Blur a copy, screen it back
Michael Orton built the effect in the mid-1980s by sandwiching two slides of the same scene: one exposed sharp, one thrown out of focus, both overexposed. Projected together they produced detail and diffusion at once. This does the same arithmetic on one file.
The image is blurred at the radius you set, brightened by the soft layer exposure, then screen-blended back over the untouched original. Glow amount controls how much of that blend survives.
Blur radius and glow amount
These two do most of the work and pull against each other. A small radius near 1% keeps texture visible and reads as a light haze. Past 4% individual shapes stop being recognisable in the soft layer and the frame turns into coloured light.
Glow amount is the blend weight. At 100% the screened result replaces the original entirely, which usually clips highlights. Most images settle between 50% and 75%.
Milky shadows
Screen blending can only lighten. Because dark pixels have the most headroom, they move furthest, and an untreated Orton pass often leaves blacks sitting around 60 to 80 instead of near 0.
Shadow protection scales the blend down where the source luminance is already low. At 40% the deepest tones keep most of their density while the midtones still glow. Raise it when a night scene turns grey; drop it to 0% for the fully lifted, high-key look.
Soft layer exposure
Orton overexposed both slides by roughly a stop each, because a normally exposed sandwich came out too dark. Soft layer exposure reproduces that step: it multiplies the blurred copy before the screen blend, so the glow gets brighter without widening the radius.
Raising exposure and lowering glow amount produces a tighter, hotter bloom. Lowering exposure and raising glow produces a broad, flat wash.
Example: backlit forest
Start at 2.4% radius, 65% glow, 35% soft layer exposure, 118% saturation, 40% shadow protection. If tree trunks disappear, cut the radius before the glow. If the canopy clips to white, drop soft layer exposure to around 20%.
Saturation runs last, on the blended result. Screen blending desaturates as it lightens, so a value slightly above 100% restores the colour the blend washed out rather than adding any.