Scatter light the way glass does
A pro-mist filter holds microscopic particles suspended in the glass. Light passing through does not all travel straight: a fraction scatters sideways and lands on neighbouring parts of the sensor. Bright sources bleed outward, deep shadows pick up a faint wash, and fine tonal separation drops without the image going soft.
That is a veil, not a bloom. Every pixel contributes, weighted by how bright it is.
Diffusion strength and veil spread
Strength is the amount of scattered light added back. Real filters are sold in grades, commonly 1/8, 1/4, 1/2, and 1, roughly matching values here from 20% to 85%.
Veil spread is the scatter distance as a percentage of the shorter image side. Small values around 1% produce a tight halo around light sources. Values above 5% behave more like shooting through fog than through a filter.
Highlight bias
At 0% every pixel contributes equally to the veil and the whole frame hazes over, close to what white pro-mist does. At 100% only bright pixels contribute and the effect concentrates on lamps, reflections, and blown highlights.
The default of 55% sits where most black pro-mist grades land: obvious around light sources, present but restrained everywhere else.
Black lift
Diffusion raises the black point because scattered light reaches areas that recorded nothing. Black lift reproduces that by moving the floor upward, up to about 42 levels at maximum, then rescaling the range above it.
This is the control that separates a diffusion filter from a glow effect. A photo with genuine 0,0,0 blacks and haloed highlights does not look like it came through glass.
Example: neon street at night
Try 58% strength, 2.8% spread, 34% black lift, 55% highlight bias, 12% warmth. Signs should bleed a soft halo while the wet road keeps its texture. If the whole image looks foggy, raise highlight bias toward 80%. If highlights blow out, reduce strength before spread.
Daylight scenes need less of everything. Strength around 25% and black lift near 15% is usually enough, since there is no dark surround for the halo to read against.