Notes on Magnification

Some people are confused about the relationship among diopter, magnification power, total power and focal length. The most common confusion is between magnification power and total power. The following is presented by Bertech to help clear up any confusion.

Diopter is a measurement of a lens’ ability to bend the light of a viewed object and thereby increase its apparent size. Each diopter represents a 25% increase in the size an object is magnified. A four diopter lens (referred to as a 4d lens) would increase the size of an object by 100%. Viewing a 2” wide object under a 3 diopter (75% increase) magnifier, would increase the size of the object by 1.5” and cause the object to appear to the 3.5” wide. The diopter of a lens can be calculated by dividing the number 40 by the focal length (in inches) of a lens. Once the diopter number is known, other parameters such as magnification power and total power can be calculated.

Magnification power is a measure of how much larger an object is after magnification. A magnification power of 1x means the object’s size has been increased by 100%. Our 2” object seen through a lens with a magnification power of 1.5x would appear to have added 150% on to its original size. So the 2” object will appear through the lens to have increased in size 3”. Its new size would be 5 inches (2” original size + 3” increase in size). Since the magnification power represents 4 times the diopter number, divide diopter by 3 to calculate it. This is the type of power that the Military Standard uses, but total power is generally used in the industry.

Total power, like diopter and magnification power, also refers to the ability of the lens to increase the size of a viewed object. However, total power relates the final size to the original size. Therefore, total power is always 1+ the magnification power. Looking at our 2” wide object through a magnifier with a total power of 4 would make it appear 4 x 2” = 8” wide, but its magnification would be 3 x 2” or 6”. Caution: Some people use magnification power and total power interchangeably. If you’re not sure which one is being quoted, ask for the focal length or the diopter value of the lens. Then use the Magnification Chart to determine the total power.

Focal length is inversely proportional to diopter. The stronger the lens, the closer the work must be to the lens to focus the object. Too large a diopter magnifier may make it impractical to work under the magnifier. The focal length can be calculated by dividing the diopter number of the magnifier into 40. A 3 diopter magnifier would have focal length of 40/3 = 13.3”. Other considerations in selecting a magnifier are lens diameter, shape, reach, lighting and price. In this section, Bertech presents a large selection of magnifiers with many of these options.

*Industry rates lenses by total power and not by magnification power. Bertech has followed that standard in the following pages. Whenever an “x” appears after a lens number, it refers to total power. The only exception is that Military Standard S-45743E, DOD-STD-2000-1-B and WS-6536E call for 4x magnification power (16 diopter) lenses for inspection of solder joints. This is equivalent to a 5x total power lens.

Magnification Chart
DIOPTER TOTAL 1 POWER TOTAL 2 POWER FOCAL LENGTH" DIOPTER TOTAL 1 POWER TOTAL 2 POWER FOCAL LENGTH"
1 0.25 1.25 40.00 2 0.50 1.50 20.00
3 0.75 1.75 13.33 4 1.00 2.00 10.00
5 1.25 2.25 8.00 6 1.50 2.50 6.67
7 1.75 2.75 5.71 8 2.00 3.00 5.00
How strong is your existing magnifier?
To determine the diopter value, place a piece of paper with writing under the magnifier, move it away until it’s out of focus, and bring it back in until it just goes back in focus. Then measure the distance (in inches) from the center of the lens to the paper, and divide that distance into the number 40. The result is the diopter of the magnifier. 		The 25% Rule
Each diopter number increases the apparent size of a magnified object by 25%. As an example, if you want to view something twice as big as it is, you would need to increase it 100%, or four 25% increments. Each diopter will give you one 25% increment. Therefore, you would need a 4 diopter lens.