How Subjects Change Light
Written by Abin Alex and Anu Joseph.
Abstract
A practical guide to how a subject changes light: transmission and refraction, absorption, diffuse and direct reflection, glare, the inverse square law and the family of angles, with three diagrams and two tables.
Keywords: how subjects change light, transmission, refraction, absorption, diffuse reflection, direct reflection, glare, family of angles, inverse square law
What can a subject do to light?
Short answer. It lets light through, takes it in, or sends it back.
- Photons move, while photographic subjects often sit still, so we tend to think of light as the active player. That attitude handicaps our ability to see a scene.
- Shine the same light on a white wall, a sheet of glass and a black cloth, and you get three very different pictures. The light was identical. Each surface did something different with it, and there are only three things it could do: let the light through, take it in, or send it back. Table 1 sets out the three.
| Subject action | What happens | Can it be photographed? |
|---|---|---|
| Transmit | Light passes through, as in clean air and clear glass | Simple transmission cannot, refraction can |
| Absorb | Light is taken in and given out as heat | Only by comparison with light that is not absorbed |
| Reflect | Light strikes the subject and bounces off | Yes, it is what makes vision possible |
Table 1. The three things a subject does to light.
Look at a glass, a black cloth and a white card under one lamp. Say what each does to the light.
Choosing a wardrobe, a set colour or a prop surface is choosing how the subject treats the light.
Cham tipAsk what the surface does to light before you place a lamp.
Cham taskList ten objects and the main thing each does to light.
Check yourself: What three things can a subject do to light?
Transmit, absorb or reflect it.
What is transmission and refraction?
Short answer. Transmitted light passes through the subject. Refraction bends it, and diffuse transmission scatters it.
- Light that passes through a subject is transmitted. Clean air and clear glass transmit light. If a subject passed every ray through unchanged, the camera would have nothing to record, so a subject we can photograph always changes the light in some way.
- Simple transmission occurs only when light strikes the surface perpendicular to it. At any other angle the light bends. This bending is refraction, and it is caused by a change in the speed of light in the material. Air refracts light very little, while the dense glass of a lens refracts it a great deal. Refraction can be photographed, which is why a completely transparent subject is not invisible (Hunter and Fuqua, 1997, p. 22).
- In direct transmission light passes through in a predictable path. White glass and thin paper scatter the rays in many random directions, which is diffuse transmission, and such materials are called translucent (Hunter and Fuqua, 1997, p. 23). A large translucent sheet over a small light makes it larger and therefore softer. A diffusion sheet in front of a flash and clouds over the sun do this job.
Put a glass of water in front of a patterned card and look at how the pattern bends.
Diffusion frames and silk in front of a light are translucent materials used to soften it.
Cham tipUse a larger translucent sheet to make a small light bigger.
Cham taskPhotograph a lamp bare and behind thin paper and compare the shadows.
Check yourself: What makes a translucent material useful to a photographer?
It scatters light and makes a small source larger and softer.
What happens when a subject absorbs light?
Short answer. Absorbed light turns to heat and is never seen again. Partial and selective absorption decide black, white, gray and colour.
- Light taken in by a subject never comes back out as light. Its energy turns into something we cannot see, usually heat. Like transmission, simple absorption leaves nothing for the camera to record (Hunter and Fuqua, 1997, p. 24).
- We only notice absorption by contrast, when a dark area sits next to light that was sent back. That is why deeply absorbing subjects, such as black velvet or black fur, are so hard to light and show detail in.
- Most subjects absorb part of the light, and that partial absorption is one factor that makes a subject black, white or some gray in between. A subject also absorbs some frequencies more than others, and this selective absorption is one factor that decides its colour.
Photograph a black cloth and a white sheet side by side and see how little detail the black one shows.
Black costumes need a rim or kicker to show shape, because they absorb most of the light.
Cham tipGive black subjects a light that grazes the surface.
Cham taskLight black velvet so that its folds show and write what you did.
Check yourself: What does absorbed light become?
Heat.
What types of reflection are there?
Short answer. Diffuse reflection, direct reflection and glare. Most surfaces give a mixture of all three.
- Reflection is light striking a subject and bouncing off. Without it we would see almost nothing, because most things give off no light of their own. What we see, and what the camera records, is the light those things send back (Hunter and Fuqua, 1997, p. 24).
- Of the three actions, reflection is the most visible, so photographic lighting is mainly an exercise in managing reflection.
- Light reflects as diffuse reflection, direct reflection or glare. Most surfaces give some of each, and the proportions in the mix make one surface look different from another. Table 2 compares the three.
| Type | How it behaves | Example |
|---|---|---|
| Diffuse reflection | Same brightness from any viewing angle | White card, newspaper |
| Direct reflection | A mirror image of the source, seen from one angle only | Mirror, polished metal, water |
| Glare | A polarized direct reflection, always dimmer than the source | Glossy plastic, glass, varnish |
Table 2. The three types of reflection.
Walk around a white card and a mirror under one lamp. Which one changes brightness as you move?
Choosing a surface texture on a set is choosing the mix of diffuse and direct reflection.
Cham tipThe source decides the type of light and the surface decides the type of reflection.
Cham taskName the dominant reflection type of ten surfaces in your room.
Check yourself: Which reflection looks the same from every angle?
Diffuse reflection.
How does diffuse reflection behave?
Short answer. It is the same brightness from every angle and depends on the distance of the light, not its angle or size.
- A diffuse reflection scatters light evenly in every direction, so it looks equally bright wherever you stand. A sheet of white card is almost pure diffuse reflection: three cameras placed at three different angles all record it at the same brightness (Hunter and Fuqua, 1997, pp. 27-28).
