Ron,
In a city scene, you can use retaining walls or building flats to transition from one scene to another. Try to match the coloration of the flats to the backdrops. These photos show foreground scenes, building flats and photo backdrops. (Kong really doesn't help the transition, but I try not to aggravate him.
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Use colors to push the background into the distance. Distant objects are lighter in tone and slightly bluish due to haze. Colors are brighter and warmer the closer you get to the viewer.
I used a light, bluish gray overspray with canned spray paint on the backdrop photos to push them into the distance. Building flats were weathered with lighter color chalks to separate them from the foreground structures.
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Here retaining walls separate the scenes so the viewer's attention is drawn first from the foreground scene then up to the more distant scenes. The difference in elevations helps separate the scenes also.
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In a rural scene, shallow relief cliffs or trees can serve the same purpose as building flats. Here you have foreground rocks, low relief cliff faces, and then the painted backdrop.
Even something as simple as a row of low hedges (chunks of clump foliage, for example) along the base of your backdrop will hide that seam and connect it to the scenes in front of it.
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The more difficult transitions might seem to be scenes placed side-by-side rather than one in front of the other. But, we seldom have the room to progress gradually from the city to the country, for example. Luckily, the viewer's eye will tend to focus on one scene at a time, not the space between them.
The more detail you have in each scene will help keep the viewer's eye focused where you want. He will tend to ignore the less detailed intermediate scenes.
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Jim