Resolving power is the ability to distinguish an object from its surroundings. Due to optical properties of the lens, as detail gets smaller, the image of the detail, softens (or blurs). Visually, this amounts to a loss of visible contrast. Digitally, using the classic black and white line pair target, this means that the light coming thru the white space has smeared to the point where the pixels no longer can portray a difference in intensity (equal grey), therefore no image.

Just before this final blurring is a critical size called threshold resolution (anything smaller and it disappears). This critical resolution is specified as "line pairs per mm", and is a true measure of the quality of the lens. This threshold resolution reflects about a 9% MTF value, and is not a crisp image, but viewable, and a good image processing system can work with it. This value may be approximated as 1/[3000*NA].

While a dark line on a white background will eventually "grey out", a white line on a dark background will always be visible (assuming there is enough light coming thru to energize a pixel over the system's noise level). The white line will never appear smaller than the system's Airy Disk diameter, which can be approximated as 2/[3000*NA].

In a well corrected, diffraction limited lens, with appropriate illumination, the limiting of fine detail is purely a function of the N.A. (Numerical Aperture) of the taking lens (the lens that initially images the object and sends it into the optical system). The higher the N.A, the more you see, and the more you pay.

The second part of "seeing" is that, once the lens system has picked up the object, it must magnify it to the point where it covers at least 2 pixels on the sensor. The smaller the object, and the larger the pixel, the higher the magnification must be. Conversely, an excess of magnification will "smear" the object across many pixels, creating a low contrast image.

CCTV lenses, forced to work close, are limited to about 5 microns resolution, and generally work at less than 1X. If a 2X extender is added to the back of the lens, and the sensor has 5micron pixels, then it may be possible to "see" the limit.

Navitar zoom lenses, operating in the macro mode, can go down to 2.4 - 1.6 microns, depending on the model, and 0.6 microns in the micro (ultra-zoom) mode. A major advantage of the zoom lens is that it lets you see a larger field to locate the object and then zoom up to cover the two pixels to resolve it.

Just for reference, 1 micron is .001mm. The human hair is about 100 microns (.100mm) in diameter.