Which type of lens has the same curvature in all meridians?

A spherical lens is characterized by having a consistent curvature across all meridians, meaning it is equally curved in every direction. This uniform curvature allows for the lens to bend light in such a way that it can effectively focus an image onto the retina, regardless of the angle at which the light enters the lens.

In contrast, cylindrical lenses have varying curvature in different meridians, which means they are designed to correct astigmatism by focusing light more effectively in one direction than the other. Aspheric lenses are designed with a more complex shape that reduces distortion and provides a flatter profile, enhancing visual quality, particularly in peripheral vision. Flat lenses, on the other hand, do not have any curvature to refract light, thus serving different functions such as in protective eyewear, rather than focusing images.

Thus, among the options provided, spherical lenses are definitive for their uniform curvature throughout, making them suitable for general corrective purposes in vision wear.