I've become a big fan of the K&F black mist filters in the recent past, adding this one to the 1/8 and 1/4 that I already own. Like the others, this is a nice quality filter. No color cast or distortion noticed at 200% inspection on a full frame image (beyond what's intentional on this kind of filter). I really like the heavy texture on the outside of a ring as it removes the need for a filter wrench if the filter gets stuck.

On the subject of strength, no one should start with a strength 1- start with 1/4. The effect of the full strength is more of a special effect; I usually have the 1/8 on my lenses just as a walkaround. The 1 is way too strong for that, unless I was shooting exclusively old Hollywood type glamour type photos and wanted that super soft look to everything. The 1/4 will give the halation on light sources while maintaining sharp in-focus lines. The 1, not so much. You can also tell that the filter reduces the amount of light transmitted. Not by a lot, and I haven't tested the exact amount, but sitting on a piece of paper, the filter is distinctly gray (like an ND2 or ND4 for example).