There are several different types of lens and combination of lenses used in magnifiers. To keep this section as simple as possible the advantages and disadvantages of only the four most common lens designs will be described.
The shapes of some of the lenses illustrated above have been exaggerated to identify the differences more clearly.
Convex lens
These lenses are inexpensive, lightweight and can be viewed through either side of the lens. They are available in powers from 1.5x to 4x. Their biggest drawback is that they tend to distort the image around the edges.
Convex lenses are useful as a “spotting” magnifier for shop prices, checking washing machine controls and cooking instructions. They are not recommended as an aid for prolonged reading activities.
Bi-Aspheric lens
The shape of this lens in the illustration on page 9 has been exaggerated to show the difference in shape from the convex lens. Bi-Aspheric lenses can be used either side and are available in powers from 1.7x to 20x.
When used at the correct distance from the page and the eye, Bi-Aspheric lenses give an undistorted image almost to the very edge of the lens. Their biggest drawback is that if they are not used at the correct distance they cause image distortion.
In other words Bi-Aspheric lenses are not very tolerant lenses – accurate focusing is essential.
Single Aspheric lens
Single Aspheric lenses are best used one way – the flatter side to the page, the more curved side to the eye. They are available in powers from 2.5x to 14x. Single Aspheric lenses tend to be heavier than Convex or Bi-Aspheric lenses.
The greatest advantage of Single Aspheric lenses is that they are extremely tolerant of changes in working distance; they will give a distortion free image from rim to rim at varying distances. This means they are ideal for those who find it difficult to get close to their magnifier.
Flat Field lens
This type of magnifier (sometimes called Dome or Bar Magnifier) is very easy to use. It simply sits flat on the page and therefore there is no focusing problem. Flat Fields are also very tolerant in terms of the eye to lens distance.