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Spherical Equivalent Formula:
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The Spherical Equivalent (SE) is a calculation that converts a sphero-cylindrical prescription into a single sphere power. It's commonly used for contact lens fitting and in certain refractive procedures.
The calculator uses the Spherical Equivalent formula:
Where:
Explanation: The formula accounts for the fact that the cylindrical power is distributed across different meridians by taking half of its value and adding it to the spherical power.
Details: SE is particularly important when converting spectacle prescriptions to contact lens prescriptions, in refractive surgery planning, and when simplifying prescriptions for patients with low astigmatism.
Tips: Enter both sphere and cylinder values in diopters. The cylinder value can be positive or negative. The calculator will automatically compute the spherical equivalent.
Q1: Why is spherical equivalent used for contacts?
A: Contact lenses typically correct astigmatism differently than glasses, so the spherical equivalent provides a good starting point for fitting.
Q2: When shouldn't spherical equivalent be used?
A: For high astigmatism (>1.50D) or when precise vision correction is critical, the full sphero-cylindrical prescription should be used.
Q3: Does the axis affect spherical equivalent?
A: No, the axis isn't used in the SE calculation since it only considers the magnitude of the cylinder, not its orientation.
Q4: Can SE be used for all contact lens types?
A: While SE works for spherical lenses, toric contact lenses (for astigmatism) require the full prescription including cylinder and axis.
Q5: How accurate is spherical equivalent?
A: For low to moderate astigmatism (≤1.00D), SE typically provides good visual acuity. For higher astigmatism, individual tolerance varies.