The position of a corrective lens determines how light is bent and focused onto your retina. Because eyeglasses sit away from your eyes and contact lenses rest directly on your corneas, we must adjust the prescription power for each to provide clear, comfortable vision.
Eyeglasses sit approximately 12 millimeters away from the surface of your eye, while contact lenses rest directly on your cornea. This difference in distance changes the way light is refracted, or bent, as it enters your eye.
The farther a lens sits from your eye, the more we need to adjust its power to achieve the same visual result. This is why your contact lens power often differs from your eyeglass lens power, even though both correct the same underlying vision problem.
Vertex distance refers to the space between the back surface of your eyeglass lens and the front surface of your eye. Our eye doctors measure this distance during your examination because it directly affects the lens power we prescribe.
As prescriptions become stronger, vertex distance becomes increasingly important. We apply a mathematical formula to adjust the lens power and compensate for this space, ensuring that light converges precisely where it needs to on your retina.
The power difference between your eyeglass and contact lens prescriptions becomes more significant when your prescription is stronger. For mild prescriptions, the two powers may be nearly identical, but for higher prescriptions, typically those exceeding plus or minus 4.00 diopters, we make more substantial adjustments.
Contact lenses require several measurements that eyeglasses do not. We measure the curvature of your cornea to select a lens that fits properly and centers correctly on your eye. We also assess the diameter of your iris and evaluate your tear film quality to determine whether your eyes can safely and comfortably wear contact lenses.
These additional specifications ensure that your contact lenses move appropriately with each blink and allow adequate oxygen to reach your cornea. Without these precise measurements, even the correct prescription power would not provide comfortable or clear vision.
Both eyeglass and contact lens prescriptions include specific measurements and abbreviations that describe your vision correction needs. While some values appear on both types of prescriptions, contact lens prescriptions include additional parameters related to lens fit that help ensure proper positioning and comfort.
Both eyeglass and contact lens prescriptions list sphere, cylinder, and axis measurements. The sphere value indicates the degree of nearsightedness or farsightedness you have, with a minus sign meaning you are nearsighted and a plus sign indicating farsightedness.
The cylinder and axis values correct astigmatism, which is caused by an irregular curve in your cornea or lens. While these terms appear on both prescriptions, the actual numbers may differ slightly due to vertex distance and how the lens sits relative to your eye.
Your contact lens prescription includes base curve and diameter measurements that never appear on eyeglass prescriptions. The base curve describes the curvature of the back surface of the contact lens, measured in millimeters, and must closely match your corneal curvature to provide a stable and comfortable fit.
The diameter indicates the width of the contact lens from edge to edge. We select this measurement based on the size of your cornea and iris, as a lens that is too small or too large can cause discomfort, blurred vision, or eye health complications.
Prescriptions use standardized abbreviations that may seem confusing when you first encounter them. Understanding these terms makes it easier to discuss your vision needs with our office.
We use a specific mathematical formula to convert between eyeglass and contact lens prescriptions. This formula accounts for vertex distance and ensures that the effective power reaching your eye remains consistent through a proportional adjustment based on your prescription strength, not a simple addition or subtraction.
For example, if you have a high minus eyeglass prescription, your contact lens prescription will typically be slightly less minus. If you have a high plus prescription in glasses, your contacts will usually be slightly more plus.
Contact lens prescriptions also specify the brand name and material of the lens. Different brands and materials have unique properties affecting how they fit your eye and how much oxygen they transmit to your cornea.
You cannot substitute one contact lens brand for another, even if the power, base curve, and diameter appear similar on paper. Different lens designs behave differently on your eye and can significantly impact both comfort and vision quality.
Getting an accurate prescription for eyeglasses or contact lenses requires different testing procedures and measurements. An eyeglass prescription focuses primarily on determining the correct lens power, while a contact lens fitting involves evaluating how lenses interact with the surface of your eye.
During an eyeglass prescription exam, we test your vision using a series of lenses in an instrument called a phoropter. We ask you to compare different lens options while viewing an eye chart, and this process, along with measurements from digital instruments, helps us determine the exact prescription that gives you the clearest vision.
