A contrast sensitivity test, checks for the ability to differentiate between light and dark. This ability, is as much an important sense as 20/20, allowing us to be able to carry out everyday tasks safely.Contrast sensitivity testing can be important in diabetes and cataract medicine. Cataracts are a clouding of the eye's natural lens, which causes light to scatter on the retina, reducing image contrast and causing dimness of vision. Many cataract patients have good visual acuity, but they still notice a loss of their visual capability. Contrast sensitivity testing can provide a true real world vision of a patient's functional vision. Tests of contrast sensitivity are normally done before and after cataract surgery, to document the degree of improvement that has been achieved.