Focal length is the
distance from the center of a lens to the point where the light rays
converge and the object is in optimal focus (focal point). This is
also known as the "working distance" of the lens. Focal length is
important when the task requires using tools with the object being
viewed. Because focal length decreases as power increases, there is
less room to perform work on an object under higher power lenses.
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What is
Field of View?
The field of view is the
size of the magnified area under the lens that is in focus. The
field of view decreases as power increases. More powerful lenses
make small details look big, but less of the total object is
visible. There is a trade-off for the viewer who must decide between
the size of the field of view and amount of magnification
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What is
the relationship between Light and Magnification
Proper lighting is equally
important as magnification in achieving a good viewing situation. A
magnifying lens would be useless in the dark, so increasing light
levels results in better vision. In fact, the better the quality of
light used with a magnifying lens, the less power is needed. When
less magnification is required, the user has a bigger field of view
and working distance under the lens. Therefore, the quality of light
should be closely evaluated when choosing an illuminated magnifying
lamp.
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What is
a Diopter?
Power refers to how much
larger an object is made to look through a magnifying lens. Power is
typically indicated by an "X" such as 2X or 4X. Unfortunately, there
are two different conventions for computing power that lead to
different results. Below is an example of how two power ratings can
be achieved for the same 8 diopter lens.
Formula: diopter/4=Power 8/4£½2X
The question is, "Does an 8-diopter lens make an object look 2 times
bigger or 3 times bigger?"
Formula a says that the 8-diopter lens makes an object look 2 times
bigger, over and above what the unaided eye already sees.
Formula B says that the 8-diopter lens makes an object look 3 times
larger than it actually is. By including the size of the orginal
object (+1), Formula B adds the size of the image from the unaided
eye to the 2X enlargement from the lens.
Dazor reports power according to Formula A. For example, a 3-diopter
lens makes an object look 75% bigger than what the unaided eye
already sees.
