What is Diopter?

First the highly technical stuff that I don’t quite understand all the way myself.  Diopter is a unit of measurement.  Easy enough.  So what does it measure?  Diopter is the unit of measurement for the optical power (a.k.a. focusing power) of a lens.  It is “equal to the reciprocal of the focal length measured in meters.” (per our friends at Wikipedia)

Like I said, that gets a bit technical for me.  But as a photographer I am more interested in the diopter adjustment feature of my camera.  Not all cameras have this feature, but on the Sony Alpha 850 that I shoot with it is a small dial to the left of the viewfinder.  The dial  adjusts the magnification of the optical viewfinder to suit the eyesight of the photographer.  That means if you wear glasses you can remove them and adjust the diopter to your vision and then shoot more confidently without your glasses.  Of course it cannot correct for your vision 100% but it is a step towards making photographing without your glasses easier.