[dha]

The real magnification of any object, in terms of size, is really a function of both your glass lens and the lens in your eye. Generally, an object appears larger the closer it gets to your eye. The image size produced upon the retina of your eye is therefore limited by how close the object can be placed in front of your eye and still be in focus. The glass magnifier adds to the focal length of your eye lens allowing an object to be brought closer to your eye and therefore appear larger. A rule of thumb is that the distance of distinct vision, about 10 inches in most people, divided by the focal length of the lens, yields the effective magnification.

So as an example, if the focal length of the lens you have is 1 inch, then you divide 1 inch into 10 inches and the answer is a magnification of 10. Of course there are variations in peoples eyes and their ability to focus at short distances, so this is only a close approximation. The focal length of a lens can be approximated by focusing the image of a distant light bulb, 20 feet away or more, onto a sheet of paper. The distance between the lens and paper at the best focus of the light bulb is the focal length.