Depth is crucial to paintings. Even in the most simple scenes, using overlapping forms to create a sense of depth and space helps your viewer understand the painting. A simple rule of thumb is F, M, B -or 'foreground, middle-ground background'. If you have three major planes in your image, you'll give your viewer a nice sense of scale. This is not limited to large, outdoor, scenes! Any image you make that involves a background should have three planes. In this post I'll talk a bit about depth and atmospheric perspective, and you'll see how much value is added with an emphasis on FMB.
Using a human in your foreground (or anywhere, really) is a great way to emphasize scale. This image by Kekai Kotaki is a great example of the 'tiny human in the foreground' principle: you know the middle-ground object is HUGE by comparison. Especially since there's a fair amount of atmospheric perspective in this scene, it's clear that the adversary in the middle-ground is considerably larger than the dragon-riding warrior in the foreground. The background in this image is the sky, though it still adds a nice sense of scale. If you're familiar with fantasy art, this might be a familiar composition. A small hero in the foreground facing a large baddie in the middle-ground tells the viewer "this is epic". If the middle-ground character were on its own without any foreground scale reference or overlapping, it would not appear so obviously huge. To learn more about this concept, make sure to watch this older Ctrl+Paint video from the Principles of Design series.
"Atmospheric Perspective" is a term describing the visual impact on objects seen from an extreme distance. This screenshot from Hawken is a great example of the concept - you can see that the background tower is very low contrast, while the foreground building is much higher in contrast. Both are receiving direct sunlight and have cast shadows, but as the objects recede into the distance they lost their definition. Additionally, outdoor scenes generally add a bit of the sky color to objects in the distance (in this case, blue) which visually separates them from the foreground. So the tricks to remember for atmospheric perspective are simple: distant object are lower contrast, less saturated, less detailed, and take on the color of the sky.
Guide Your Audience
As Simple as 1,2,3
One way to harm your painting is to add too many elements. Simplicity is a powerful tool when it comes to composing images. If you keep in mind three distinct levels of depth - F, M, B, you'll avoid a confusing image. This is not a 'rule', but it will come in handy when you're creating thumbnails. With each sketch you make, ask yourself: "do I have a foreground, middle-ground, and a background?"