During the training session in Golden, Co., I mentioned the terms "high-frequency image" and "low-frequency image" a few times ... mostly to blank stares. I thought I'd take a moment to jot down some good definitions for future reference.
High-frequency images can be seen as having lots of abrupt tonal transitions in a small space. An example of one such image would be a urban scene with lots of bright colours and tons of edges. Conversly, low-frequency images are those where the tone remains relatively constant throughout these small areas and the transitions are gradual. A great example of a low-frequency image would be a facial portrait, the changes in tone being subtle and smooth.