We also measure the distance from your eyes to where your eyeglass frames will sit. Our eye doctors may evaluate your eye alignment and focusing ability to ensure your prescription addresses all your visual needs, not just basic clarity.
Contact lens fitting requires several tests beyond a standard eye exam. We measure the curvature of your cornea using instruments such as a keratometer or corneal topographer. Advanced technology like the Pentacam provides detailed corneal mapping that helps our eye doctors, including Dr. Thanh Mai, OD, FSLS, FIAOMC, who has extensive expertise in corneal tomography, determine the optimal lens parameters for your eyes.
We also evaluate other factors that affect contact lens wear.
We place trial lenses on your eyes to assess how they fit and move during normal blinking. A properly fitted contact lens should center well on your cornea, move slightly with each blink, and feel comfortable throughout the day.
During this trial period, we verify that your vision is clear with the lenses and observe how they perform during various activities such as reading or looking from side to side. This helps us determine whether any adjustments are needed before finalizing your prescription.
New contact lens wearers typically return for a follow-up visit within one to two weeks of beginning lens wear. During this appointment, we evaluate how well the lenses are performing after you have worn them in real-world conditions and check that your eyes remain healthy.
Even experienced contact lens wearers benefit from regular follow-up visits. We can detect early signs of complications that you might not notice on your own, allowing us to make adjustments before small issues develop into larger problems.
Eyeglass prescriptions typically remain valid for one to two years, depending on your age and eye health. Contact lens prescriptions expire after one year in most cases because contact lenses carry higher risk of complications if not properly monitored.
We need to examine your eyes regularly while you wear contact lenses to ensure your corneas remain healthy and your lenses continue to fit properly. Your prescription may change over time, or you may develop conditions that require a different lens type or wearing schedule.
Several factors can make the difference between your eyeglass and contact lens prescriptions more pronounced. Understanding these factors helps explain why your two prescriptions may look quite different, especially if you have a strong correction or specific vision conditions.
If you have a strong prescription, the power difference between your eyeglasses and contact lenses will be more noticeable. The mathematical conversion formula has a greater effect as prescription strength increases, and people with prescriptions stronger than plus or minus 4.00 diopters typically experience the most significant differences between their eyeglass and contact lens powers.
We take extra care when fitting contact lenses for high prescriptions because even small errors in power or fit can cause noticeable vision problems or discomfort. We may try several lens options to identify the best combination of power, base curve, and diameter for your specific eyes.
Astigmatism adds complexity to contact lens fitting because the lens must align at a specific angle to correct your vision properly. Toric contact lenses have design features such as weighted zones or varying thickness that help them rotate to the correct position on your eye.
We may need to adjust the axis value on your contact lens prescription compared to your eyeglass prescription. This adjustment accounts for how the lens positions itself on your eye and any rotation that occurs during normal wear, as small changes in axis can significantly affect vision quality with toric lenses.
Presbyopia is the normal age-related change that makes focusing on near objects difficult, typically beginning around age 40. Both eyeglasses and contact lenses can correct presbyopia, but they use different optical designs, with bifocal or progressive eyeglasses creating distinct zones for distance and near vision while multifocal contact lenses blend these zones across the lens surface.
The unique shape of your cornea affects which contact lenses will fit correctly and provide clear vision. Some people have steep corneas while others have relatively flat corneas, and we select base curves that closely match your corneal shape to ensure the lens centers properly and moves appropriately with blinking.
If you have irregularly shaped corneas due to conditions such as keratoconus, you may need specialty contact lenses fitted by eye doctors experienced in complex cases. Dr. Nathan Schramm, OD, FSLS, FBCLA, uses advanced technology including WaveDyn and SMap systems for precise keratoconus management and specialty lens fitting to provide the clear, stable vision that standard soft lenses cannot achieve.
Recognizing when your prescription is no longer providing optimal vision helps you maintain clear, comfortable eyesight and healthy eyes. Whether you wear eyeglasses, contact lenses, or both, certain symptoms indicate it may be time for an updated prescription.
