The CTF is a measure for the information transmission capability of an optical system. It is a somewhat abstract concept that shall not be discussed in detail here. Basically it is a function that relates the contrast produced by object features in the image and their spatial frequency, i.e., the inverse of their size. The contrast continuously drops with increasing spatial frequency, that means, smaller features are always depicted in the image with less contrast than larger features. The Rayleigh Resolution gives the spatial frequency (and thus inversely the size of features) where the contrast has dropped to 25%.