Contrast is the difference in brightness that separates an object from its background. It is the difference in brightness and/or color that makes an object (either real or an image) distinguishable from other objects and from its surroundings.
Contrast sensitivity is a person’s ability to see these differences. The better able one is to detect objects of low contrast, the higher one’s contrast sensitivity.
In seeing everyday objects, our eyes collect several bits of information using various visual channels. These visual channels collect information about size, shape, contrast, color and motion. Each channel collects and feeds this information individually to our eye/brain system. The Snellen letter chart tests only one of these visual channels.; it measures one’s ability to see well-defined black letters on a white background, which is a very high contrast environment. While most objects we see are larger than the letters on a letter chart, they are much lower in contrast.
“I’m 20/25 but I can’t see well a lot of the time”
Many people with normal or near-normal high-contrast acuity have low contrast sensitivity. Low contrast is common among the elderly, as it is a common characteristic of the aging eye.
Low contrast sensitivity often goes un-diagnosed, in part because a contrast sensitivity test is often not administered as part of a routine eye exam.
When contrast sensitivity is not measured, those who are experiencing vision difficulties become confused and uncertain about what they are experiencing. People with glaucoma, for example, often complain of “poor” vision despite testing normal or near-normal for visual acuity. And without the understanding of how their contrast vision has changed, they are not able to become aware of the challenges to their safety and daily living that reduced contrast sensitivity brings, such as night driving or moving through a dimly lit room.
Without testing it can be difficult for someone to distinguish between vision challenges from reduced acuity (blurring) and reduced contrast sensitivity.