The magnification is either fixed, or you can change it by changing the lenses, e.g. you have a choice of 10X or 30X magnification. So you could repair a very small object under 10X, and switch it to 30X to inspect the joints.
You can vary the magnification (continuously) by turning a knob, e.g. to choose any magnification between 7.5X and 36X. This is good if you work on objects of various sizes, e.g. from untangling the spring of a small wrist watch to dismantling a large pocket watch.