The camera lens normally forms an image which is a faithful representation in scale and perspective of the various elements of the scene in front of it. Any alteration in the shape or proportions of the normal image at any stage of the photographic process is known as distortion.


3rd Harmonic: The introduction of an unwanted 3rd harmonic of the fundamental signal (3 octaves higher than the fundamental signal).


Distortion that results when two or more pure tones produce new tones with frequencies representing the sums and differences of the original tones and their harmonics.