Fringer EF-FX Pro II with X-T4

Just got my new X-T4 (very nice) with the 16-80 kit lens (better than average kit), so far very enjoyable.

Was also looking for max zoom range lens for travel, and having checked many reviews, decided on a Tamron 18-400 (EF-S mount) with a Fringer adapter.

First impressions on this combo: it works (just about) - don't expect 100% AF hits, and don't try low light at any zoom setting.

Most important (didn't know this in advance): Fringer prevents X-T4 from using IBIS with an OIS lens! As Tamron OIS offers about 2.5 stops (compared to X-T4's claimed 6.5), you can expect noticeable camera shake when handheld at anything slower than 1/250 at 400 mm and perhaps 1/30 at 50 mm, unless your hands are as steady as a tripod:). Remarkably - with the kit 16-80 even 1/4 handheld is feasible at 80 mm (if you stop breathing, that is:)!

Quote from Fringer support:

...

[me]

Now I understand why IS is quite weak - the Tamron has 2.5 stops, while X-T4 has 6.5 stops. For example - at 50 mm I can shoot hand-held up to 1/4 shutter speed with my kit 16-80 lens, but have to go to 1/30 with the Tamron.
Is there any chance you could change this in a future firmware release?

Kind regards,

...

[fringer]

No, so far the adapter’s IBIS support is weak. Lenses longer than 100m won’t get reliable IBIS at all. That’s why we always use lens IS first. I’m sorry.

The above notwithstanding - Fringer works, and is a nice product. Just don't expect an ultra-zoom used in this way to match the performance of the OEM lens.