How this calculator works
Optical power measures how strongly a lens changes ray convergence. One diopter is one reciprocal metre, so focal length must be converted to metres before taking its reciprocal. The sign carries through the reciprocal. This paraxial relation describes effective focal length; combining separated lenses requires more than adding their powers.
Formula
P = 1/f, with f in metres and P in diopters (m^-1); f = 1/P.
Worked example
A +0.25 m focal length is +4 D. A -2 D diverging lens has a focal length of -0.5 m.
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Frequently asked questions
Can focal length be negative?
Yes. It denotes a virtual focal point for a diverging lens under the usual lens convention.
What is a zero-diopter lens?
It has no net focusing power and an infinite effective focal length.
Can I add two lens powers?
Powers add approximately for thin lenses in contact. Separation adds another term.