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 under the lens that is in focus. The field of view decreases as power increases. More powerful lenses make small details look big, but less of the total object is visible.
Proper lighting is equally important as magnification in achieving a good viewing situation.
A magnifying lens would be useless in the dark, so increasing light levels results in better vision. In fact, the better the quality of light used with a magnifying lens the less power is needed. When less magnification is required the user has a bigger field of view and working distance under the lens. Therefore, the quality of light should be closely evaluated when choosing an illuminated magnifying lamp.