Why doesn’t the apparent brightness of a specular reflection on metal change with light distance?
Asked 10/4/2022
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In Light: Science & Magic, the chapter on metal says the brightness of the direct reflection on a metal surface is not affected by the distance between the object and the light source. That seems to conflict with the inverse-square law, since moving the light closer or farther should change how much light reaches the object. Why can the reflected highlight appear unchanged? What am I missing?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
3y ago
2 Answers
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**It is the inverse square law at work... it is the same reason the exposure settings for a picture of a light source (e.g. a light bulb/window/etc) do not change as you move closer to, or farther from, that light source.
As the distance increases you do get less light, but it is also concentrated into a smaller area; so it's brightness (light/area) does not change. It doesn't really matter if the source is reflected or not... just that metal doesn't absorb (nor polarize) any of the light.
This is also the same reason that larger sensors get more light for any equivalent image (composition/exposure) and tend to perform better in lower light situations.
(**I have not read that book)
Edit based on your comments above: Note that, as long as the metal is completely/evenly illuminated, the reflection of the softbox is a larger/smaller portion of the softbox face; because the reflective surface has a fixed/limited angle of view/incidence that will reflect back to the camera. So again, the exposure does not change due to the ISL.
It is the same as moving closer to, or farther from, a white wall that fills the camera's FOV at all distances... as you get closer to it you get more light from a smaller area, and the total light (light/area, exposure) doesn't change.
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
3y ago
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Generated from our catalog & community — verify before relying on it.
You’re comparing two different things: total light reaching the metal, and the apparent brightness of the reflected source.
The inverse-square law says a farther light delivers less total illumination to the subject. But the reflected image of the light source also becomes smaller as distance increases. So although less light arrives, it is concentrated into a proportionally smaller reflected area. The result is that the highlight’s luminance (brightness per unit area) can stay essentially the same.
This is similar to photographing a visible light source itself: if you move farther away, it occupies less of the frame, but its apparent brightness does not automatically dim just because you stepped back.
For polished metal, the direct reflection is basically an image of the light source, and metal reflects most of that light specularly rather than scattering it diffusely. So the highlight depends more on the source’s apparent brightness than on subject distance alone.
A practical caveat: this holds for the specular reflection itself, assuming the metal is evenly illuminated and the geometry stays the same. Other parts of the scene, diffuse surfaces, or imperfect reflections can behave differently.
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