Pentax HD DA 20-40 mm f/2.8-4.0 ED Limited DC WR
8. Vignetting
At 20 mm and by f/2.8 the brightness loss in the frame corners amounts to 30% (-1.02 EV). On stopping down to f/4.0 it decreases to 20% (-0.63 EV) and it becomes practically imperceptible (10% and –0.30 EV) by f/5.6. In the middle of the focal range the situation is even better. At the maximum relative aperture the vignetting is 24% (-0.81 EV) and it decreases to 22% (-0.74 EV) by f/4.0. Employing f/5.6 allows you to diminish that aberration further to 16% (-0.50 EV). The problem becomes negligible by f/8.0, where the vignetting is just 10% (-0.32 EV).
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At the maximum focal length and by f/4.0 the light fall-off in the frame corners gets to 28% (-0.96 EV). It is an interesting fact that stopping down helps very little with the reduction of vignetting at that focal length: by f/5.6 it still amounts to 26% (−0.87 EV), by f/8.0 it is 22% (−0.72 EV) and by f/11 - 18% (−0.59 EV).
The measurements above concern of course just one precise area within the frame and they don’t tell us everything about vignetting; after all its function graph is important as well. That’s why in our tests we always present such graphs as the one, shown below, with isophotes so contour lines joining points of equal surface brightness; they tell you how fast the brightness loss is as you move away from the frame centre.
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In order to make these graphs more reliable and competent we decided to show you another presentation. This time these are the averaged out values of brightness loss in concentric circles as you move away from the frame centre. The percentage numbers, presented in every picture, state the size of the real surface area, limited from above and from the right with a value of 100% and from below by the presented curve. That’s how you know how much light, overall, you lose due to the vignetting. You shouldn’t mix up those percentage values with the ones, given at the beginning of the chapter because they are not directly connected with each other.
