At the full aperture (which is now f/2.8) there is a decent sharpness throughout the entire frame.
At the optimum aperture, f/8, the lens performance is good, reaching maximum resolution in 2 center charts (resolution of "37" is the max. theoretical number in this test). A little troublesome is sharpness dependence on the direction of lines, but this effect is not present in the frame center. There is a 2.7% reduction of the circle diameter at the sides of the frame compared to the circle size at the center and the perfect circle became an ellipse on the side, with 4.6% diameter variability. Chromatic aberration is low. Using it at f/32 and above is pointless due to lack of sharpness.
Analysis with the 400 mm f/5.6 lens at various f stops is given here. The resulting 560 mm lens gives best results at f/16. Dependence of the image quality in various parts of the full 24x36mm frame is shown below, with the average resolution in hundreds of line widths per picture height (LW/PH) given below each chart.
There is unexpectedly (compared to the 2x Extender) disappointing loss of sharpness. The resolution is consistent throughout the entire frame, and is in the mid-2000 LW/PH, however it is difficult to precisely estimate the sharpness because it is uneven, depending on the lines direction.. The loss of contrast is significant. There is a 3.3% reduction of the circle diameter at the sides of the frame compared to the circle size at the center and the perfect circle became an ellipse on the side, with 6.4% diameter variability. Chromatic aberrations are well visible.
Compared to the 2x Extender, this Extender performs surprizingly poor. Despite slightly lower resolutions achivable with it, it also shows distortions, not observed in the 2x Extender. Compared to the Tamron 1.4x Tele-Converter, its performance is only slightly better.
