Even at the full aperture (which is now only f/4) there is a decent sharpness throughout the entire frame, but the contrast somewhat suffers, being still better than the 100-400mm zoom can deliver.
At the optimum aperture, f/5.6, the lens performance is even better than 100-400mm zoom reaching maximum resolution in 3 center charts (resolution of "37" is the max. theoretical number in this test), however, the zoom lens is more sharp at the frame sides. There is no reduction of the circle diameter at the sides of the frame compared to the circle size at the center. Notice the roundness of the circles at the sides - they are perfect, the same as produced by the original lens. Chromatic aberration is low. The Extender does not introduce any noticeable distortions. For photo stitching purposes, this is a better composite lens than the 100-400mm zoom at the corresponding focal length. Using it at f/32 and above is pointless due to lack of sharpness. This test also suggests that the real resolution of the 135mm lens is at least double of that reported by most sources (it's over 7400 LW/PH or 154 lp/mm), but this remains to be confirmed by at least 80M pixel full frame camera that does not yet exist.
Analysis with the 400 mm f/5.6 lens at various f stops is given here. This 800mm lens gives reasonable 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.
Even at the full aperture there is some loss of sharpness, but it is consistent throughout the entire frame. The worst part is loss of contrast (see the unevenness of the colors as well), that is significantly enhanced at f/16. At the optimum aperture its performance is reasonable, and certainly you will get more details than using the 400mm lens alone. There are no distortion intoduced by the Extender, so the total distortions are nearly nonexistent. Chromatic aberrations are doubled and become a problem.
Analysis with the 400 mm f/4.0 lens at various f stops is given here. The best performance is ia 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.
The most obvious problem at the full aperture is low contrast and unevenness of the colors. This composite lens loses also sharpnes at the picture sides. At the optimum aperture the sharpness starts out even higher than with the previous f/5.6 lens (as expected from the single 400mm lens charts) but drops considerably towards the frame sides. Also, it is difficult to estimate the sharpness because it is uneven (comparing the circle to a clock, the 10th to 4th hour axis looks the best at the lower left corner of the frame). Chromatic aberration are similarly bad as with the 400 f/5 lens.
