Difference in diopters and magnification

For the purposes of selling via Amazon (or any other online website) a diopter and magnification refer to the same thing: the power of magnification provided by the lens. A +1.0 diopter is the same as a +1.0 magnification. Just FYI, a +1.0 is considered as a mild (weak) magnification, usually ordered by first-time reading glass customers. As we age, our cornea flattens out a bit (a condition known as presbyopia) and you'll need a stronger strength. A +3.0 is considered a strong power, usually for folks in their more advanced years (60+ ish). Finally, just a little bit of geek here, every 1/4 of diopter difference (or magnification difference) equates to 1" of focal distance. If you typically hold your Ipad 14" from your eyes and a +1.5 works for you, then a +1.75 would allow you to hold your IPad 15" away. Your brain quickly compensates for this difference, but we never recommend getting a stronger strength than you need since your eyes will get accustomed to this and you really want to fight getting stronger readers than you actually need! I hope this was helpful. Neil at Reading Glass World.

What would be the head circumference maximum that would comfortably wear these glasses?

The beauty of these glasses is that the TEMPLE (ear) part is adjustable! They "clic" and stretch to fit almost any size head!

Where are these readers made?

USA patented invention; company now owned and operated in Wisconsin; lenses made in South Korea; frames and final assembly in Taiwan.

If you are referring to the size of the frame, then this is the standard or Original frame which fits most people. The length is adjustable on the sides by about 10 mm. If you need a larger frame then you need to look for the CliC XXL (also called XL or Expandable in different listings). The XXL is wider, longer, and has an expandable section in the back of the headband, and is available in black, dark tortoise, and transparent smoke gray. If you only need a little bit longer frame and do not need the extra width, then you can also look for the CliC Long. The CliC Long is usually only available in black. A few very small women may need an even smaller frame than the Original, in this case you would look for the CliC Petite.If by size you are referring to the magnification of the lenses, the standard magnifications for the Original frame are 1.25, 1.50, 1.75, 2.00, 2.50, 3.00, and 3.50.