We measure MTF, or Modulation Transfer Function, as part of our resolution test suite. MTF indicates the pixel-by-pixel contrast across a border between a black line and a white background. When we photograph our test chart, the contrast should be high, and the transition should be abrupt.
Small apertures have an enormous negative effect on both lenses at all focal lengths. Their best performance is consistently within a stop of f/4, and wide open is never significantly worse than their best performance. On the other hand, both lenses lose as much as half their scores at f/22. A word about putting f/22 on these lenses: mistake. On a simple lens, the f-ratio is the ratio between the diameter of the aperture to the focal length of the lens. So at 7mm, f/22 translates into an aperture of 0.318mm. That acts like a pinhole – the optical effect of the edges of the hole gets significant in comparison to the effects of the glass. At such short focal lengths, it's possible to get plenty of depth of field at f/8 or wider, and we strongly recommend keeping the aperture as wide as possible.
The strength of these lenses is their ultra-wide settings. Though it's a convenience to be able to zoom, if they don't give good results at 7mm, they aren't worth the trouble. Both Olympus and Panasonic seem to agree: their lenses have pretty consistent performance across their zoom ranges.
In our resolution tests near the center of the frame, the Olympus lens delivered much better scores for SQF (subjective quality factor), MTF (modulation transfer function) and line-widths per picture height. Subjective quality factor integrates MTF and the human eye's particular response to contrast to indicate how sharp a print will look, and the Olympus scored nearly five percent better than the Panasonic. In MTF and line-widths, Olympus's advantage at f/4 was in the range of 17 percent. It's more significant that the Olympus performs much better off-center. Though we don't have quantitative data on resolution toward the corners of the images, our sample shots show the Panasonic lens falling apart off center.
At the least wide setting, the Panasonic lens delivered marginally better resolution at the center of the frame than the Olympus at f/4. As it did at the widest setting, though, our field tests show the Olympus lens holding up much better than the Panasonic in the corners. The Olympus trails the Panasonic by about one percent in SQF score at 14mm.
At 10mm, the Olympus and Panasonic lenses resolved very comparably near the center of the frame – there was a roughly two percent difference in their scores in our lab testing. Again, though, field tests show that the Olympus has better correction in the corners.