What microscope is the least powerful?

Answer 1

Short Answer: The unaided human eye which has a magnification of 1X and can resolve details down to about 0.2 millimeters.

Longer Answer:

Depending on the type of lens a single lens magnifier can magnify up to about 12X and resolve down to 17 micrometers.

A multiple lens achromatic magnifier can go up to about 25X and resolves down to about 8 micrometers.

Stereo microscopes that can easily view 3-D objects go up to about 1000X and resolve down to about 150 nanometers.

Likewise, any optical microscope is limited by the diffraction limit of light to about 1300X of usable magnification. More magnification will just produce a larger version of the image that is blurry since it is not possible to sharpen the focus beyond the 150 nanometer diffraction limit.

Microscopes that use ultraviolet light, which has a shorter wavelength that can focus tighter, can resolve down to 10s of nanometers or about 10 times better than visible light microscopes for a magnification of 10,000 X.

Modern Scanning Electron Microscopes (SEM) are capable of resolving down to about 0.2 nanometers which translates to a useful magnification of 1,000,000 X. Specialized types of electron microscopes (Field Emission Transmission Electron Microscopes) can resolve down to 0.05 nanometers which translates to a magnification of 2,500,000 X. For reference, a carbon atom is about 15 nanometers in diameter.