Most lenses including the human lens are not perfect optical systems. As a result when visual stimuli are passed through them they undergo a certain degree of degradation. The question is how can this degradation be evaluated? Before we can answer this question we need to define "modulation."
Recall in a square wave grating there were dark bars and light bars. We can measure the amount of light coming from each. The maximum amount of light will come from the light and the minimum from the dark bars. If the light is measured in terms of luminance (L) we can define modulation according to the following equation:
Now, let's assume that you have a square wave grating of a specific frequency (v) and modulation (contrast) and this stimulus is passed through a lens. The modulation of the image can now be measured.
The modulation transfer function (MTF) is defined as the modulation, Mi, of the image divided by the modulation of the stimulus ( the object), Mo, as shown in the following equation.
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