Magnifiers and Other Optical Devices
Simple magnifiers: usually consisting of a high power
positive lens mounted in some form of handle or stand, magnifiers
are available in magnifications from 1.5x through to 20x. They are
available in non-illuminated, illuminated, stand, hand held, spectacle
mounted and hands free mountings. Up to 4x magnification the simple
magnifier is a relatively easy device to use, being tolerant of
working distance and usually of a relatively large area.
Once the magnification begins to exceed 4x the use
of the magnifier becomes more precise. The main problems experienced
when trying to adapt to using a stronger powered magnifier are principally
related to poor understanding of the technique and limitations of
the unit. When using magnifiers of 4x power or above, it is important
to forget the ‘Sherlock Holmes’ approach to using a magnifier. Strong
magnifiers need to be held quite close to the subject matter, the
actual distance being dependent upon the lens power.
This distance is typically in the range of 15mm to
45 mm. Similarly the distance from the magnifier to the user's eye
is critical to ensuring optimum magnification and field of view.
Again the actual distance is dependent on the magnifier
power but typical examples are an eye to lens distance of 150mm
for a 4x unit reducing to 40mm for a 10x unit. It must also be emphasised
that the magnifier will magnify spaces as well as text so the actual
reading process through a magnifier will be slower than previously
experienced with ‘normal’ vision.
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