How we define frequency in an image? How we define frequency in an image? 


Does the repetition of a single gray level defines the frequency?(Example: can a black(single color/constant region) background image be called high frequency image?) I think it is not so.


Or


Does the frequency depends on number of changes in the gray levels in an image? If so Why? I couldn't able to relate the image frequency with the signal frequency where we say high frequency signal when the same cycle is repeated many times in a unit time.
If an image has large values at high frequency components then the data is changing rapidly on a short distance scale. e.g. a page of text

If the image has large low frequency components then the large scale features of the picture are more important. e.g. a single fairly simple object which occupies most of the image.

Thus low frequency correspond to slow varying information ( continuous surface)
high frequency correspond to quickly varying information ( eg. edges)