Subject: Day/Night Vision
I am nearsighted but I seem to be able to see fairly well when I'm outside on a bright sunny day. When the day is cloudy my vision isn't so good. Does the sun have something to do with improving a person's vision?
Answered by Richard E. Gans, M. D.
This question relates to "depth of field". Ask any photographer whether focusing is more critical when the camera aperture is wide open or closed down -- they will tell you that the smaller the aperture, the greater the depth of field. This means that the smaller the opening in the lens, the larger the range of distances that objects are in focus.
The iris in the eye serves the same purpose as the aperture, (f stop) in a camera. In bright sunny conditions, the pupil (the opening in the center of the iris) gets smaller. The smaller the pupil, the greater the range of distances that objects are in focus. The larger the pupil, as in cloudy or dark conditions, the more critical the focus must be controlled. Thus, people who need to wear glasses in dimmer lighting conditions may be able to see well without glasses in brighter light.
PLEASE NOTE: The information contained on this system is not intended to supplant individual professional consultation, but is offered as a community education service. Advice on individual problems should be obtained directly from a professional.
Copyright, 1994. Richard E. Gans, M.D.
Last Modified: August 23, 1996