The Optical performance of a lens (or at least some aspects of its performance) can be expressed on a Modulation Transfer Function Chart (MTF). They are an objective measurement of lens performance, which can be useful for making comparisons between lenses. Of course, sometimes objective measurements miss certain factors; we all have the experience of two pieces of equipment with identical specs giving different results. Nonetheless, these MTF charts are useful.
MTF charts compare a hypothetical lens that allows 100% light transmission to a real lens, which has less than 100% transmission. We can see how the light transference characteristic of the real lens varies from the centre of the lens to the outer rim. A fairly flat line represents even light transmission across the whole area of the lens. A line that dips towards the right shows there is less light entering through the rim than there is in the centre. Flatter responses are superior, but no real lens has perfectly even light transference.
As the centre of the lens gives the best performance we can adjust the aperture of the camera so that only this centre of the lens is used. This will give sharp images. On the other hand, we might want to have our background out-of-focus, in which case we use a wider aperture which corresponds to more of the lens diameter. The softness of the out of focus background is called the bokeh. This is shown on the chart by the distance between lines taken at different aperture settings. The closer the lines are together for different aperture setting the softer the out of focus section will be. This is better bokeh.
In simple terms, the higher the lines are on the MTF chart the better. Flatter lines show a lens has more even light transmission across its whole diameter. The closer the lines are together at different apertures the softer the focus.