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Lens Testing

The best way to find the sweet spot of your lenses is to run some tests. You can buy lens charts for this purpose, but they're not very useful to non-technicians, as the lines on the lens charts don't reflect the subject matter that you photograph on a day-to -day basis. The most instructive method is to take some test photos of a subject that you photograph regularly. For instance, if you take a lot of landscape photos, carry out your lens test in the landscape.

The testing procedure is very simple:

  1. Take a sequence of photos, starting at the widest aperture of your lens and closing the aperture one stop at a time until you reach the minimum aperture setting.
  2. Set your camera to the lowest ISO setting and mount it on a tripod. Use a cable release and mirror lock-up to eliminate image softening caused by camera shake or noise levels.
  3. Take three photos at each setting (in case one is spoiled by camera shake).
  4. Focus the lens on the most important part of the subject; then switch to manual focus so that the focusing point remains unchanged throughout the series.
  5. If you're testing a zoom lens, test each end of the zoom range separately, as well as any midrange focal length that you use regularly.
  6. Use the JPEG format so that you can examine the images as they come straight out of the camera.

After taking your test photos, compare them at 100% magnification on your computer monitor.

When I test my lenses, I look at both edge and center sharpness (see Figures 5–11) and record the results in a table. Table 1 shows the results for my Canon EF 17–40 mm f4 L lens at the 17 mm focal length. The images taken at both extremes of the aperture range—for example, f1.8 and f22—are significantly softer than the others. The images taken at the aperture settings of f8 and f11 (boldfaced in the table) are the best; they form the sweet spot for this lens. I should try to use these apertures when image quality is my main priority.

Table 1 Canon EF 17–40 mm f4 lens @ 17 mm

Position

f4

f5.6

f8

f11

f16

f22

Center

excellent

excellent

excellent

excellent

good

fair

Edge

fair

fair

good

good

fair

fair

Figure 5

Figure 5 I tested my Canon EF 17–40 mm f4 L lens by taking a series of photos at the 17 mm focal length at apertures from f4 to f22, comparing the results at the edge and the center of the frame. The red rectangles show the approximate areas enlarged in the following photos below.

Figure 6

Figure 6 A 100% enlargement from the upper-left corner of the image, at f4.

Figure 7

Figure 7 This shot, taken at f11, shows a visible increase in image quality versus the shot in Figure 6. The image is much sharper, and the chromatic aberrations are reduced. The photo taken at f4 is also darker at the edges due to vignetting.

Figure 8

Figure 8 Softening caused by diffraction is clearly visible at an aperture of f22.

Figure 9

Figure 9 This enlargement is taken from near the center of the image taken with the aperture set to f4.

Figure 10

Figure 10 A comparison of Figure 9 with this test photo, taken at f11, shows no difference in image quality in the center of the image.

Figure 11

Figure 11 The test photo taken at f22 again shows the softening effect caused by diffraction.

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