How Do I Use A Magnifier and What Is Focal Length?

How to Use a Magnifier

A magnifier works by bending light to make objects appear larger. You might remember using a magnifier as a child to focus sunlight into a single point to burn a piece of paper. To use a magnifier for vision, however, you need to do the opposite of that. Instead of holding the magnifier at the focal point where the light converges, you want to hold it just slightly lower than that point. This will give you a full field of view where everything appears bigger and clearer.

The key to using a magnifier correctly is understanding its focal length, which is the distance from the lens to that single focal point. The focal length is determined by the lens’s power, or diopter. For example, an 8 diopter magnifier has a focal length of about 12 centimeters (1/8 of a meter), while a 39 diopter magnifier has a much shorter focal length (1/39 of a meter). The stronger the power, the shorter the focal length, and the closer you must hold the magnifier to the object.

A common mistake is pulling the magnifier past its focal distance. When this happens, the light rays flip, and the image you see will appear upside down. To get the best results, you must always hold the magnifier short of its focal distance to achieve maximum magnification and a full field of view.

The Importance of a Quality Magnifier

When choosing a magnifier, quality and precision are paramount. Most acrylic magnifiers are made using a process called injection molding. The quality of the final lens depends heavily on the mold’s calibration. Many companies use molds that lose their precision over time, leading to inconsistencies in the lens. A magnifier labeled as an 8 diopter, for instance, might not actually be an 8 diopter. These inaccuracies can also cause visual aberrations that distort the image.

This is why it’s important to choose a brand known for quality and precision. A reputable company will regularly re-polish and re-calibrate their molds to ensure every lens they produce is exactly as specified, providing you with a reliable and accurate magnification tool.

Charlie Saccarelli

President, Chadwick Optical

As President of Chadwick Optical, Charlie Saccarelli is the driving force behind the company’s mission to help every patient left behind by the current health care system. Under his leadership, Chadwick has grown from a simple optical lab into a trusted resource for practitioners around the world looking for ways to help the patients that “can’t be helped.” He is a master optician, a father, a bit of a nerd, and a passionate patient advocate who has lectured worldwide on all things optical.