Magnification, also referred to as magnifying power, depends on the focal length of the lenses used in an optical device. At Carson Optical, we calculate the magnification based on measurements of the actual product, not the theoretical properties of the lenses. We use optical measurement equipment, such as a lensometer or lens clock, to measure the power of a lens. This provides the user with much more accurate results of magnification compared to other methods, which are based upon the lens mold or intended design, that might not correspond to the real-life product.
Our advertised magnifying power (MP) is based upon the standard industry equation (also referred to as “trade magnification”) for the maximum magnifying power corresponding to ideal viewing conditions, and it depends on the diopters of a lens or lens system. The diopters of a lens are equivalent to the inverse of the focal length in meters.
MP = D/4 + 1
The above magnifying power is related to the nominal magnifying power as follows: MPnominal=MP-1=D/4. The diopter value (D) used for these calculations is based on empirical measurements from actual samples of the individual lens or lens system, using a lensometer and/or lens meter with a confirmed zero and calibrated to at least two points using known reference standards. The diopter measurement is made according to the back focal length (BFL) specified by the directionality of actual usage of the lens or lens system. The testing is repeated over a sufficiently large sample set to calculate the average actual magnification. The results are converted to magnifying power and rounded to the nearest half power. For example, magnifying powers between 2.25-2.74x are rounded to 2.5x and 2.75-3.24x are rounded to 3.0x.
For magnifying lens systems worn directly over the eyes like reading glasses, the convention is slightly different. These types of products are both designed and marked based on diopters as the primary indicator of optical strength, instead of magnifying power. As is customary in the eyeglass industry, diopters shall be written in format as +X.XX, and converted to the nearest equivalent quarter power of magnifying power as a reference for the consumer.
Magnifying power provides the maximum magnification for spherical lens-based magnifiers, where actual magnification depends on the distances from the object to the magnifier. For our ball and cylindrical lens systems that have a set focal distance or position to the object, we measure the magnification directly at their pre-set positions.