How to Choose a Prescription Lens
Snorkel Down offers optical lenses for
people requiring optical correction. The lenses we offer are known as "Step
Dioptre" lenses, which basically means stock or pre-fabricated lenses.
Although we do offer (+) Positive Dioptre lenses, the majority of snorkelers
who require optical correction will benefit from our (-) Negative Dioptre
lenses for Myopic or negative sphere correction, in other words... for
nearsighted people. Step Dioptre lenses will not correct for cylinder or
astigmatism. If you have a cylinder greater than +1.0 or less than -1.0 you
will need to consider a custom made lens specifically for your prescription.
Our Negative Step Dioptre lenses are available from -1.5 to -10.0 in
increments of -0.5... Our Positive Step Dioptre lenses are available from +1.0
to +4.0 in increments of +0.5.
HOW DO I CHOOSE THE PROPER
STRENGTH OF LENS? You will need to know what your current eyeglass
prescription is. This is usually given to you on a piece of paper when you
have your eyes tested. If you cannot locate the prescription, we suggest
contacting the optometrist who last tested your eyes or the company where you
last bought your glasses. Your prescription may contain some terminology that
we may need to clarify. OD (Ocular Dexter) refers to your right eye. OS
(Ocular Sinister) refers to your left eye. Sphere Refers to the degree of
correction needed by your eye, measured in dioptres. For nearsighted people
this is always (-) negative. Cylinder Refers to the amount of astigmatism in
your eye (the degree by which your eye is not uniformly round). Although step
dioptre lenses do not correct for cylinder, the cylinder number does affect
which lens will best suit you. To best select, you must add one half (1/2) of
the cylinder number to the sphere number. Note: The cylinder may be either (+)
positive or (-) negative, which can make the math a little confusing. You need
to be careful with the (+) and (-) signs.
Here are some examples:
|
EYE
|
SPHERE
|
CYLINDER
|
|
OD (Right Eye)
|
-3.0
|
0
|
|
OS (Left Eye)
|
-2.5
|
0
|
OD : Sphere of -3.0, plus ½ of the cylinder 0 : -3.0 + 0 = -3.0
OS : Sphere of -2.5, plus ½ of the cylinder 0 : -2.5 + 0 = -2.5
So in example # 1 the best Step Dioptre lens for the right eye would be
-3.0, and the best lens for the left eye would be -2.5.
Example #2
|
EYE
|
SPHERE
|
CYLINDER
|
|
OD (Right Eye)
|
-4.5
|
+1.0
|
|
OS (Left Eye)
|
-4.0
|
+1.0
|
OD : Sphere of -4.5, plus ½ of the cylinder +0.5 : -4.5 + 0.5 = -4.0
OS : Sphere of -4.0, plus ½ of the cylinder +0.5 : -4.0 + 0.5 = -3.5
So in example # 2 the best Step Dioptre lens for the right eye would be
-4.0, and the best lens for the left eye would be -3.5.
Example #3
|
EYE
|
SPHERE
|
CYLINDER
|
|
OD (Right Eye)
|
-2.5
|
-1.0
|
|
OS (Left Eye)
|
-3.0
|
-1.0
|
OD : Sphere of -2.5, plus ½ of the cylinder -0.5 : -2.5 + -0.5 = -3.0
OS : Sphere of -3.0, plus ½ of the cylinder -0.5 : -3.0 + -0.5 = -3.5
So in example # 3 the best Step Dioptre lens for the right eye would be
-3.0, and the best lens for the left eye would be - 3.5.
As Step Dioptre lenses increase in increments of -0.5 it may not be possible
to exactly match your requirements. For example if you are in need of a -2.75
lens, the closest Step Dioptre lens would either be -2.5 or -3.0. In this case
you would always move down and choose the weaker power -2.5. Water will also
serve to magnify somewhat, therefore objects viewed underwater will appear to
be slightly nearer and larger than actual. © Deep Blue Gear 2001 - 2006