Confused by SPH vs CYL on a Glasses Prescription? Start Here
Two columns, a few decimals and a mysterious AXIS number. Most people only understand the first column.
Photo by Ksenia Chernaya / Pexels
SPH (sphere) is the main power of your lens, in dioptres. A minus value means short-sighted and a plus value means long-sighted. CYL (cylinder) is the extra power that corrects astigmatism, and it only works together with AXIS, an angle from 1 to 180 degrees. If CYL is blank, no astigmatism correction was prescribed.
What does SPH mean on a glasses prescription?
SPH stands for sphere. It is the main power of your lens, measured in dioptres (D) in steps of 0.25. A minus sign, such as -2.50, means you are short-sighted: distant objects blur, and the lens spreads light out slightly to compensate. A plus sign, such as +1.75, means you are long-sighted: the lens adds focusing power. Ignoring the sign, a bigger number means a stronger lens. The name comes from the shape: sphere power is the same in every direction across the lens, like a slice of a ball. Every prescription lists an SPH value for each eye, with OD for the right eye and OS for the left. If you see PL or plano, the sphere power is zero, which happens when only astigmatism needs correcting. What the numbers mean for your eye health is a question for your optician.
Two small details save confusion later:
- The right and left eye almost never match, so do not worry if OD and OS differ by a full dioptre.
- A minus SPH does not mean "bad" and a plus SPH does not mean "good". They are simply opposite directions of focus.
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What does CYL mean, and why does it need an AXIS?
CYL stands for cylinder, and it measures the extra power needed to correct astigmatism. In astigmatism the front surface of the eye is shaped a little more like a rugby ball than a football, so light focuses at two different points instead of one. Cylinder power is also given in dioptres and 0.25 steps, but it is applied along one direction only. That direction is the AXIS, a number between 1 and 180 degrees, where 180 is horizontal and 90 is vertical. Think of CYL as a second lens laid over the first, with its power switched on along a single line. Without an axis, a cylinder value means nothing, so a prescription with CYL but no AXIS is incomplete. Your optician can tell you whether your reading is normal for you. A blank CYL field simply means no cylinder was written.
Cylinder can be written with a minus or a plus sign. Both notations describe the same lens, which we cover below.
How do SPH and CYL differ side by side?
SPH changes focus equally in every direction, while CYL changes focus along one line only. SPH corrects short-sightedness or long-sightedness, and CYL corrects astigmatism. Both are measured in dioptres in 0.25 steps, and the lab combines them in one lens: for example -2.00 SPH with -1.00 CYL at axis 90. SPH is always present on a prescription, whereas CYL appears only when needed. The practical consequence is that a sphere-only lens is a simple curved surface, while a lens with cylinder needs a toric surface that must be rotated to the exact axis. That is why fit matters more than people expect. If a frame sits tilted on your face, the cylinder rotates away from where your eye needs it, and the world looks slightly smeared even though the numbers are correct.
| Feature | SPH (sphere) | CYL (cylinder) |
|---|---|---|
| Corrects | Short- or long-sightedness | Astigmatism |
| Unit | Dioptres (D), 0.25 steps | Dioptres (D), 0.25 steps |
| Direction | Same in every direction | One direction only, set by AXIS |
| Sign meaning | Minus: near-sighted. Plus: far-sighted | Describes notation, not better or worse |
| Needs AXIS? | No | Yes, always |
| If blank | Plano, zero sphere power | No astigmatism correction written |
How do you read a prescription line, step by step?
Read each eye left to right: SPH, then CYL, then AXIS. Take the line OD -2.00 / -1.00 / 90. It says your right eye needs -2.00 for short-sightedness, plus -1.00 of astigmatism correction along the vertical 90-degree line. An OS line reading -1.50 with nothing after it means the left eye needs sphere only. After those three columns, look for the extras: ADD (the extra reading power used in varifocals), PD (pupillary distance in millimetres, typically 54-74 mm for adults) and occasionally prism. When you copy values into an order form, enter them exactly as written, keep the minus and plus signs, and never guess a missing AXIS. If the axis is missing, go back to your optician rather than filling in a number.
