Lens review

Sony 35 mm f/1.4G

2 July 2009
Arkadiusz Olech

5. Chromatic aberration

At the beginning, some words of explanation and a reminder in what way and in what units we present the chromatic aberration results. The Imatest software draws a vertical black-to-white profile edge placed near the side or corner of the image, separately for each of the prime colours.The chromatic aberration measure is best expressed as a maximum difference between profiles drawn in two different colours. You can determine it by measuring the field between two curves and it's given in pixels at the beginning. More about it can be found on Imatest software site.

There is a problem, though, because if you take the measurement in pixels you favour the reflex cameras with bigger pixels. To make the result more objective, you should convert the area in pixel into more rational units like square millimeters. The result still won’t tell us everything, although it will be more objective, because it will eliminate the differences between a full frame and smaller detectors. So the best way is to express aberration by a percentage scale - you measure the aberration in pixels and then you divide the result by a half of the picture diameter, also expressed in pixels. This operation allows us to eliminate partially the influence of the pixels’ size and the detector’s size as well. How it works in practice can be seen in the chart presented below, which shows the Sony 1.4/35 chromatic aberration, measured on the A100 and the A900 picture fringe.

Sony 35 mm f/1.4G - Chromatic aberration

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At first glance the results are strange because the full frame aberration seems to be lower than the small matrix aberration. When we remind ourselves what kind of operations are made to express the results as percentages, we can understand that the A900 aberration is divided by a number 1.5 times bigger than in the case of the A100. If you wanted to compare directly the results from both cameras, you would have to multiply the A900 results 1.5 times, which would give the 0.22-0.23% level, so about 0.02-0.03% more than for the A100.

The problem is that any 1:1 excerpts or similar are not a good way to assess the lateral chromatic aberration because in practice they are taken, if at all, mainly from the center of the frame and this aberration doesn’t exist there. You must take the whole frame into account to assess the aberration well and in this perspective it will bother us less on A900 than on A100, what can be clearly seen in our chart.

It doesn’t change the fact that the Sony 1.4/35 chromatic aberration is not the lowest and it will be perceptible on the pictures. The proof can be seen in these excerpts from the two cameras used in the test.

Sony 35 mm f/1.4G - Chromatic aberration