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Defining your own light source can help make miniatures feel bigger by deemphasizing the bounced light form the ground making the surface feel farther away. This method achieves that in a way the is both easy and defuse.
As things get smaller light becomes in effect more defuse. Because of this some lighting effects must be added with paint: dry brushing adds a combination of rim light and ambient occlusion by specifically targeting edges allowing for larger range in crevices and providing a targeted application for sharp rims that would normally refract light. In addition dry brushing can be used as a weathering effect imitating the wear and tare to the more delicate corners of brittle materials such as stone and paint. However, in weathering applications, special care should be taken to ensure the color choice marches the weathered state of the material.
The reason for an ink wash is two-fold: first, it adds a layer of dark grime to the crevasse of a model and second it increases the range available for ambient occlusion in a similar way to dry brushing. When applying a n in wash the color should be selected by considering a combination of weathering and scale, with the environment influencing the hue and scale the depth.