For white balance, the key is not “gray” specifically. The key is neutral and not clipped. A black, gray, or white target can work if it is spectrally neutral and properly exposed. Gray is popular because it lands in the safest zone.
A white card is intuitive for white balance because you’re saying: “this should be neutral white; remove whatever color cast makes it not white.”
A gray card is often preferred because it is usually a more reliable neutral reference.
Why gray can be better:
- White can clip
- A white card may overexpose in bright light.
- Once one or more RGB channels clip, the camera no longer knows the true color balance.
- Example: if red/green/blue all max out at 255, there’s no useful color information left.
- Gray stays in the safe middle
- A gray card reflects less light, so it is less likely to blow out.
- The camera can measure the actual RGB relationship more accurately.
- Gray cards are made to be spectrally neutral
- A good gray card reflects all visible wavelengths evenly.
- Many “white” objects are not actually neutral: paper often has optical brighteners, blue tint, warmth, gloss, etc.
- Gray cards also help exposure
- Traditional camera meters are calibrated around middle gray, often approximately 18% reflectance.
- Some people say 18%, some say 20%; practically they mean “middle neutral gray.”
- So the card can help with both color balance and exposure.
- Less affected by glare
- White cards can show specular highlights or surface shine.
- A matte gray card is less likely to produce misleading bright reflections.
white card = easy intuitive WB target, but can clip / be non-neutral
gray card = neutral target with safer exposure headroom
“20% gray” refers to where the card sits on the scale between black and white, but specifically in terms of light reflectance, not just visual appearance.
black dark gray middle gray light gray white
0% ---------- 20% ------------ ~50%? ------------ 80% ------------ 100%
But here’s the tricky part:
A photographic 18–20% gray card does not look like “20% brightness” on a screen. It often looks like a fairly normal middle gray.
Why? Because there are two different scales:
- Physical reflectance
- 20% gray = reflects 20% of incoming light.
- This is the camera/exposure meaning.
- Perceptual/screen brightness
- Human eyes do not perceive brightness linearly.
- Screens use gamma curves.
- So 20% physical reflectance may look closer to “middle gray” visually.
So yes: it’s a graydation between black and white, but measured by how much light bounces off the card.
Tiny correction to the mental scale:
Reflectance scale:
0% black
18–20% photographic middle gray
80–90% light gray / white-ish
100% ideal perfect reflector
The reason 18–20% is “middle gray” is that it is roughly perceptual middle, not mathematically halfway in reflected light.