Photochromic Lenses vs Separate Sunglasses: Which Makes More Sense for You?
21 July 2026
16:47
You’ve had your eye test, you’ve been handed a card with a grid of numbers and abbreviations, and nobody has explained what any of it means. Sound familiar? You’re not alone – most patients walk out of an appointment with a prescription they couldn’t read if they tried. Here’s what it all actually means.
A glasses prescription is split into right eye (usually labelled RE or OD) and left eye (LE or OS). Each eye gets its own row, and within each row you’ll see some or all of the following: SPH, CYL, AXIS, ADD, and occasionally PRISM. Let’s go through them one at a time.
SPH (Sphere) – Your Main Correction
This is the core of your prescription. A minus number (e.g. −2.50) means you’re short-sighted – distance vision is blurry, close-up is fine. A plus number (e.g. +1.75) means you’re long-sighted – close-up is harder, distance may be clearer. The bigger the number either way, the stronger the correction your eyes need. Zero (or “plano”) means no correction needed for that component.
CYL (Cylinder) and AXIS – Astigmatism
If there’s a number in the CYL column, it means you have some degree of astigmatism – the front surface of your eye isn’t perfectly spherical, so light focuses unevenly. The CYL value tells you how much correction is needed, and the AXIS (a number between 1 and 180) tells the lab which angle to orient the correction at. Astigmatism is extremely common, completely correctable, and not something to worry about.
ADD – The Reading Addition
If you’re roughly 40 or over, you’ll probably see an ADD value – this is the extra focusing power added to the lower part of your lenses for reading and close-up work. It’s what makes varifocal lenses work: the top of the lens uses your distance prescription, and the ADD power is blended in towards the bottom for reading. A higher ADD means you need more help for close-up than you did a few years ago, which is completely normal.
PRISM – Less Common, More Specialist
Most prescriptions don’t include prism. When it’s there, it means your eyes aren’t aligning perfectly, and the lens needs to redirect light slightly to compensate. It’s measured in prism dioptres with a direction (base in, base out, base up, base down). If yours includes prism, we’ll explain exactly what it’s correcting and why.
The one thing worth remembering A stronger prescription doesn’t mean your eyes are unhealthy. It just means they need more correction to focus light properly. The health of your eyes is a completely separate question – and that’s what the rest of the examination checks. |
Your prescription is only one part of how your glasses perform. Lens quality, coatings, frame fit, the position of the lens in front of your eye, and whether the optical centre is aligned with your pupil all affect how clearly and comfortably you see. That’s exactly why a properly dispensed pair of glasses from an experienced optician can feel noticeably different from an online order using the same prescription numbers.
Prescriptions aren’t permanent – they shift over time, especially in children, in your early 40s when ADD values start appearing, and later in life as the lens inside the eye changes. That’s why regular eye examinations matter even when your current glasses still feel fine – a small change caught early means a comfortable update, not a jarring adjustment later.
If you’ve got a prescription card and you’re not sure what it’s telling you, bring it in. We’re always happy to walk through it and explain what each number means for your eyes specifically. Get in touch or call 0141 427 4040.
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A minus SPH value means you're short-sighted - distance vision needs correction. The larger the number, the stronger the correction.
Astigmatism appears as a CYL (cylinder) value with an AXIS. It means the front surface of your eye isn't perfectly round, so an additional correction at a specific angle is needed. It's very common and easily corrected.
ADD is the extra focusing power for reading and close-up, added to the lower part of varifocal or bifocal lenses. It typically appears from your early 40s onwards as the lens inside the eye becomes less flexible.
No - prescription strength measures how much correction your eyes need to focus light, not the health of the eye itself. Eye health is assessed separately during your examination.
It varies. Children's prescriptions can change every year, adults typically stabilise until their early 40s when ADD values begin appearing, and later in life further changes are common. Regular eye tests catch changes early.
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