The Fotodiox 11 mm extension tube, is quite nice, but I still think it is overpriced at 165 €

Yes - it does impact the overall optical quality / sharpness with Fujifilm GF lenses, knocking them down quite a bit. This is however highly dependent on the individual lens design; Lenses with floating elements take a stronger hit. Which most Fujifilm GF lenses have...(i assume)

I would certainly not want to use a longer extension tube, given how much worse the quality would get ! So, I am glad that i chose the slimmest extension tube on the market available for Fujifilm GFX cameras.

Was it necessary to buy ?

Was this expensive extension tube necessary for me to buy ? No, not really to be honest. I could easily have lived without that expensive tube... while instead focusing on the native capabilities of native Fujifilm GF lenses. But nothing is just black or white. It makes sense for SOME lenses, still !

However - if you wish to do real Macro / closeup photos with high quality - use a real Macro lens. Or switch system for macro photography with appropriate lenses. Most people who bought a Fujifilm GFX, usually have a mirrorless fullframe camera... which is more flexible. Or why not a Micro Four Third camera like Olympus / OM System + lenses - which is an extremely capable Macro system !

Works better on non-floating lens designs

Also, perhaps not surprising is, that when combined with lenses that does not exhibit floating lens elements, the quality is good. Both the Mitakon 65mm ƒ1.4 and the Contax Carl Zeiss Sonnar T* 100mm ƒ3.5 benefit from this little extension tube, getting closer than normal without that the image quality is falling apart.

After all, both lenses exhibit quite a long min focus distance; the Sonnar only focus down to 1 meter, and the Mitakon 65/1.4 to 0.70 meter.

It almost seems as if this extension is better for vintage lensed (which means; those that are performing well on the Fujifilm GFX camera system). Forget about inferior lenses to begin with.

The image above

taken with the GF 55mm ƒ 1.7 lens at ƒ1.7 together with the 11mm extension tube - i sharpened up. And since the photo is smaller than the original, it appears sharper than it was out of the camera. The flowers are super tiny. Around 3 mm in diameter.

Sure, with some digital voodoo in post processing, you can chisel out finer details and make them appear sharper. I mean, why not.

Mitakon 65 mm ƒ1.4 on Fuji GFX
with Fotodiox 11mm extension tube

Contax Carl Zeiss T* 100/3.5 on Fuji GFX
with Fotodiox 11mm extension tube

With Fujifilm GF lenses

Only the GF 55/1.7 and the GF 35-70 mm lenses showed potential with the Fotodiox 11mm extension tube.

The combo with the GF 30mm however, did not impress me. Well it is after all, a 24mm wideangle lens - and i am not into going close to subjects with a strong wideangle focal length... I will not even try to test it with the even wider GF 23mm ƒ4 lens (which has the equivalent of a 18 mm extreme wideangle lens). *LOL*

The light wasn't so strong outside, being already evening. I will of course test this extension tube with various lenses in bright daylight in the upcoming weeks, in order to get steady images without blur/shake.

Given that the Mitakon 65mm ƒ 1.4 lens works so well with the 11 mm extension tube - now allowing me to get closer than 0.7 meter - without that the image quality is falling apart - is a big bonus in my opinion. Making the lens far more flexible in the closer range.

Pretty amazing, that the optical quality can hold up ! Likewise with the Sonnar 100/3.5 lens; the optical quality is absolutely useful in the now closer range.

Definitely something I have use for !