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Back vertex power (BVP) is a specific measurement of an optical lens's strength. It measures the focusing power at the back surface of the lens, which is the side closest to your eye. In the world of optometry, when a doctor writes a prescription for glasses, they are specifying the back vertex power.
read more about back vertex power in lenses ...To ensure you see clearly, the power of your lens must be precise. Optical standards state that the power must be measured from the back vertex, not the front. When an optician checks your glasses using an instrument called a lensometer, they place the lens so the back surface rests against the machine's stop. This measures the light exactly as it leaves the lens to enter your eye.
Every lens has two surfaces: a front and a back. The power measured at the front (front vertex power) is often different from the power at the back. For very thin, low-power lenses, the difference is tiny. However, for thick, high-power lenses, the difference can be significant. Because your eye sits behind the lens, the back vertex power is the only one that matters for your vision correction.
The goal of any prescription lens is to move the focal point of light directly onto your retina. The position of the lens relative to your eye changes where that focal point lands. By using the back vertex power as the standard, lens makers ensure that the light is focused correctly regardless of how thick or curved the front of the lens might be.
Because back vertex power depends on the position of the lens, the distance between the lens and the eye is important. This is called "vertex distance." This is why a contact lens prescription is different from a glasses prescription. A contact lens sits directly on the eye (zero distance), while glasses sit about 12mm away. This change in distance changes the effective back vertex power required to focus the light.
When switching from glasses to contacts, doctors use a formula based on back vertex power to adjust the prescription. For prescriptions over +/- 4.00 diopters, the power must be changed significantly. For a nearsighted person, the contact lens power will be lower than the glasses prescription. For a farsighted person, the contact lens power will be higher. This compensation ensures the light hits the retina at the exact same spot despite the change in vertex distance.
Yes. The numbers written on your prescription by your eye doctor refer specifically to the back vertex power needed to correct your vision.
It is measured using a device called a lensometer (or focimeter). The lens is placed in the device with the back side facing the eyepiece to get the correct reading.
The front curve, or "base curve," affects the magnification and the physical fit of the lens, but the final total power is always adjusted to match the required back vertex power.
You do not need to worry about measuring back vertex power yourself. This is a technical standard handled by your eye doctor and the optical lab. However, if you have a strong prescription and you switch between contacts and glasses, ask your optician to explain how the change in vertex distance affects your power. They can show you why the numbers on your two prescriptions are different.