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Does distance affect luminance?

The closer a little light source is to the subject, the more enlightened it will be. Reducing the distance of the light to the subject by half makes lighting four times brighter. This is because the light is concentrated in an area one-quarter of the size.

For example, if you use a small flashlight or studio lamp to light a portrait, reducing its distance to the subject by half gives you four times the light.

Likewise, moving the light twice the distance makes the light fall one-quarter of the brightness. A similar effect applies to print exposures when changing the magnifier height and when a close-up is required.

In practice, this "inverse square law" means that you should be particularly careful when lighting multiple objects at different distances in a small studio, by using a compact and hard light source.

This can make it difficult to obtain good lighting for the elements closest to and furthest from the light with a single exposure setting.

One solution is to move the light source much further so that the ratio from the closest to the most distant becomes less, or switch to multiple light sources or diffusers that will significantly reduce the "fallout" effect of lighting.

The same problem does not occur with direct light from the sun in the open air.

The sun is so far away that two places on earth – whether at the seaside or the top of the mountains – are almost equal in distance from the sun.

Variations in luminosity in landscape photography can be created by local atmospheric conditions, but not by the distance of the sun.

If you photograph indoors, however, using sunlight coming in from a small window, the window itself can act as a compact light source.

The intensity will then change with the distance in the same manner as if you had a lamp of that size in the same position.
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