Great weather today, with enough good targets to test! I didn't retest focus in the corners yet - just the central horizontal axis in landscape orientation. Here's what I can confirm about my copy of the 100-400mm lens with the aperture wide open:
- The field of focus is much like that of my Lumix 12-60mm lens: much more area in front of the selected AF target is in focus than behind it.
- Generally, quite sharp from the center to the far edges at all focal lengths when the lens is aimed correctly at a target.
- At 100mm, typically sharp across the entire frame. Zooming in loses some sharpness and depth of field at the extreme edges and corners, as expected.
- My lens is moderately decentered, but after repair seems to handle real-world subjects well enough.
- Except with very near targets, With targets that are level with the camera, my upper left corner is sharpest and my lower right corner gets soft zoomed in past 200mm. It doesn't seem to be a concern with real-world images due to shallow depth-of-field. (My Panasonic 100-300 II is similar.)
- Consequentially, past 150mm the extreme right edge loses sharpness while the extreme left edge remains very sharp. Not a problem at all.
- With very near targets targets lower or higher than the camera, near or far away, results are opposite and significantly worse. Decentering appears past 100mm. The whole left quarter of the frame is soft at around 300mm, while the right side remains very sharp. Sharper than the center (where I'm focusing).
- While this is a real problem, it improves enough at 400mm, where all edges have softness at close distances.
- I bought this lens mainly for photographing wildlife at a distance. For near targets, my Panasonic 100-300 II is more reliable despite having more fall-off at the edges.
- Flat wall targets are difficult to test with, especially at close range, because of the need to precisely align the lens. My most exact tests consistently show a sharp left edge/upper left quadrant with decentering adversely affecting the far right edge/lower right quadrant when zoomed in past 200mm. Not bad enough to be a concern for me, but something I want to keep in mind. Were I to get any closer to the target, I expect the left side would become soft and the right side very sharp. But testing a flat target that close is too difficult and unrealistic. When the lens is not perfectly perpendicular and the focus target is at center, corner focus varies greatly with focal length even when the target isn't so close to the lens.
UPDATE: Further testing proved that distance doesn't matter, just angle to the target. Olympus has offered to try a third repair.
