Lens (optics) distance from this point to the lens is also known as the focal length, though it is negative with respect to the focal length of a converging lens. Convex-concave (meniscus) lenses can be either positive or negative, depending on the relative curvatures of the two surfaces. A "negative meniscus" lens has a steeper concave surface and is thinner at the centre than at the periphery. Conversely, a "positive meniscus" lens has a steeper convex surface and is thicker at the centre than at the periphery. An ideal thin lens with two surfaces of equal curvature would have zero optical power,
Lens (optics) object off the optical axis of the lens is imaged, where rays pass through the lens at an angle to the axis θ. Rays that pass through the centre of a lens of focal length "f" are focused at a point with distance from the axis. Rays passing through the outer margins of the lens are focused at different points, either further from the axis (positive coma) or closer to the axis (negative coma). In general, a bundle of parallel rays passing through the lens at a fixed distance from the centre of the lens are focused to a ring-shaped