HD Pentax-DA 20-40mm F2.8-4 Limited
Vignetting
Vignetting, or light falloff, is the darkening of corners or edges of the frame being photographed. While virtually all lenses exhibit some degree of vignetting, it ranges from being unnoticeable to being very pronounced, depending on the lens model.
To test the 20-40mm's vignetting, we photographed the sky and ran through the whole aperture range at one-stop increments at five different representative focal lengths. The results, converted to grayscale, are shown in the table below.
20mm | 25mm | |
| F/max |  |  |
| F4 |  |  |
| F5.6 |  |  |
| F8 |  |  |
| F11 |  |  |
| F16 |  |  |
| F22 |  |  |
30mm | 35mm | |
| F3.5 |  | --- |
| F4 |  |  |
| F5.6 |  |  |
| F8 |  |  |
| F11 |  |  |
| F16 |  |  |
| F22 |  |  |
40mm | ||
| F4 |  | |
| F5.6 |  | |
| F8 |  | |
| F11 |  | |
| F16 |  | |
| F22 |  |
Based on these results we can conclude that the 20-40mm exhibits a moderate amount of vignetting. The vignetting is nowhere near as prominent as it would be on a superzoom lens or even the 18-55mm kit lens, but it is nevertheless quite easy to spot in everyday images, especially toward the longer end of the zoom range. We find that the 20-40mm produces more vignetting than we'd expect of a premium zoom lens, especially one with a predominantly "normal" focal length range.
The table below quantifies the corner light falloff as a percentage of the center brightness. We have also subjectively categorized the vignetting as being negligible, discernible, or easily-visible.
20mm | 25mm | 30mm | 35mm | 40mm | |
| F2.8 | 86% | - | - | - | - |
| F3.5 | 88% | 88% | 87% | - | - |
| F4 | 92% | 91% | 88% | 85% | 84% |
| F5.6 | 96% | 94% | 92% | 89% | 86% |
| F8 | 97% | 96% | 93% | 92% | 88% |
| F11 | 97% | 96% | 95% | 95% | 90% |
| F16 | 97% | 96% | 96% | 95% | 93% |
| F22 | 97% | 96% | 96% | 97% | 95% |
Legend | |||||
| Negligible | Discernible | Visible | |||
At 20 to 25mm, you're in good shape after stopping down by just one stop to F4. At 30 to 35mm, approximately a stop and a half is required (F5.6). At 40mm, vignetting doesn't become hard to spot until F11, which is somewhat discouraging.
Verdict
Even though the 20-40mm has more falloff than we had expected, this degree of vignetting is still very easy to correct: either via in-camera lens corrections, or during post-processing. We are therefore not too concerned with this aspect of the lens, though it certainly does add a bit of overhead when editing photos, especially for RAW shooters.
Full-frame photo taken at 40mm and F8
On the other hand, visible vignetting at F8 isn't something we like to see in a premium lens. After mounting this lens on the full-frame Sony A7, we saw why the perceived vignetting on APS-C is so significant: the lens barely covers the APS-C image circle in the corners at certain focal lengths and apertures. One example of this is shown above.
