I ordered a Fotodiox extension tube designed specifically for Fujifilm GFX cameras. I chose the shortest version out of those available on the market (from different companies; Fujifilm, Fotodiox, Viltrox - coming in 11mm, 18mm, 20mm, or 45mm length). I only want to focus a not closer sometimes; No macro or anything like that - just a slight closer focus when needed.

Marumi achromat 300 close-up lens

I tried to use the Marumi achromat close-up lens on the Fujifilm GF 55 mm ƒ/1.7 lens - but the resulting quality wasn't good at all. At wide open aperture, the fine details where extremly mushy as well stopped down to ƒ 2.8;in both cases sharpness left a lot to be desired. I immediately knew that the Marumi is no good solution to get a closer focus.

As you can see below, the normally excellent Marumi close-up lens, didn't perform well at 100% screen size resolution on the Fujifilm GF 55mm ƒ1.7 lens.

11mm

I went for the shortest offering, ciming from FotodioX - and its rather expensive for just being a simple extension tube; 1770 SEK (165 EUR). Fujifilm is even worse with twice the price.

My thought is, that the FotodioX 11mm extension tube might be a bit more flexible than I originally thought because I can use it with many other lenses (via adpater), not just native Fujifilm GF lenses. Here in mind, I have for example the Contax Carl Zeiss Sonnar T* 100mm ƒ3.5 lens, with its 1 meter min focus distance, should gain some closer focus, coming in handy.

A word of caution
(lenses with floating lens elements)

Using an extension tube with a lens design that exhibits floating lens elements (FLE) - often results into worse optical quality / sharpness - compared to fixed lens designs.

For example that is the case with the Fujifilm GF 120/4 Macro lens, which achieves out of the box (only) 1:2 magnification. Fuji offers an additional expensive extension tube in order to get 1:1 true macro. BUT - the quality is compromised, especially in the corners, while flatness isn't flat anymore...

The Fujifilm GF 120mm "Macro" lens is a bit of a strange offering, and caused quite a stir in the Fujifilm GFX community. I mean, you pay around 3,000 € for an exclusive macro lens that isn't really 1:1 macro. Then, you have to add €350 for an extension tube, but the lens quality deteriorates.

This is the reason i chose the smallest 11mm tube, so that any potential degradation due to floating lens elements would in theory be negligible because the extension tube is so short.

Sharpness would get a whole lot worse if I would used a longer tube, such as 45mm for example. I don't need "macro" or very close focus, because I already have the Pentax 645 SMC 120mm ƒ4 Macro lens which goes all the way down to 1:1.

Well, we will see, how the 11mm tube quality turns out into.