Flash Color Temperature Gels: Color temperature calibration of CTO and CTB in mixed and complex ambient light
In-depth analysis of the physical nature of "color pollution" caused by photography flash under complex mixed light sources such as indoor tungsten lights, restaurant warm light and blue dusk, and systematically dismantling the precise color temperature calibration conversion and white balance collaborative workflow of the orange temperature filter (CTO: Full/Half/Quarter) and blue temperature filter (CTB).
1. Core physical principle: Let the two beams of light be "coherent first, then calibrated" in the spectrum
The light emitted by the xenon tube of the photography flash (Xenon Tube) or modern high-power LED fill light is pure daylight color temperature (approximately 5500K - 5600K pure white light).
Indoor tungsten lamps, ceiling spotlights, and candlelight are warm light (about 2700K - 3200K rich yellow light).
When two beams of light with completely different blackbody radiation spectra hit the same scene, any camera's global white balance algorithm will inevitably take one into consideration and lose the other.
[Wrong approach]: Directly use 5500K flash to hit the face + 3200K background light ──> forming two irreconcilable opposite color temperature conflicts
[Two steps of correct industry]:
The first step [Physical Coherence]:
Install a CTO orange filter in front of the 5500K flash to filter the light emitted by the flash into [3200K warm yellow light]!
At this time: the main human light (3200K) in the entire field and the background ambient light (3200K) in the entire field are absolutely consistent and at the same frequency at the physical level!
The second step [Unification within the machine]:
Unifiedly lower the camera's in-camera white balance color temperature (K value) to [3200K (tungsten light mode)].
A miracle happened: the camera accurately restored all the 3200K warm light in the entire scene into transparent, white, slightly warm, natural and pure colors at once!
2. CTO and CTB filter family specifications and color temperature conversion table
The color filter industry follows strict Mired: Micro Reciprocal Degree optical absorption standards:
$$\Delta \text{Mired} = \left( \frac{10^6}{T_{\text{target}}} - \frac{10^6}{T_{\text{source}}} \right)$$
| Color filter model | Chinese name | Optical color temperature conversion effect | Light intensity attenuation compensation (Light Loss) | Typical application scenarios |
|---|---|---|---|---|
| Full CTO (1/1) | Full-level orange temperature filter | 5500K $\rightarrow$ 2900K ~ 3200K | About -1.1 EV | Indoor traditional old-fashioned tungsten chandelier, retro cafe, fireplace candlelight banquet |
| 1/2 CTO | Half-step orange temperature filter | 5500K $\rightarrow$ 3800K ~ 4000K | About -0.6 EV | Modern warm white halogen lamp shopping mall, sunset golden hour (Golden Hour) backlighting |
| 1/4 CTO | Quarter Orange Filter | 5500K $\rightarrow$ 4500K | About -0.3 EV | On cloudy days outdoors, it can add a weak warm color to the face to prevent the portrait from becoming cold and gray |
| Full CTB (1/1) | Full-step blue temperature filter | 3200K $\rightarrow$ 5500K | About -1.6 EV | Convert traditional movie tungsten filament constant light into daylight, and cooperate with daylight diffusion outside the window |
| Plus Green (window green) | Fluorescent green correction film | Supplementary green peak spectrum | About -0.5 EV | Environmental color temperature correction of low color rendering fluorescent tubes in subway stations and old factories |
3. Classic creative application: "Reverse cold and warm contrast blockbuster" for dusk blues portraits
Mastering CTO, photographers can not only calibrate reality, but also “fake” a shocking blockbuster atmosphere out of thin air:
"Fake deep blue night scene" practical magic:
- The sun has just set in the evening, and the sky outside is a bland gray-white (not yet into deep blue).
- Place a piece of Full CTO (Full CTO) on the top of the camera or in front of the off-camera flash and aim it at the model's face.
- Manually force the camera color temperature to 3000K (or even 2800K in extreme cold mode).
- Visual Transformation:
- The model's face received CTO orange light, and after being processed by the camera at 3000K, it was restored to a fair, transparent and healthy skin color with a strong three-dimensional effect;
- The originally plain and gray vast sky and distant mountains in the background were instantly forcibly rendered into rich, deep and breathtaking Royal Twilight Blue under the violent reverse pull of 3000K extremely cold white balance**!
- A Hollywood-level portrait of warm and cold complementary colors taken in bad weather was born.
4. Avoid pits and maintain durability
[!CAUTION]
- Risk of high temperature melting of color filters:
When a high-power hot shoe flash is shooting continuously at 1/1 full power at high speed, the instantaneous focusing temperature generated by the Fresnel lens of the lamp head can exceed 150℃! Cheap thin plastic color filters will instantly blister, turn black and carbonize when heated, or even stick to the flash lens.
Countermeasure: Be sure to purchase high-temperature-resistant professional polyester optical color paper (such as Lee Filters or Rosco Cinegel), and use high-temperature-resistant silicone magic tape to maintain a tiny 3-5mm air insulation gap with the lamp head.- Flash Exposure Compensation (FEC) Calculation:
After installing Full CTO, the flash will lose more than 1 stop of physical light energy. When controlling the flash in manual M mode, remember to manually increase the output power from 1/16 to 1/8 or even 1/4+0.3 to make up for the light transmission loss.
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