Dioptre & Magnification

Knowledge 18/04/2019
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Dioptre & Magnification

Dioptre refers to the curvature of the lens. As the dioptre increases, the lens become thicker and the curvature greater. As the curvature increases, light rays are redirected to fill a greater portion of the viewer’s retina which makes the object look bigger.
Power (magnification) refers to how much larger an object is made to look through a magnifying lens. Power is typically indicated by an X such 2X or 4X. Daylight uses the most common formula to calculate power: dioptre / 4 + 1

For example, to calculate the magnification (power) of a 3 dioptres lens:
3 dioptres divided by 4 = 0.75 + 1 = 1.75X (the original object appears 1.75X bigger)
5 dioptres divided by 4 = 1.25 + 1 = 2.25X (the original object appears 1.75X bigger)
12 dioptres divided by 4 = 3.0 + 1 = 4.00X (the original object appears 1.75X bigger)

A 3 dioptres lens makes an object look 75% times larger, over and above what the unaided eye already sees. The figure “1” added to the final figure is the original object.

Dioptre vs Magnification power

Dioptre Magnification power % increase
3 1.75X 75%
5 2.25X 125%
8 3.00X 200%
12 4.00X 300%
15 4.75X 375%


Example of magnification

What is focal length? 
The focal length is the optimal focus point from the centre of a lens to the magnified object. It is the distance at which the object is at its sharpest. The focal length is measured in millimetres (mm).

Because the focal length decreases as the dioptre or magnification power increases, an object under a high magnification lens has to be positioned closer to the lens. Reducing the space between the object and the lens.

The higher the magnification power of a lens, the more important the positioning of the object will be in order to see it sharply. The object will go in and out of focus quickly on a high power lens versus a weaker lens. The user has to be more accurate in positioning the object under the lens.

A 3 dioptres (1.75X) lens is the most popular standard magnification lens because it offers good magnification and ease of use.

What is the field of view?
The field of view is the area under the magnification lens that is in focus.

The field of view decreases as the dioptre or magnification increases. A higher magnification lens makes smaller details look bigger, however, less of the total object is in focus because the field of view is reduced. When choosing a magnifier, there is a trade-off between the field of view and the amount of magnification required.