PD’s & Lens Measurements Explained for Glasses
Understanding your lens measurements is an important part of ordering prescription glasses, replacement lenses and varifocals correctly. At Ilkeston Factory Specs, we professionally measure pupil distance (PD), fitting heights and prescription lens settings in-store to help ensure accurate, comfortable vision for your glasses and lenses.
What Is a PD (Pupil Distance)?
PD stands for pupil distance and is measured in millimetres. It is the distance between the centre of one pupil and the other and helps position your prescription lenses correctly in front of your eyes.
PD measurements may be recorded:
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as one total measurement
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or separately for each eye
For example:
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Total PD: 60mm
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Or Right 30mm / Left 30mm
Accurate PD measurements are especially important for:
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varifocal lenses
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bifocal lenses
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stronger prescriptions
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thinner lenses
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prescription sunglasses
Why Accurate Lens Measurements Matter
Correct lens measurements help ensure your prescription lenses provide clear and comfortable vision. Incorrect measurements can affect:
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visual clarity
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lens comfort
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varifocal performance
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distance vision
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reading zones
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lens alignment
At Ilkeston Factory Specs, we take professional measurements in-store when supplying complete prescription glasses, replacement lens and reframe services.
What Are Fitting Heights?
Fitting heights are measurements used to correctly position lenses vertically within your chosen frame. They are especially important for:
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varifocal lenses
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bifocal lenses
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occupational lenses
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some specialist lens designs
Fitting heights may vary depending on:
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frame size
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frame shape
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prescription type
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lens manufacturer recommendations
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individual wearer requirements
Large, oversized or specialist frame styles may sometimes require adjusted lens positioning for the best visual results.
Prescription Terms Explained
Understanding common prescription abbreviations can make ordering glasses and replacement lenses easier.
SPH (Sphere)
The SPH measurement refers to the main magnifying power of your prescription lens used to correct short-sightedness or long-sightedness.
CYL (Cylinder)
Cylinder measurements correct astigmatism and help sharpen your vision. Not all prescriptions include a CYL value, but it is very common.
Axis
The axis works alongside the CYL measurement and is recorded between 1 and 180 degrees to position astigmatism correction accurately within the lens.
ADD (Reading Addition)
An ADD measurement is used mainly for:
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varifocal lenses
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bifocal lenses
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reading glasses
It provides additional magnification for close-up vision and reading tasks.
Prism
Prism corrections are less common but are essential for some prescriptions where eye alignment support is needed. If your prescription contains prism values, accurate measurements are extremely important during glazing and lens fitting.
Lens Measurements for Varifocals & Strong Prescriptions
Lens Measurements for Varifocals & Strong Prescriptions
Varifocal lenses and stronger prescriptions usually require more precise measurements than standard single vision glasses. Accurate PD measurements, fitting heights and frame positioning all help improve:
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visual comfort
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adaptation speed
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reading performance
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distance clarity
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lens alignment
Some thinner and high-index lenses may also benefit from more precise measurements depending on prescription strength and frame size.
Can You Measure Your PD Yourself?
Some customers may already have PD measurements from a previous optician or online order. However, professional in-store measurements are usually more accurate, especially for:
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varifocals
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bifocals
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strong prescriptions
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specialist frame styles
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reglazing services
If you are unsure about your measurements, our experienced team can take accurate measurements in-store when your buying our new glasses or lenses.
Onsite Lens Fitting & Optical Laboratory
Unlike many online-only retailers, Ilkeston Factory Specs operates an onsite optical laboratory where many prescription lenses are glazed and fitted professionally in-house. This allows us to:
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assess frame suitability
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take accurate measurements
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fit lenses professionally
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advise on lens positioning
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help with replacement lenses and reglazing
We regularly help customers from Ilkeston, Derby, Nottingham and surrounding East Midlands areas with prescription lenses, replacement lenses and specialist eyewear services.
Why Choose Ilkeston Factory Specs?
✔ Independent optical specialists since 1991
✔ Onsite optical laboratory
✔ Professional PD & fitting measurements
✔ Varifocal and specialist lens fitting
✔ Replacement lens experts
✔ Same-day services available on selected lenses
✔ Personalised eyewear advice