Can you convert a glasses prescription to a contact lens prescription? The answer is not directly, but it can be approximated with a vertex conversion formula.
Read on to know the main differences between the two types of eyewear, and find out how to get the right contact lens prescriptions for various refractive errors.
Are glasses and contact lens prescriptions the same?
Though eyeglass and contact lens prescriptions look similar to one another, there are a lot of differences between the two, which can lead to variances in visual acuity depending on the refractive error you have. Remember: eyeglasses sit roughly 1 cm away from your eyes, whereas contacts are placed directly on your eyes. This distinction is so important because the perceived power of your lens will depend on how near or far the lens is from your eye.
What Happens If I Use the Same Prescription?
When taking an eye test for glasses, lenses are placed approximately 12-13 mm from the eyes to mimic the distance between your glasses and your eyes. Increasing the vertex distance (or the distance between your eyes and the lenses) would add more positive power to the lenses, whereas reducing the vertex distance will add more negative power to the lenses. Since contact lenses sit directly on the eyes, they have no vertex distance which adds more negative power to the lenses.
That being said, contact lens prescriptions will need to be adjusted accordingly, otherwise nearsighted individuals can end up with contacts that are too strong, while farsighted individuals may get lenses that aren’t strong enough. This may result in discomfort, blurry vision, and eye strain, which we want to avoid at all costs.
Glasses and contact lens differences
When looking at an eyeglass prescription and contact lens prescription side by side, a very obvious difference you’ll notice would be the cylinder and axis value on a glasses prescription, which you won’t normally find in a contact lens prescription (unless you’re getting a toric or multifocal lens).
Similarly, because contact lenses need to be customized to fit your eye, they require certain measurements that a glasses prescription won’t have. One of these specifications would be the base curve or curvature of the lens that is based on the shape of your eye. Another would be the diameter or size of the lens, which also needs to be tailored to your eye.
Vertex conversion chart for contact lenses
A handy tool that opticians use to save time on computations and eye tests is a vertex conversion chart, which includes several figures for different lens powers. If you’re nearsighted, you’ll notice a negative or minus (-) sign on your prescription, but if you’re farsighted, you’ll have a positive or plus (+) sign on your prescription.
How vertex conversion charts work
Vertex conversion charts are pretty simple to use. You just have to know where to look and understand what the columns mean. The ‘glasses lens power’ column refers to the power on your current glasses prescription. If this figure is negative, then you’ll be converted to the figure on the left of this column. If the figure is positive, then your converted figure is on the right.
One important thing to know about vertex conversion charts is that most of them start at eyeglass prescriptions of +/-4.00D. The reason why they often exclude lower lens powers is because the converted power is not that far from the original. If the vertex distance conversion formula yields a difference of less than 0.25, then it won’t make a significant difference in visual acuity. In other words, if your prescription falls within this range, you can get away with using the same power as your glasses prescription. The only exception to this rule would be if you have astigmatism, since additional measurements will have to be taken to account for differences in the shape of your cornea.
Getting Prescription Contact Lenses
Now that you’ve learned how to get a contact lens prescription through a vertex conversion chart, you can easily shop a wide selection of contact lenses here at EyeCandys. We’ve got clear lenses for those who want their natural eye color to shine through, as well as a variety of prescription colored contacts for those who want to switch up their looks. Check out our stunning range and find your new favorite pair today.
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