Quick answer

Most filter holder vignetting is not a filter fault. Check three things in order: the adapter ring must be the one made for your exact lens, the holder must sit square on that ring, and the filter stack must be no thicker than the holder was designed for. If all three are correct and corners are still dark, the lens is wider than the holder can support and you need a different system.

Separate the symptom from the cause

Dark corners can come from the lens itself, the adapter ring, the holder, or the filters. Shoot one frame at your widest aperture with the holder attached but no filters loaded. If the corners are clean, the problem is in the filter stack or its alignment. If they are already dark, the holder or adapter ring is the cause. This one test saves you from buying filters you do not need.

Check the adapter ring before anything else

An adapter ring is cut for one lens, not a mount. A ring made for a 20mm lens will not sit correctly on a 15mm lens even if both use the same camera mount. The ring controls how close the holder sits to the front element, and that distance is what keeps the holder out of the frame. If you are using a ring that was not made for your lens, no amount of adjustment will fix the corners.

Align the holder so it sits square

Once the correct ring is fitted, the holder must sit flat against it with no tilt. A holder that is slightly rotated or not fully seated pushes one corner into the frame. Rotate the holder through its full range and watch the corners in live view. If the dark corner moves as you rotate, the holder is not square. Reseat it and tighten the locking screw evenly.

Reduce the filter stack

Every filter you add moves the front of the stack further from the lens, and the holder can only clear so much. If corners are clean with one filter and dark with two, the stack is the cause. Use the thinnest filters you have, or move to a larger holder system that gives the stack more room. Do not force a third filter into a holder that was not built for it.

When to stop adjusting and change systems

If the correct adapter ring is fitted, the holder is square, and you are running a single thin filter, and corners are still dark, the lens is wider than the holder can support. At that point, stop adjusting. Move to a larger holder system or accept that the lens needs a different approach. Continuing to shim or force the holder will not recover the corners and may damage the ring.

Vignetting diagnosis

  • Dark corners with no filters loaded: The holder or adapter ring is the cause. Confirm the ring matches your exact lens model, then reseat the holder until it sits flat and square. If corners are still dark after a correct ring and a square holder, the lens is wider than the system supports.
  • Corners clean with one filter, dark with two: The filter stack is too thick for the holder. Remove one filter or switch to thinner filters. Do not add a third filter to a holder that already vignettes with two.
  • Dark corner moves when you rotate the holder: The holder is not square on the adapter ring. Loosen, reseat, and tighten evenly. If it still moves after reseating, the ring or holder is damaged and needs replacement.

Products that solve specific vignetting causes

  • NiSi S6 150mm Filter Holder Adapter Ring for Sigma 20mm f/1.4 DG HSM Art: Designed for Sigma 20mm f/1.4 DG HSM Art; for NiSi S6 150mm Filter Holder Adapter Only – requires filter holder and filters sold separately; no vignetting; 360-degree rotating gears for use with CPL; aluminum alloy construction. If your dark corners come from using a non-dedicated ring on this lens, this is the correct part. The 360-degree gear rotation lets you turn a CPL without shifting the holder, which helps you keep the holder square while adjusting.
  • NiSi S6 150mm Filter Holder Adapter Ring for LAOWA FF S 15mm F4.5 W-Dreamer: Designed for LAOWA FF S 15mm F4.5 W-Dreamer; for NiSi S6 150mm Filter Holder Adapter Only – requires filter holder and filters sold separately; no vignetting; 360-degree rotating gears for use with CPL; aluminum alloy construction. A 15mm lens is exactly the kind of wide angle that exposes a wrong or misaligned ring. This dedicated ring is the starting point before you blame the holder or the filters.