The most popular models and options cost about the same as a good pair of prescription eye glasses. Diopter refers to the curvature of a lens. As the diopter increases, the lens becomes thicker and the curvature greater. As the curvature increases, light rays are redirected to fill a greater portion of the person's retina which makes the object look bigger. Higher diopter lenses have greater magnification power.
Power refers to how much larger an object is made to look through a magnifying lens.
Focal length is important when the task requires using tools with the object being viewed. Because focal length decreases as power increases, there is less room to perform work on an object under higher power lenses. The field of view is the size of the magnified area that is in focus under the lens. The field of view decreases as power increases. More powerful lenses make small details look big, but less of the total object is visible.