# Resolving Power

by Dr.Brain
Tags: power, resolving
 Sci Advisor P: 1,474 If you had an infinite numerical aperture, you could resolve an object with infinite precision. The problem is, the best numerical apertures available to us are around 1.5. This means that the resolution we are able to acheive in the far-field is approximately $\lambda/2$, so if we are imaging something using a 500 nm source, the maximum resolution we can acheive is 250 nm. There are a few theories as to why this is so, essentially the theory depends on what criterion you use to define an object as being resolved. Basically if your Numerical Aperture is finite you cannot image something with infinite precision because you have lost some of the scattered light and hence some of the information about the object (This is commonly referred to as Abbe's theory of imaging). Note that these restrictions only apply only to the far-field. In the near-field (roughly defined as distances smaller than $\lambda$), resolution is only limited by the aperture of our detector and the distance from the source. Provided the signal we are trying to detect is reasonably stable with time, we can obtain images with resolutions that exceed the maximum resolution allowed in the far-field. For more info, I suggest doing a google on SNOM (or NSOM) which stands for Scanning Near-field Optical Microscopy. Claude.