Irix 15 mm f/2.4 Blackstone review - Vignetting

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Irix 15 mm f/2.4 Blackstone

27 September 2016

Szymon Starczewski

8. Vignetting

First let’s check the vignetting performance on the smaller sensor of the Canon 50D - appropriate thumbnails are shown below.

Canon 50D, f/2.5	Canon 50D, f/2.8

The vignetting is noticeable only in the area closest to the maximum relative aperture. By f/2.5 it is 28% (−0.95 EV), and by f/2.8 it reaches 22% (−0.73 EV). Stopping down the aperture to f/4.0 and f/5.6 eliminates the problem practically completely because the results we got amounted to, respectively, 12% (−0.36 EV) and 9% (−0.26 EV).

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Now let’s see how the situation changes after passing to full frame.

Canon 5D III, f/2.5	Canon 5D III, f/2.8

Canon 5D III, f/4.0	Canon 5D III, f/5.6

Unfortunately the small front element combined with good aperture fastness for this angle of view makes itself felt. As a result you get one of the highest vignetting values we’ve seen in our tests so far. At the maximum relative aperture it amounts to as much as 72% (−3.64 EV) and deserves to be called monstrous. The f/2.8 aperture is equally weak, with a result of 66% (−3.10 EV). Even by f/4.0 you still deal with a high level of that aberration as it gets to 48% (−1.87 EV).

Also by f/5.6 and f/8.0 you won’t have any problems with noticing the vignetting in photos - the results we got are, respectively, 40% (−1.46 EV) and 36% (−1.28 EV). You can approach the moderate levels by applying f/11 and f/16, with the brightness loss being 30% (−1.04 EV) and 27% (−0.93 EV) respectively. As you see even on very significant stopping down you cannot get rid of that aberration completely.