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Filmmaking Dictionary: Number 1 / Fractional Diffusion, Gel and Filter Strengths
Definition
Fractional filter strengths, such as half, quarter or eighth, identify grades within a manufacturer’s filter or material range. They do not automatically state a fraction of an exposure stop or a percentage of light transmission.
The meaning depends on the product family. Half diffusion, half colour correction and a fractional camera diffusion filter describe different effects. Read the material’s full name and specification rather than treating the fraction as a universal measurement.
Diffusion Strength Is Not Stop Loss
Diffusion changes how light is distributed. A stronger material may create a more even illuminated surface, scatter light over a broader angle or conceal the original lamp’s structure more effectively. Its effect also depends on how it is lit and how large it appears from the subject.
The reduction in measured illumination is a separate property. A product called ‘quarter diffusion’ does not promise a quarter-stop loss. The result can also vary with the source, geometry and measurement method.
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Colour Correction Uses Another Convention
A fractional CTO or CTB gel indicates a correction strength within that range. It does not mean that the gel transmits the corresponding fraction of the light. Its colour change and transmission should be considered separately.
Likewise, a half-strength camera diffusion filter is not equivalent to a half-strength lighting diffusion sheet. One modifies the image through the lens, while the other changes light before it reaches the subject.
Why ND Numbers Are Different
Neutral-density optical-density values have a defined logarithmic relationship to transmission. A nominal 0.3 ND removes approximately one stop; 0.6 removes two stops and 0.9 removes three. Those numbers are not the same system as half, quarter or eighth product grades.
Practical Checks
- Identify the exact product family and manufacturer.
- Check stated transmission or stop-loss information separately from the strength name.
- Compare the light’s shape, spread and shadow quality in the intended setup.
- Recheck exposure after changing material.
- Record the full product name in lighting notes, not only ‘half’ or ‘quarter’.
For example, replacing a half grid with a different manufacturer’s half white diffusion may change spill and exposure even when both names include ‘half’. A matching fraction is not proof of an interchangeable result.
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Frequently Asked Questions
Does quarter diffusion remove a quarter of the light?
No. The name alone does not define its transmission.
Are fractional strengths consistent across brands?
Not necessarily. Compare the actual products and their measured or stated performance.
Related Terms
- Diffusion
- Gel / Gels
- CTO (Color Temperature Orange)
- CTB (Color Temperature Blue)
- 0.3 ND
- 1x Net / Single Scrim
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