All images and pictures contain spatial frequencies. Most of us are familiar with some type of frequency such as the 60-cycle, 110-volt electricity in our homes. The voltage varies in time as a sinusoid, and the sinusoid completes a full cycle 60 times in a second-a frequency of 60 Hz. Images have spatial frequencies. The gray level in the image varies in space(not time),i.e. it goes up and down.