After shooting and analizing all real-life and resolution target pics, huge discrepancy between measured and visual sharpness was noticed with Sigma 30 mm f1.4 EX DC. There were two possible causes:
1)The lens has lower resolution when shot near (target is placed at 1 meter distance) than when close to infinity (theatre building was some 100 meters away), and
2)Sigma 30f1.4 has problem of nonconsistent focusing with Nikon D70 body. The later problem was already reported by some users.
In order to find the cause and conclude this comparison with exact resolution figures for the Sigma lens, I have performed another test where test target was shot again, but in a different way. Every aperture setting was shot 3 times, using different focusing method for each shot.
First method was to turn manually focusing ring to the closest possible setting (0.4 m) and then engage AF. Second was autofocusing after the ring set to infinity. Third shot in a row was performed after focusing manually, relying on D70 green focus indicator.
Results are shown in this chart. Three conclusions could be drawn from this test:
1) Sigma 30mm f1.4 has potentialy great resolution
2) It is often hard to reach that high resolution as it depends on focusing method and the sheer luck
3) Highest and most consistent resolution figures could be reached by turning focusing ring to infinity prior to activating AF (via the shutter button or other means).
All observations were done using proven good Nikon D70 body. Nikkor 35mm f2.0 tested along with Sigma 30f1.4 didn't show such pecularities.