The magnifying glass is the simplest system of increase. The most normal ones are usually formed by a convergent lens. If we place an object between the magnifying glass and its focus (it is that point where the rays that come from far away focus), the image will be increased more. More when it's on the focus and the less the closer the lens is.
When we look at an object and we need to observe the details, we will approach it as much as possible. A 10-year-old boy is capable of focussing at 10 cm, while an adult of forty years, at 40 cm. For this reason, by agreement, it was decided at 25 cm as standard distance. The augmentation of a magnifying glass means the number of times that an object is seen larger than when we see it without magnifying glass at 25 cm.
Normally, the usual magnification is usually increased between 2x and 6x. From here, spherical aberration usually appears, which degrades the quality of the image. In order to solve this obstacle, loupes are designed with two and three lenses.