Definition of "diopter" [di•op•ter]

• A unit of measurement of the refractive power of lenses equal to the reciprocal of the focal length measured in meters. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "A diopter is a unit of measurement that indicates the amount of correction needed to change your vision to be as close to 20/20 as possible."
• "The diopter is a little adjustment that you can make to how your viewfinder works - it's particularly useful for people with poor eye sight - it's usually a little wheel next to your viewfinder."
• "Also connected with this is checking the 'diopter' on your camera (if it has one."