Tamron 20-40 mm f/2.8 Di III VXD
8. Vignetting
| A7R III, APS-C, 20 mm, f/2.8 | A7R III, APS-C, 20 mm, f/4.0 |
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| A7R III, APS-C, 30 mm, f/2.8 | A7R III, APS-C, 30 mm, f/4.0 |
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| A7R III, APS-C, 40 mm, f/2.8 | A7R III, APS-C, 40 mm, f/4.0 |
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In this case at no focal length there are no problems whatsoever. If you apply the widest angle of view available and the f/2.8 aperture you have to take into account the loss of just 17% (−0.54 EV) of light in the frame corners. The problem disappears practically completely by f/4.0, where it is just 13% (−0.41 EV) and by f/5.6, where it is 12% (-0.40 EV).
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Even better results you can observe in the middle of the focal range – by f/2.8 and f/4.0 the vignetting was, respectively: 16% (−0.49 EV) and 10% (−0.31 EV).
At the maximum focal length and the maximum relative aperture vignetting amounts to 19% (-0.61 EV) and it decreases to an imperceptible level of 9% (-0.27 EV) on stopping down the aperture to f/4.0.
Now let's see how the tested lens fares on a full frame detector – appropriate images are shown below.
| A7R III, FF, 20 mm, f/2.8 | A7R III, FF, 20 mm, f/4.0 |
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| A7R III, FF, 30 mm, f/2.8 | A7R III, FF, 30 mm, f/4.0 |
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| A7R III, FF, 40 mm, f/2.8 | A7R III, FF, 40 mm, f/4.0 |
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In case of the most difficult combination of the wide angle and maximum relative aperture the loss of light in the frame corners reaches 63% (−2.89 EV); then it decreases to 48% (−1.88 EV) on stopping down the aperture to f/4.0 and to 40% (−1.48 EV) after applying the f/5.6 aperture. By f/8.0, f/11.0 and f/16.0 we got, respectively: 35% (−1.26 EV), 34% (−1.19 EV) and 33% (−1.16 EV).
The situation gets noticeably better in the middle of the focal range. By f/2.8 vignetting amounts to 40% (−1.47 EV) and decreases to a value of 28% (−0.94 EV) on stopping down the lens by 1 EV. By f/5.6 we got a result of 26% (−0.86EV), and by f/8.0 and f/11.0 the same values, reaching near 25% (−0.85 EV).
Additional slight decrease of vignetting can be seen at the maximum focal length. In its case by f/2.8 the brightness loss in frame corners amounts to 37% (−1.34 EV), and by f/4.0 it decreases to 26% (−0.88 EV). Any noticeable problems end by and large by f/5.6 and f/8.0, where the results keep the same level of about 22% (−0.73 EV).
| Sony A7R III, 20 mm, f/2.8 |
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| Sony A7R III, 30 mm, f/2.8 |
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| Sony A7R III, 40 mm, f/2.8 |
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