Which vision strength do I need?

In short

Exactly the values from your prescription

Inside the mask, your eyes look through air, not through water – the mask holds an air pocket in front of your eyes, exactly like glasses do. That's why only the lens itself matters: Khroom masks have flat lenses. The image reaches you undistorted and at true size. The exact same correction applies as on land – nothing to recalculate, nothing to add.

Why do you read something different online?

Because online sources often refer to masks with curved lenses. Only those cause the notorious fisheye effect: everything looks magnified and distorted at the edges. That's not a property of water, but a design flaw of the mask – it distorts permanently and causes headaches for many wearers. With a flat lens, this problem doesn't exist – so there's nothing to correct for either.

How to choose correctly

◾ Take the sphere value (SPH) from your prescription – separately for left and right
◾ Use the values 1:1 – don't add to them, don't round up
◾ If your strength isn't available: choose one level lower
◾ Order two individual lenses (left + right) – together they form your corrective lens set

Do you have astigmatism (cylinder / CYL)?

Our lenses correct only near- and farsightedness, not astigmatism. Please don't try to compensate for astigmatism with additional diopters – that won't make the image sharper, it will make it blurrier. Simply go by your SPH value; with mild to moderate astigmatism, vision remains clear in practice. If your astigmatism is pronounced, you can pop the lenses out of the frame and have matching lenses made up by your optician.