluating SEM’s you are wise to take specifications regarding magnification with some skepticism.  Pay close attention to RESOLUTION and how well the tool allows you to reach magnification to observe the limits of resolution.  This is best evaluated with resolution standards and monitor samples like Gold Islands or Tin Spheres on Carbon resolution standard samples from suppliers such as those on our RESOURCE LINKS page or follow the direct links below.   These resolution standards are typically under $200 and quite useful when considering an expensive purchase like an SEM.  Real-world samples might not possess details fine enough to make this determination or they might require special preparation in order for them to be revealed.

Resolution Standards:

Gold Islands – Gold on Carbon Resolution Standards

Tin Spheres on Carbon Resolution Standards

References:

Magnification and Resolution at Science Learning.