MTF is a measure of the contrast of a lens at any particular spatial frequency (lp/mm reading). It is related to resolution, since resolution is defined as the spatial frequency at which MTF drops below some given value.
We can see that the resulting intensity is made of a sinus law with a high "spatial frequency" (wave number) which is the average of the spatial frequencies of the two patterns, ...
Noise also varies in both magnitude and spatial frequency. Noise occurring over short distances has a high frequency (it's 'fine-grained'), while noise occurring over long distances has a low frequency.
(2) In technical terms this means that the spatial frequency of the subject is higher than the resolution of the camera which we defined by the Nyquist frequency.
The spatial frequency at which a digital image is sampled (the sampling frequency) is often a good indicator of resolution. This is why dots-per-inch (dpi) or pixels-per-inch (ppi) are common terms used to express resolution for digital images.
Film's sharpness decreases gradually as the pitch (spatial frequency of fineness of detail) increases.
Seen as an MTF curve, film's response to detail gradually becomes less as the details get finer.
 See also: Image, Frequency, Resolution, Value, Light
  
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