A new sky changes the light in the whole photograph

The commonest composite, and the commonest failure

Estate agency, landscape, architecture, car photography, wedding. A flat white overcast sky is replaced with a dramatic one, and nine times out of ten the result is obviously wrong.

The reason is that a sky is not a backdrop. It is the light source for everything else in the frame. Change it and every consistency relationship in the photograph breaks at once.

The four things that must agree

1. Light direction. A dramatic sky with the sun low on the left, over a building whose shadows fall to the left, is a contradiction. Check the shadows in the plate first and choose a sky whose bright area matches.

2. Light quality. The original overcast day produced soft shadows and low contrast on the ground. Drop in a clear blue sky with a hard sun and the ground still has soft shadows. The scene now looks like a bright day in which nothing casts a shadow, which the eye reads instantly even if it cannot articulate it.

3. Exposure relationship. A bright dramatic sky means a lot of light in the scene. If the buildings stay at their original overcast exposure they look dark and dull against it. Conversely, a dark moody sky over a brightly lit foreground reads as fake because the foreground is lit by something the sky does not explain.

4. Colour spill. Everything in the scene is lit by the sky. A golden sunset sky means warm light on every upward-facing and sun-facing surface. A deep blue sky means cool light. Without that, the sky and the ground look like two photographs, which they are.

The method

1. Choose the sky for the plate, not the other way round. Look at the shadows, note the direction and hardness, and pick accordingly. This is the decision that determines whether the rest is possible.
2. Mask the existing sky. Colour Range on the blue or the white usually gets 90 per cent in one click; the channel technique from earlier handles trees against sky better than anything else. Check the edges around foliage and aerials at 100 per cent.
3. Place the new sky underneath and scale it. Watch the perspective: a sky photographed with a wide lens has clouds that recede to the horizon, and stretching it distorts that recession visibly.
4. Match the horizon. The new sky’s horizon should sit at the plate’s horizon, not at the top of the buildings.
5. Add atmospheric haze at the horizon. Real skies are paler and less saturated near the horizon because of the depth of air. A subtle white or pale-blue gradient across the bottom of the sky layer is almost always needed and is the single quickest improvement.
6. Grade the foreground to match. A Photo Filter or Colour Balance layer over everything below the sky, warming or cooling towards the sky’s colour, at low strength.
7. Add the spill. A colour layer set to Soft Light, masked to the upward-facing and sky-facing surfaces: roofs, the tops of cars, water, shoulders.
8. Reconsider the foreground exposure against the new sky’s brightness.

Automatic sky replacement

Photoshop’s Sky Replacement, Luminar’s version and several phone apps do steps 2 to 6 automatically, including an attempt at the colour grade and the spill. They are good starting points and they mask foliage better than most people do by hand.

What they do not do: choose a sky whose light direction matches your shadows, or fix the light quality mismatch. The automatic grade adjusts colour, not the hardness of shadows that are already in the pixels.

So the automatic tool does the tedious part and leaves you the judgement, which is the correct division of labour. Use it, then check the four agreements.

The free equivalent is to do it manually, which is what this lesson describes, and GIMP handles every step. There is no free one-click sky replacement of comparable quality, and that is an honest gap.

Where the honesty question bites

Sky replacement is a good case study because the answer depends on the use.

• Advertising and illustration. Expected, unremarkable, nobody is misled.
• Estate agency. Widely done, and in several markets it has become a point of complaint: a listing showing a house under a blue sky that was photographed on a grey day is describing the photograph rather than the property. Some agency standards now require disclosure. Check what applies where you work.
• Landscape competitions. Most major competitions prohibit it, and entries have been disqualified over it.
• News and documentary. Prohibited outright. Adding or removing content, including sky, ends careers.
• Weather, science, evidence. Obviously not.

The test from earlier applies cleanly: would the viewer be surprised? For a magazine cover, no. For the photograph accompanying a story about a storm, yes.

A line in the delivery note costs nothing: “sky replaced, source file supplied”.

The technical limit

Some plates cannot take a new sky. Specifically:

• Where the sky reflects in the scene: windows, water, wet roads, car paint. Replacing the sky without replacing every reflection of it leaves the old sky visible in the glass. This is a lot of work and it is where most sky replacements fail on inspection.
• Where the sky is blown to pure white. The edge pixels around fine branches are pure white and hold no information about coverage, so the mask has nothing to separate. The result is a halo of white around every twig.
• Where the subject is backlit against the old sky, so its edges carry a bright rim from a sun that is now somewhere else.

In each case the honest answer is to reshoot, or to leave the sky alone and improve it with tone instead: a graduated Curves layer can do a surprising amount to a flat sky without replacing anything, and it raises none of the questions above.