Canon LTM Canon 25mm F3.5!

[38Deardorff]

I have a 40mm->Series VI adapter. Series VI is 44mm -close enough to 43mm to answer your question.
The adapter itself does add vignetting in the very far corners, probably usable as a hood, but adding any filter will definitely be a problem.

Quick testing with a 49mm filter and it looks like that is not quite wide enough either, unless you find a slim filter -maybe.

Looks like 52mm would be OK. So if you can find a 40mm->52mm step up adapter, that would be the smallest i would be comfortable with.

Any chance the vignetting is typical fall-off while wide open? I can't imagine using a tiny lens like the 25 (or 28) with a 52mm filter.
Many decades ago i had the 25mm w finder & liked it a lot. These days i use 21mm & 28mm....

 

[Darinwc]

Its a hard stop. This lens is wide and has a (relatively) long barrel so as to act as a shade.
1: no filter
2: slim 40mm filter
3: 40mm->Series VI (44mm)
4. 40mm->Series VI (44mm) + filter/retaining ring. -exactly what you would get from a 43mm step-up ring+filter.
5: 40mm->Series VI (44mm) + 49mm filter held against it.