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According to the book Light Science & Magic: An Introduction to Photographic Lighting, metal chapter, the brightness of the direct reflection on a piece of metal is not affected by the distance from that object to the source.

However, the brightness increases or decreases as we move the light closer or farther away, following the inverse square law. So why is the brightness of the reflection remains unchanged? Did I miss something?

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  • \$\begingroup\$ Without seeing the text in question, I'm pretty sure it is referring to the position of the reflector in the overall total optical distance from the light source to the illuminated subject. It matters not if the two distances (light to reflector + reflector to subject = total distance) are 2m + 18m = 20m total distance or 15m + 5m = 20m total distance or any other combination of two distances as long as the total optical distance is the same in both cases. (assuming the reflector is large enough that there is no 'cutoff' so that the reflection covers the entire subject) \$\endgroup\$
    – Michael C
    Commented Oct 4, 2022 at 8:02
  • \$\begingroup\$ @MichaelC "We expect the closer light to illuminate the background more brightly, but the brightness of the metal does not change because the brightness of direct reflection is not affected by the distance to the source.". The subject is about lighting the background while maintaining the exposure on the metal. There is an illustration showing moving a soft box either closer or far away, as long as it covers the family angle of incidence, exposure won't change on the metal \$\endgroup\$
    – Long Doan
    Commented Oct 4, 2022 at 8:07
  • \$\begingroup\$ Is the metal being used as an off-camera reflector? Or is the metal object the subject inside the frame? \$\endgroup\$
    – Michael C
    Commented Oct 4, 2022 at 8:08
  • \$\begingroup\$ In this case, it is the subject on top of a white background. The scenario in the book is about lighting both the metal piece and the white background appropriately. The book demonstrated, with pictures, that by simply pulling the light closer, will illuminate the background while not blowing the highlight, which is a direct reflection on the subject \$\endgroup\$
    – Long Doan
    Commented Oct 4, 2022 at 8:13
  • \$\begingroup\$ I do not have access to the book, other than the first few pages that are available on google books. What do the following sections in Chapter 3 say about this? 'Direct Reflection' and 'Breaking the Inverse Square Law?' \$\endgroup\$
    – Michael C
    Commented Oct 4, 2022 at 8:18

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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.

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