By "stress" I mean "to use the lens in such way that its weaknesses are exposed." But first....
I. How Not to Stress a Lens
Most lenses in history, and especially now, are capable of excellent results—as long as they aren't stressed. Here's how to see the best performance of your lens under optimum conditions:
1. Shoot in strong white light with the camera pointed away from the light source, with no specular reflections or light sources in the picture and no impinging light shining directly on the objective (outermost lens element). Use at least a short lens hood to block light impinging from a radical angle.
2. Use the widest aperture at which aberrations are minimized, which usually involves stopping the lens down several stops from wide open.
3. Use a fine-grained, low-contrast pictorial film, or your digital imaging sensor at its optimum or "base" sensitivity ("ISO"). With many digital cameras, the base ISO is around 200, even if the camera can be set at lower values.
4. Minimize blur caused by subject and camera motion—this might be accomplished with the use of a tripod or other means of mechanically steadying the camera—and arrange your subject so as to minimize both foreground and background blur.
5. Don't focus too close—lens designs vary, but perhaps stick within a magnitude of 2 or 3 of (50 • ƒ) (where ƒ = the lens focal length). Thus, for a 50mm (2-inch) lens, you would want to place your main subject perhaps 50 to 200 inches (~4–16 ft.) from the camera.
6. Focus carefully. Focus errors are a very common cause of image degradation—much more common than most people realize. (I see focus errors in published work and "great master" photographs regularly.)
7. Don't enlarge the resulting picture in the print or onscreen past a modest limit.
8. (Digital only) to minimize "chromatic aberration" (i.e., purple fringing), don't ask your lens to image complex, fine-lined subjects with high-contrast edges, such as bare tree branches against a white sky, especially near the edge or corners of the frame.
9. (Zoom lens only) use an intermediate setting in the zoom range. Generally, a kit-type zoom will show its strongest performance if you zoom in roughly a third of the way in from its widest setting. (Even though the actual optimum setting will vary from lens to lens, this will usually be close enough.) Doing this will mainly ameliorate geometrical distortion, and falloff (also called "vignetting").
10. Make sure the lens is clean.
With these ten conditions met, you'll probably find that your lens is a very good performer indeed. Even lenses that are very far from state-of-the-art will perform well. This includes many very old lenses and lenses with significant budget compromises, or even the lens permanently attached to an inexpensive consumer camera. Some of the prettiest prints I ever saw were pictures made with a tripod-mounted single-coated Miranda lens from the 1960s. They were just made under conditions pretty close to those described above.
II. How to Stress a Lens
"Stressing" a camera lens basically involves moving incrementally away from these ideal conditions, such that the lens will be less and less likely to maintain peak performance. Here are a few of the ways this can be done....