The power and strength of magnifiers
The way the power and strength is marked on magnifiers may vary, depending
on the make and brand. Usually we describe the power of a magnifier in terms
of “x” (times), but you may also notice that magnifiers are given a dioptre rating
“d or D”. A dioptre is the technical description measuring the power of a lens
and may differ across different manufacturers.
Predicting the required power of a magnifier
The power of a lens directly governs the size of the magnifier. In other words the stronger the power of a lens the smaller the diameter will be.
There is no such thing as a large powerful magnifier. As magnifiers get stronger, their working distance gets shorter. This means that the stronger the power of a lens, the closer you have to hold the magnifier to your eye, and the closer you have to hold the print to the magnifier.
The prediction chart (please click this link: Prediction Chart) will help you predict the level of magnification that you are likely to require in order to read newspaper size print. Obviously this prediction can only be used as a rough guide since many other factors – including the level of lighting or the strength of your current reading glasses, can affect the level of magnification needed.
To get the most out of a prediction chart:
• Always wear your reading glasses, even if you have found them of little benefit in the past.
• Use a desk light to add additional light onto the page. For best results direct the light in front of you onto the prediction chart.
• Hold the chart at 25 centimetres (10 inches) from your eyes or at your preferred distance where you feel most comfortable and can see the chart easily.
• Starting with the largest print, read down the various print sizes until you find it too difficult. The smallest print size that you can comfortably read should then be used for your prediction.
• Look at the recommended lens power written on the right hand side below the last print size that you can read.
For example if the smallest print size you can read at 25cm is N32, then you require approximately +16d (5x) magnification in order to read newsprint.
(Above text taken from the Royal National Institute for the Blind's website)