The resolution of the human eye represents the ability of the eye to distinguish two points or two lines, which is commonly referred to as vision. It must be noted that when performing visual acuity examinations, even the same subject will get different results. Of course, these changes are known to have clues for analyzing visual function and making inferences, but as far as representing visual acuity there is a lack of generality, and the basic meaning of the values themselves is still very poorly understood. This situation is very similar to the problem that occurs when performing resolution tests with various patterns in order to evaluate photographic objectives. In a word, this kind of inspection is not a basic physical quantity of the universality of the measurement, but is only a comparative evaluation of a certain ability under certain specific conditions, which has great limitations.

In order to overcome this shortcoming, the concept of modulation transfer function in the field of optics is introduced, so that a more accurate evaluation of image transfer and reproduction can be made.

An image with any shape and brightness distribution can actually be regarded as a combination of basic spatial frequencies in an appropriate proportion (called the frequency component or spectrum of the original image). If the image is imaged with a lens with a known modulation transfer function (MTF), the image condition can be calculated. Unlike resolution or visual acuity, MTF is not just a certain threshold under specific conditions, from MTF can also be obtained from the imaging situation, which contains more information, so it is a general representation.

The advantages of using MTF to evaluate visual properties can be listed as follows:

1) From the shape of the MTF curve, it is possible to infer visual functions that cannot be known by simple visual acuity measurement, such as inferring the characteristics of amblyopic eyes.

2) The characteristic of retinal information processing system can be unified mathematically.

3) If the feasibility of Fourier transform is recognized, it is also possible to infer the image quality characteristics and perceptual characteristics of various images according to MTF.

4) With the latest laser interference technology, it is possible to separate and evaluate the imaging system of the eyeball and the brain processing system behind the retina, which will be very important.