Lens review

Sigma A 20 mm f/1.4 DG HSM

3 November 2015
Arkadiusz Olech

8. Vignetting

First let’s check how the Sigma 1.4/20 performs in the vignetting category on the small sensor of the Canon 50D. The appropriate thumbnails are presented below.

Canon 50D, f/1.4 Canon 50D, f/2.0
Sigma A 20 mm f/1.4 DG HSM - Vignetting Sigma A 20 mm f/1.4 DG HSM - Vignetting

The problems are slight here, noticeable in fact only at the maximum relative aperture where that aberration reaches 26% (−0.86 EV). On stopping down the aperture to f/2.0 you see it decrease to an imperceptible level of 9% (−0.27 EV).

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Now let’s check how the situation changes when you employ the full frame.

Canon 5D III, f/1.4 Canon 5D III, f/2.0
Sigma A 20 mm f/1.4 DG HSM - Vignetting Sigma A 20 mm f/1.4 DG HSM - Vignetting
Canon 5D III, f/2.8 Canon 5D III, f/4.0
Sigma A 20 mm f/1.4 DG HSM - Vignetting Sigma A 20 mm f/1.4 DG HSM - Vignetting

There are no surprises here. Already in their press release connected to the launch of the tested lens the producer honestly admitted the level of vignetting was high. Our measurements confirm that much. At the maximum relative aperture you lose as much as 65% of light so −3.02 EV. After stopping down the lens to f/2.0 the situation improves a lot but the vignetting still remains distinct, amounting to 43% (−1.61 EV). The moderate level of that aberration, that of 24% (−0.70 EV) can be observed by f/2.8. The problems end by f/4.0 and  f/5.6, where the vignetting reaches respectively 15% (−0.47 EV) and 11% (−0.34 EV). Further stopping down doesn’t have any measurable influence on the vignetting.

Sigma A 20 mm f/1.4 DG HSM - Vignetting

Below you can find a function graph of an averaged light fall-off presented in concentric circles, depending on the distance from the frame centre. The percentage value in every of the pictures states the size of an area limited at the top and on the right by the 100% value and at the bottom by that curve. It indicates how much light is overall lost due to the vignetting. Please, do not mix those percentages with the ones you saw at the beginning of this chapter; these two values are not connected in any direct way.

Sigma A 20 mm f/1.4 DG HSM - Vignetting

f/1.4 f/2 f/2.8 f/4