Miniatures are usually painted with the light falling from above, however, I usually angle my airbrush, which represents the light source, to the front or to one side of the front so as to cast the most ‘light’ on the most viewed part of the miniature but for this explanation, the light is shining from the zenith of the objects represented. Even using an airbrush, you must finish the blending with a brush as an airbrush will always leave the miniature looking airbrushed whereas a normal brush will create more natural variation in tone as you blend.
The diagram to the right shows how a cylinder should be highlighted when lit from directly above. As the tangential angle approaches horizontal at the top, and thus faces the light source more fully, so it catches more light and should therefore be painted a lighter colour. The cylinder requires roughly equal zones, arbitrary though they are, of highlight, mid-tone and shadow. For a curved object such as this, which could be an outstretched arm firing a pistol, a good blending technique is required to keep the transitions smooth. It is also worth noting that where the edge around the circular face faces upwards, it reflects more light to the viewer and so this line will need to be blended down to the sides. It should not be highlighted much, if at all, underneath.
This dodecagonal prism follows similar rules, however, each face will be a more uniform colour than the blending on the cylinder and the difference between the luminosity of each face will give the shape form. Looking at a cylinder as a shape like this can help to think about the colour mixes required and how to space them in order to create your blending but it also demonstrates how light is caught on a shape with flat faces. The upward facing, horizontal surface at the top catches the most light and the downward facing faces underneath are in shadow. Again, the upward facing edges reflect more light, represented by pale lines, but any downward facing edges will not receive any extra light. The vertical edges will receive some extra light but less than the horizontal ones at the top.
Painting a tank like this will help to really pick out its shape and a miniature with flat surfaces rather than curved ones will require less blending, though a little variation will add interest and creating a transition on even a flat surface, as has been done here, will help to confirm your light source and can be used to draw the eye to specific details. This is sometimes known as colour modulation but be careful not to overdo it as it can end up looking a bit silly if the gradient is too severe.
If we take the cylinder again and squash it horizontally, we get the shape to the immediate right, which is an elliptic cylinder. Following the same rules, we get a smaller zone of highlighting than on the cylinder, due to the narrower area facing upwards and then a larger zone of mid-tones down the steep vertical sides and a smaller zone of shadow underneath.
By rotating this shape through 90° (or squashing the cylinder vertically) we get the shape on the far right, another elliptic cylinder. This time, the long, shallow curves are on the top and bottom giving very wide zones of highlight and shadow leaving very narrow sections of mid-tones along the sides.
The same principles should be applied to all of the shapes on your miniature. For example, this sphere, which could be a helmet or the hair on a head, has an area at the top which is highly lit, a gentle transition into the mid-tone around the equator and then another, even transition into the darkest shadow at the bottom.
Using an airbrush is definitely a help and it will allow you to try a light source from any direction and even two directions at once with different colours, for example, natural light from above and the glow from lava underneath. It should always be noted though, that pre-highlighting with the airbrush is only a guide, you will always have to finish with a brush if you want a good result so you must have a well practised blending technique and it can also be useful to keep a lamp close by while you paint so that you can hold the miniature under it from time to time to get an idea of how the light hits its topography and to check your work.