Sph means sphere and determines the spherical power of the lens. If the value is negative it means that you are myopic or "short-sighted", if the value is positive you are hyperopic or "long sighted".
BC means base curve and determines the radical curvature of the back surface of the lens.
Dia means diameter and tells us how far it is straight across the lens from edge to edge.
If the lens corrects astigmatism besides myopia or hyperopia there are additional markings on the box.
Cyl means cylinder. This number tells us how big astigmatism the lens corrects.
Axis determines in which direction the astigmatism lays.
Here I will only talk about soft lens material since soft lenses are dominating the market today. There are a lot of materials to choose from since every manufacturer has their idea of how the best lens is made. To bring some order, FDA have categorized the materials in 5 categories.
I will now try to give you some pros and cons on these different materials. The siliconhydrogel lenses are so different from the hydrogels so I choose to compare the siliconhydrogel to the hydrogelmaterials 1-4 as one group, but the pros and cons given on each hydrogelmaterial is a comparison towards the other three hydrogelmaterials.
-Great oxygen transmissibility to the cornea.
-Good resistance against drying up in the eye.
-High stiffness in the material.
-Low surface wettability, noticeable is that the newest siliconhydrogels are better in this point of view.
Hydrogel lenses with high water content
-Low stiffness in the material.
-Good visual acuity due to thick lens.
-Easy to damage.
-More likely to attract debris.
Hydrogel lenses with low water content
-Less likely to dry out in dry environments.
-Can be made thinner.
-The materials have low oxygen-transmissibility.
-Hydrogellenses with ionic vs non ionic material.
-Ionic materials have larger water transmission through the lens.
-Ionic materials are more likely to attract protein debri.
So which lens should I choose? Well, there is no simple answer to that question, it depends on how you wish too use the lenses, your need of correction and the environment you spend time in.
Soft lenses are the most common type of lens and are what you normally relate to when you talk about lenses. A soft lens has a predetermined length of life, from one day to a year. The soft lens is almost always instantly comfortable but a rigid lens takes some getting use to, from a couple of days up to a month. After the breaking in time the rigid lens will also be comfortable. A rigid lens doesn“t have any predetermined length of life. Instead it's the condition of the lens that determines when it's time to change the lens.
There are several reasons to why the two prescriptions don't match. If you have correction for astigmatism in both prescriptions they are almost never the same and that depends on that glasses are made with much smaller steps in astigmatism correction.
If you have a low need of astigmatism correction you will most likely not have any correction for astigmatism at all in your lenses. If you have high myopia or hyperopia the power on the prescription will vary depending on that the distance from your eye to the glass differs from the distance between your eye and the contact lens.
Yes it is, if you have mild astigmatism of 1-2 diopters the result will be very good, but if you have more astigmatism like 3-4 diopters you will most likely not have as good visual acuity as with your glasses. If you have severe astigmatism like 6-7 diopters, a rigid lens would probably be your best option.
Yes it is, there are two different ways to deal with presbyopia. The first is to have correction for distant vision on one eye and for reading distance on the other eye. The second option is to wear progressive contacts which have optics for both distant and near vision in each lens.
You should not wear contacts when you have a cold, are ill, if your eyes feel strained or are more red than usual. It could also be ill advised to wear contacts in certain environments such as bakeries or construction sites because of the amount of dust in the air.
To take a nap for let's say half an hour is not a medical issue regardless which lens żou use, but it could feel a bit dry in your eyes when you wake up again. If you wish to sleep a whole night with your lenses you must have a certain type of lens that is designed for that kind of use. You should also consult your optometrist before you start to sleep with your lenses.
It's not certain that two different lenses behave the same on the eye just because the parameters are the same due to different materials and thickness. You should never use a lens that you haven't been advised too use by your optometrist.