- Find OD (right) and OS (left) and treat them as two separate lines.
- Copy SPH, including the sign.
- If CYL has a value, copy AXIS as well. If CYL is blank, leave both empty.
- Add ADD only if you want varifocal or reading lenses.
- Add PD, either one number or two (one per eye).
A trick most explainers skip: spherical equivalent
To compare strength between two prescriptions with different cylinders, add half the CYL to the SPH. A lens of -2.00 / -1.00 has a spherical equivalent of -2.50. It is a rough guide to overall strength and to how thick the lens will be, not something you enter on an order form.
Why your prescription may be written two ways
Some practitioners write cylinder in minus form and others in plus form. The same lens can be transposed: add the CYL to the SPH, flip the CYL sign, and rotate the axis by 90 degrees. So -2.00 / -1.00 x 90 is identical to -3.00 / +1.00 x 180. If two printouts look different, they may well describe the same lens. Just never mix one notation's SPH with the other's CYL.
Do higher SPH and CYL values change which lenses you need?
Yes. The stronger the combined power, the thicker the lens: minus lenses grow thick at the edge, plus lenses grow thick in the centre. A higher lens index counters this by bending light more efficiently, so the same power fits in a thinner, lighter lens. Standard 1.56 suits low powers. As a rough rule of thumb, we would look at 1.61 from around ±3.00, and at 1.67 or 1.74 from around ±5.00. Cylinder adds thickness along one direction only, so a lens with a large CYL can look uneven at the edge. Since thickness follows the largest power in any direction, count SPH and CYL together, not SPH alone. A smaller frame also helps, because there is simply less lens edge to be thick.
- Low prescription (under about ±2.00): 1.56 is usually fine.
- Medium: 1.61 or 1.67 keeps edges tidy in most frames.
- High: 1.67 or 1.74, ideally in a compact frame.
One more point: if your CYL or AXIS changed at your last eye test, expect an adjustment period. Floors may look slightly tilted and straight lines may look bent for the first days. If it has not settled after a couple of weeks, go back to your optician.
Can you order glasses online with SPH and CYL values?
Yes, as long as you have a current prescription with every value written out. Enter SPH, CYL and AXIS for each eye, add your PD, and pick the lens type. At Lensdy you can choose single vision or progressive lenses, with optional blue light filtering, photochromic or anti-reflective coating, in indices from 1.56 to 1.74. With over 200 frame styles and affordable prescription lenses, you can match a thinner index to a smaller frame without paying for a full designer-shop upgrade. We would test a few shapes in our Prescription Glasses collection. For a first look, Log Black and Skyward Silver are good reference points. Citron and Julian Silver give two more styles to compare. Shipping is free on orders over €59, and most orders arrive within 1-2 weeks.
Before you order, check that the paper is complete: a CYL value always has an AXIS, and both eyes are listed. If anything is unclear, ask your optician before you buy.
Frequently Asked Questions
What if the CYL box on my prescription is empty?
It means no astigmatism correction was written for that eye, so you only need SPH. Leave CYL and AXIS empty on the order form. If you expected a value there, check with your optician.
Is a higher SPH worse than a higher CYL?
They are not directly comparable. SPH is the main focusing power, and CYL is a directional adjustment. Both add to lens thickness, so consider them together, for example through the spherical equivalent (SPH plus half of CYL).
Can I ignore AXIS if my CYL is small?
No. Any CYL value, even -0.50, needs its AXIS. Without it, the lab cannot position the cylinder, and the correction can end up in the wrong direction.
Why does my new prescription show +CYL when my old one showed -CYL?
Probably only the notation differs. Plus and minus cylinder forms can be converted without changing the lens: add CYL to SPH, flip the CYL sign, and move the axis by 90 degrees.
Got your numbers in front of you? Browse the full range at lensdy.com, choose a frame, and enter your SPH, CYL and AXIS exactly as written.