Canon 8-15/4 test: Canon RF1,4 (Commlite adapter) vs. Kenko 1,4 (Canon adapter)

In case any Canon full-frame mirrorless photographers are interested:

I tested a modified Commlite RF/EF adapter with the Canon RF 1.4x extender against the Canon RF/EF adapter with the Kenko 1.4x Teleplus Pro 300 DGX extender on my R5 and the EF 8-15/4 fisheye lens, unfortunately so far only above water.

My primary goal was to determine whether the overall image quality with the Canon extender (which I already owned) was better than with the (relatively old) Kenko, which I've been using for many years on Canon DSLR cameras and for the last three years on my R5 with the 8-15/4.

I tested at both 15mm (180° diagonal) and 21mm focal lengths at f/5.6, f/8, f/11, f/16 and f/22.

Generally speaking, the Canon RF 1.4x is significantly superior to the Kenko 1.4x in the corners at both focal lengths and all apertures. In the center of the image, there's hardly any difference (you can see something at larger apertures) - and I even examined all the images at 200% and 400% zoom to be able to make a truly precise comparison.

Furthermore, the Kenko vignettes significantly more than the Canon and has a cooler color tone.

Naturally, the difference in image quality will be less noticeable and significant behind a dome port!

It's somewhat annoying that the Commlite doesn't display the aperture correctly (the camera only recognizes the aperture without the extender) and the focal length is also incorrectly displayed (15mm is recorded as 11mm and 21mm as 15mm in the EXIF data).

The Kenko, like the Commlite, also displays the aperture incorrectly, but at least the focal length is correct.

With the Commlite/Canon combination, you should also deactivate the in-camera IBIS, otherwise the sensor will wobble strangely.

Since I also own the Canon RF 2x extender, I took the opportunity to test the combination with the 8-15/4 at 16mm and 30mm.

Generally speaking, at 16mm, the corner performance is noticeably worse than with the 1.4x extender at 15mm. At 30mm, the overall image quality (including the corners) is surprisingly good – best at f/11.

Therefore, if you expect to only shoot shy big fish and not wrecks or reef scapes, this combination could be a solution.

Please note that these test results are only valid for above-water!

I now need to find some zoom rings for use with both extenders.
If anyone knows of alternatives to Nauticam (they don't sell one for the 2x extender - as far as I know - and the one for the 1,4x is incredibly expensiv), please feel free to comment here.

Sorry that I can't provide any images from this test at the moment due to time constraints.