Most of the effect that a lens has on the light passing through it comes from the surface curvature of its elements. This curvature has to be both precise and very consistent as even the smallest deviations can introduce significant image distortion.
The picture shown here is another example of something that I happened to snap when I spotted an interesting optical effect. It records the view seen through a window of two garage doors. Panes of glass usually have zero curvature (which means they are flat) but only the left-hand pane is actually flat in this case. The right-hand pane is rippled (has variable curvature) and as a result it is clear that the image seen through that part of the window has become distorted.
This distortion would not normally be noticed as our eyes are very good at adapting in response to the information they receive and our general expectations of how the world should look. In effect, the eye sees the distortion caused by the variable-curvature on the surface of the glass and simply ignores it!
Another very common example of this effect comes from the fact that a white sheet of paper looks white to the naked eye regardless of whether it is viewed under artificial light (indoors) or natural sunlight (outdoors). As anybody who has played with a dSLR’s White Balance control will know, or anybody who experienced the days of film will recall, the colour temperatures of these two illuminants are very different. This means a piece of paper that looks white outdoors will actually look orange when viewed without correction under artificial lighting.
Returning to our random-curvature pane of glass, it is clear that any smooth surface, whether it has zero curvature (is flat) or some other profile, needs to be smooth to a very high degree of accuracy if image distortion is to be avoided.
It is therefore interesting to ponder whether the elements in lenses might degrade over time. The window shown above contains a new pane of glass in the left-hand frame and an original (Victorian) pane in the right-hand pane. Glass, being a (very stiff) liquid rather than a true solid, has a finite viscosity and will creep over time, resulting in a change in curvature. In theory, that might happen to elements within a lens, leading to image distortion, but I have to be honest and say that I have never seen evidence of this effect in practice.
The variable-curvature pane of glass did not get into that state through aging but rather was almost certainly manufactured using a process that made surface imperfections more likely than is the case in modern sheet-glass manufacturing. This too might indicate that aging creep is a tiny effect and that our lenses should remain in-tact for many, many years to come.
Nevertheless, variable-curvature distortion is interesting to observe as a reminder of the extent to which the surface of an element determines the optical quality of any image that is viewed through it.