Lens review

Pentax smc DA 17-70 mm f/4.0 AL [IF] SDM

8. Vignetting

Universal wideangle zooms with a big focal length range have usually a lot of problems with vignetting, which is especially visible at the ends of the range. Sigma 17-70 mm for instance, at the widest angle lost 40% of the corner illumination. So much praised Zeiss 16-80 mm had its biggest slip-up exactly in this category, losing 42% of illumination. Also the Nikkor 16-85 mm VR at 16 mm reached about 49% of illumination loss.

Compared to these performances, the Pentax 17-70 mm fares very well. At minimum focal length and maximum relative aperture the frame corner illumination loss is 29% (-1.0 EV) and, what is rare, at wideangle it decreases quickly on stopping down. By f/5.6 the vignetting is diminished to a convenient level of 15% and by f/8.0 it practically disappears (9%).

To be honest with Sigma, we must note that at 17 mm that lens is faster than Pentax by 1 EV so it had a more difficult task when it comes to vignetting correction. On the other hand even if we stop it down to f/4.0 the vignetting still reaches 20% so the same level as with Pentax.

At 30 mm there are almost no vignetting problems. Even wide open Pentax has only a 12% (-0.36 EV) score which is very good and doesn’t bother at all. By f/5.6 the vignetting decrease to an imperceptible level of 9%.

At 50 mm and wide open the loss of frame corner illumination is 21% (-0.70 EV). Fortunately, on stopping down by one stop the problem disappears entirely (7%).

At maximum focal length the vignetting increases again and by f/4.0 it reaches 24% (-0.80 EV) but here also this aberration decreases quickly with stopping down, by f/5.6 dropping to 13 % and by f/8.0 – to 5%.

In this category Pentax undoubtedly can be only praised, having one of the best results in the universal zooms class.