- Moving the light round a diffuse surface, or making the light bigger or smaller, hardly changes how bright the surface looks. Moving the light closer or farther does. Bring it in and the surface gets brighter, so at the same exposure it prints lighter.
- The inverse square law says intensity is inversely proportional to the square of the distance. A light at one distance lights a subject four times as bright as the same light twice as far away (Child and Galer, 2008, p. 15), and nine times as bright as the same light three times as far. Figure 1 shows why.
- The word diffusion has two meanings, so keep them apart. Diffusing a lamp makes its light softer. A diffuse reflection is something the surface does. A soft source can still make a mirror-like reflection on glass, and a hard source can still make an even, diffuse one on card. The light decides hard or soft, and the surface decides which kind of reflection you get (Hunter and Fuqua, 1997, p. 29).
- Distance also decides how even the light is across a group. With a light 3 ft from the front row, the back row at 6 ft is two stops darker. Move the light to 12 ft and the back row, at about 15 ft, is only about half a stop darker (Marr, 2004, p. 32).
Move a lamp from one metre to two metres from a white card and meter the card at both distances.
Moving a light back by double its distance loses two stops.
Cham tipDouble the distance and the light falls to a quarter.
Cham taskMeter a white card at 1 m, 2 m and 3 m and compare with the inverse square law.
Check yourself: How bright is a light at twice the distance?
A quarter as bright.
How does direct reflection behave?
Short answer. It is a mirror image of the source. The angle of incidence equals the angle of reflection.
- A direct reflection is an image of the light itself. Swap the white card for a small mirror and only the camera in the right spot sees a dazzling patch of light; the other two see nothing. Figure 2 shows this.
- On a smooth surface, light leaves at the same angle it arrives. The angle of incidence equals the angle of reflection (Grey, 2004, p. 10), so where a direct reflection appears depends only on where the light, the subject and the camera sit. Real surfaces are never perfect mirrors, but polished metal, still water and glass come close.
- The brightness of a direct reflection does not change with the distance of the source, but the size of the reflection does, so the inverse square law is not broken (Hunter and Fuqua, 1997, p. 33).
- Every point on a shiny surface has its own angle to the camera. Together, those angles form the family of angles: the zone a light must sit in to show up as a reflection. Put the light inside that zone and you see it on the surface; put it outside and you do not (Figure 3). This tells you where to place a light to include or avoid a reflection (Hunter and Fuqua, 1997, p. 36).
- A large source fills the family of angles, so the reflection fills the whole mirror or glass, and a small source fills only part of it. To keep a reflection off a subject, move the light, or the camera, until the light sits outside that zone (Hunter and Fuqua, 1997, p. 37).
Hold a mirror and a lamp. Move the camera until the lamp appears and then until it disappears.
Reflections in glass, water and eyes follow this rule, so the camera position decides what appears in them.
Cham tipTo remove a reflection, move the light out of the family of angles.
Cham taskPhotograph a glass sheet with a light inside the family of angles and then outside it.
Check yourself: What equals the angle of incidence?
The angle of reflection.
What is glare and how do you control it?
Short answer. Glare is a polarized direct reflection, and a polarizing filter on the lens can block it.
- A glare reflection is a polarized direct reflection. It looks like an ordinary direct reflection but is always dimmer than the light source (Hunter and Fuqua, 1997, p. 37).
- A polarizing filter on the camera lens blocks glare, and because the filter also absorbs some light, the exposure has to be opened to compensate. Diffuse reflection is not blocked, so a feather stays visible while glare on black plastic disappears.
- Electrical insulators such as plastic, glass and ceramics are more likely to give polarized reflection, and bright metal gives plain direct reflection. The conclusive test is to look through a polarizer. If it removes the reflection it is glare, if it has no effect it is ordinary direct reflection, and if it reduces the reflection it is a mix (Hunter and Fuqua, 1997, p. 42).
- You can also create glare on purpose. Put a polarizing sheet over the lamp so its reflection becomes polarized, then turn a polarizer on the lens to dial the reflection up or down. Light from the open sky is already partly polarized.
Look at a glossy surface through a polarizing filter. Rotate it and watch the reflection come and go.
A polarizer on a lens cuts reflections from car windows, water and wet leaves.
Cham tipRotate the polarizer until the reflection disappears.
Cham taskPhotograph glass with and without a polarizer and note the exposure change.
Check yourself: What blocks glare on a lens?
A polarizing filter.
Questions beginners ask
What can a subject do to light?
A subject can transmit, absorb or reflect light.
What is refraction?
The bending of light when it passes from one material into another at an angle, caused by a change in the speed of light.
What is the difference between diffuse and direct reflection?
Diffuse reflection looks the same from every angle. Direct reflection is a mirror image of the source seen from one angle only.
What is the inverse square law in photography?
A light at twice the distance gives a quarter of the intensity on the subject.
What is the family of angles?
The set of angles at which a light produces a direct reflection on a surface for a given camera position.
References
- Child, J. and Galer, M. (2008) Photographic lighting: essential skills. 4th edn. Oxford: Focal Press.
- Grey, C. (2004) Master lighting guide for portrait photographers. Buffalo, NY: Amherst Media.
- Hunter, F. and Fuqua, P. (1997) Light: science and magic: an introduction to photographic lighting. 2nd edn. Boston: Focal Press.
- Marr, D. (2004) Beginner's guide to photographic lighting: techniques for success in the studio or on location. Buffalo, NY: Amherst Media.
Further reading
- Hirsch, R. (2008) Light and lens: photography in the digital age. Burlington, MA: Focal Press.
How to cite this guide. Alex, A., & Joseph, A. (2026). How subjects change light: A guide for photographers and cinematographers. Creative Hut Academy. https://creativehut.org/library/how-subjects-change-light/
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