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What does ‘Diopter’ mean?
When looking at various magnifiers, you’ll come across the term ‘diopter’. This refers to the amount of curvature a lens will have. More curvature means a thicker lens, more magnification and a higher diopter number.
To find the magnification level of a lens, simply divide its diopter by 4, and add 1.
For example, if you’re looking at a 3-diopter lens, it’s magnification = ¾ + 1… or .75 + 1 = 1.75x. Objects viewed under a 3 diopter lens will appear 175% bigger than normal.
A 5-diopter lens = 5/4 + 1… or 1.25 + 1 = 2.25x. Objects viewed under a 5 diopter lens will appear 225% bigger than normal.
What does focal length mean?
Focal length is defined as the distance from the lens to the point where an object is in focus (focal point) and it becomes important if you need space above the object in which to work.
It’s kind of like shining a flashlight on a dark wall. As you move the flashlight (magnifier) closer to the wall, you are reducing its distance (focal length). As you move the flashlight back from the wall, the distance (focal length) increases.
Unfortunately, you can’t have lots of magnification and lots of room below the lens (focal length).
If you need lots of space to work, you won’t have as much magnification available. If you don’t need much working space, you can get stronger magnification, and in fact, magnifiers with higher power are generally reserved for close-in inspection and measurement…
3 diopter = 1.75x magnification at 13” focal length
5 diopter = 2.25x magnification at 8” focal length
7 diopter = 2.75x magnification at 5.5” focal length
As a general rule of thumb, when your magnification gets larger, your lens and focal length get smaller.
What is meant by ‘Field-Of-View’ (FOV)?
The field of view is the size of the magnified area that is in focus under the lens. The higher your magnification, the smaller your field-of-view. Lets go back to our flashlight example. As you move the flashlight closer to the wall (stronger magnification), the spot of light (field-of-view) will shrink.
Tips for optimum use
1. Use both eyes when using a magnifying lamp, just like you would use a comfortable pair of prescription glasses.
2. Make sure the lens is positioned to keep your object in focus, with your eyes are 8” to 10” away from the glass. This will give you the best magnification with the least amount of distortion.
3. For best results, keep your chair height and working surface positioned to maintain good posture. You shouldn’t be leaning away from the lens when viewing the object.