A customer recently asked Microscope World to help them determine what would be the appropriate microscope for their application. They needed to capture what happens to a metallic sample while it is being treated with a specific water-soluble compound. The customer was looking at the SMZ-168 stereo microscope on a track stand with LED ring light.

When we asked how large (or small) an area they needed to resolve, the customer said it was anywhere between 10µm to 350µm. We told them that was going to present a problem if using a stereo microscope. The stereo microscope will handle 350µm with no problem, but you will not get the kind of clarity needed if you need to view 10µm with a stereo microscope.

The customer was hopeful there must be a way to get the image they needed with a stereo microscope. They only needed 100x total magnification that the SMZ-168 provides when a 2x auxiliary lens is used with the 10x eyepieces and they did not want to move into a metallurgical microscope. After all, they asked, what is the difference between what you get from this stereo microscope setup and what you get from a 10x metallurgical microscope lens?

The problem comes down to the numerical aperture (NA) provided by the two different types of lenses and the resolution they provide. Resolution is the clarity of an image expressed as a numeric value. NA is a numeric quantification of a lens' ability to resolve an image. There are a few different equations that express this relationship, including: (r) = 0.61λ/NA, where λ is the imaging wavelength (550nm on average). So, if an objective lens has NA of 0.30, the resolution is (0.61x550nm)/0.30, or 1.12µm. This is the closest together two objects can be and still be distinguished from one another using that lens.

Resolution, then, is directly tied to the NA of a lens. The NA of a lens is directly tied to its working distance (that is an explanation for another time), so a stereo microscope lens, with its inherently greater working distance, will have poorer resolution than a metallurgical microscope with its relatively shorter working distance. The NA for the SMZ-168 stereo microscope at 10x objective magnification is 0.15. The NA for the 10x objective lens on the Fein Optic M40 metallurgical microscope is 0.30, or twice as good as the stereo microscope.

That's all well and good, but the customer wanted to know what it all means, practically.  Can we show them what the difference in resolution means in pictures? 10x with a stereo microscope lens versus 10x with a metallurgical microscope lens?

Here are two images of the same sample - a scratch on a flat, metallic object, viewed with the 10x objective lens:

With a stereo microscope, the zoom was set to 5x using a 2x auxiliary lens, NA = 0.15 (for perspective, the distance across the widest point of the swirl is about 1mm).

With a metallurgical microscope, the 10x objective lens was used with NA = 0.30.

Although the science behind it may be hard to understand, these images capture the difference between magnification and resolution. These have the same magnification (100x), but the better resolution with the metallurgical microscope objective lens is clear. If you need to view metallic objects with high resolution, you're going to need a metallurgical microscope, rather than a stereo microscope.

If you have questions regarding whether a stereo microscope or a metallurgical microscope would be better for your application, contact Microscope World and we will be happy to help.