making a double exposure
The effect is one blend mode and three supporting controls. Most of the work is in choosing the two photographs, and almost every disappointing result can be traced back to that choice rather than to a setting.
A double exposure is an addition. On film the shutter opened twice onto one frame and light accumulated: the dark parts of each scene contributed nothing, the bright parts contributed everything, and there was no decision anywhere about which photograph won. Screen is that arithmetic written down — one minus the product of the two complements — and it is the only way of combining two frames that adds light without ever exceeding white. Everything else in this recipe exists to make a pair of digital files behave the way that piece of emulsion did.
Choosing the two photographs
This is the part that decides whether the rest works. The base wants a subject the eye can complete from an outline alone, sitting against a background much brighter than it is. The layer wants structure and real blacks and no subject of its own, because the moment the layer has a subject you are looking at two competing photographs rather than one picture.
- Good base. A profile against a window. A figure on a beach under a flat sky. Anything where you could trace the subject with one line.
- Poor base. A group shot in a lit room, where subject and background are the same brightness and the key has nothing to find.
- Good layer. Bare branches, a skyline at dusk, wet tarmac, a shoal of windows. Dense detail with genuine shadow in it.
- Poor layer. A bright, evenly lit wall or a pale sky. Screen can only add, so a layer with no dark areas simply lifts the whole frame.
The six moves
- Choose a subject with a step in itThe effect needs a clear boundary between subject and background. A head and shoulders against a bright window or a pale sky is the reliable case, because the edge of the subject is a single steep change in brightness.
- Choose a texture with real blacksBare branches, a skyline at dusk, rain on glass, a crowd. The texture's dark areas are what will become transparent under screen, so a layer that is bright everywhere produces a pale result no control can rescue.
- Flatten the layer to luminanceScreen adds light, and colour added to colour lands somewhere neither photograph was. Flattening the layer to grey leaves the base in charge of hue and the layer in charge of structure.
- Lift the black point until the texture has weightRaise it in small steps. Small numbers give the texture presence in the base's midtones; past roughly a quarter it flattens the layer to a sheet and lifts the whole frame with it.
- Key the effect to the base's highlightsRaise the key from zero and the subject comes back, because the key restricts the layer to the parts of the base that were already bright. This is the step that separates a double exposure from a texture laid across a face.
- Fade one side if the frame needs airTurn on the gradient mask and drag the two handles so the texture is strongest where the background is and thins toward the subject. Then export at the base frame's dimensions.
When the texture wins
The commonest failure is a face disappearing under branches, and it has three cures which should be tried in this order. Raise the highlight key first: it costs nothing on the background and it is the only control that knows where the subject is. Then lower the black-point lift, because a lifted black point is exactly what gives the texture presence in the darker parts of the base, which is where a face lives. Only then touch the opacity, which thins the effect everywhere including the places that were already right.
If all three are exhausted, the answer is usually the gradient mask rather than another number. Set the first handle in the background and the second over the subject, and the texture will run out before it reaches the face. This is a manual decision about a particular photograph, which is what the mask is for.
None of these controls knows what a subject is. The key reads brightness and only brightness, so a subject wearing something pale will collect texture on it no matter how the key is set. Pulling a subject away from its background is a separate operation owned by a separate tool, and a recipe here that implied otherwise would be describing a feature this build does not have.