The task of the lens-maker is to fabricate a lens that projects a faithful image. Only one glass lens element will project an image, but that image is flawed. This lens will be fashioned convex – convex. This shape resembles a lentil seed hence the name lens. This is because the job of the lens is to gather light rays and then redirect them so that an image is formed on the flat surface of film or digital image sensor.
Alas, every lens ever made is flawed. The optical world is plagued by seven major lens errors called aberrations. The lens maker strives to eradicate each aberration, but, so far, this has proved to be impossible. The best we can do is to greatly mitigate each one. Technically is will take seven lenses working in consort. Some of these will be glued to partners; some will be air-spaced. Some will be made from dense glass, and others from lightweight glass.
The basic shape of each lens element forms a segment of a sphere. Some of the lenses in the group will be convex (bulge outward) some concave (curved in). Try as we might, the shape (figure) of the polished glass surfaces will have some flaws. A common flaw is an error in the steepness of the curve. When the steepness is different left-to-right as opposed to up-down, a circle will image, not as a perfect circle but slightly squished (oval). This is the classic indication that the aberration called astigmatism is under-corrected.
The up-down vs. left-right figure inconsistency can actually be in any direction like slanted etc. When imaging graph paper (cross lines) horizontal lines vs. vertical lines will image with different degrees of sharpness. The shaper lines display increased contrast thus the upright of a cross might image more distinct than the cross piece.