If your vision is blurry or seems to fluctuate throughout the day, your prescription may have changed or your lenses may need replacement. Blurry vision with eyeglasses often means your prescription has shifted or the lenses have become scratched or damaged.
Fluctuating vision with contact lenses can have several causes. Your lenses may be dirty or torn, they may not be fitting correctly, or your eyes may have insufficient tears for comfortable lens wear.
Frequent headaches or eye strain while wearing your corrective lenses often signal that your prescription is not optimal. You may be working harder than necessary to focus, causing the muscles in and around your eyes to become fatigued, especially when using digital screens or reading for extended periods.
These symptoms can develop with both eyeglasses and contact lenses. Sometimes the issue involves not just prescription power but also how your two eyes work together or how well your current lenses address your specific visual demands.
Contact lenses should feel comfortable and remain stable on your eyes throughout the day. If your lenses move excessively, slide off center, or feel uncomfortable, they likely do not fit properly, and toric lenses that rotate can cause vision to vary as the lens shifts position.
Certain symptoms require immediate attention from an eye doctor. If you experience sudden vision changes, sharp eye pain, significant redness, sensitivity to light, or discharge from your eye, contact our office right away, and contact lens wearers should remove their lenses immediately if any of these symptoms develop.
Do not wait for your regularly scheduled appointment if you notice signs of possible infection or injury. Early treatment prevents serious complications and protects your long-term vision health.
Patients often have questions about the differences between their eyeglass and contact lens prescriptions and when each type requires updating.
No, you cannot use your eyeglass prescription to purchase contact lenses. Contact lens prescriptions require specific measurements including base curve and diameter that do not appear on eyeglass prescriptions, and the lens power itself often needs adjustment to account for the different position of contact lenses on your eye. Ordering contacts with an eyeglass prescription can result in poor vision, discomfort, and potentially unsafe fitting, so always use a current, valid contact lens prescription when ordering contacts.
Contact lens prescriptions typically expire after one year because contact lenses sit directly on your eye and require regular monitoring to ensure your eye health and safety. During annual contact lens exams, we check for subtle changes to your cornea, signs of infection or inflammation, changes in your tear film, and whether your lens fit remains optimal. Your eyes and their needs can change over time, and conditions you might not notice yourself can develop with contact lens wear, so this annual check protects your vision and catches small issues before they become serious problems.
Yes, we can typically determine both your eyeglass and contact lens prescriptions during one comprehensive visit. We perform the standard vision testing and measurements for your eyeglass prescription first, then complete the additional corneal measurements, fitting evaluation, and trial lens assessment needed for contact lenses. Scheduling both together is convenient and ensures that both prescriptions reflect your current vision and eye health, though the appointment will take longer than a glasses-only exam.
Yes, switching contact lens brands typically requires evaluation by an eye doctor even if the basic parameters look similar. Different brands use unique lens materials, designs, and manufacturing processes that affect how the lens fits your eye and how your eye responds to the lens. A lens that works well in one brand may not provide the same fit, comfort, or vision quality in another brand, so we need to place the new lens on your eye and evaluate its performance before confirming it is appropriate for you and writing a prescription for that specific brand.
Wearing contact lenses with incorrect power causes blurry vision, eye strain, headaches, and difficulty with activities such as driving or reading. While this typically does not cause permanent eye damage, it creates unnecessary discomfort and can compromise your safety in situations requiring clear vision. If you suspect your contact lens power is not correct, whether due to an error or a change in your vision, schedule an exam so we can verify your prescription and make any necessary corrections, as continuing to wear an incorrect prescription can also mask symptoms of eye conditions that need attention.
Our Costa Mesa eye doctors provide comprehensive eyeglass and contact lens prescriptions tailored to your unique vision needs and lifestyle. Using advanced diagnostic technology including Pentacam corneal imaging, digital measurement systems, and specialty fitting expertise, we ensure you receive accurate prescriptions that deliver clear, comfortable vision. Contact our office to schedule your comprehensive eye examination and discuss your eyeglass and contact lens options with our experienced optometry team.
