How do you set a 3:1 main-to-fill lighting ratio correctly?
Asked 11/28/2013
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I'm confused about portrait lighting ratios. I thought a 3:1 lighting ratio means the fill light should be one stop lower than the main light, for example main at f/8 and fill at f/5.6. But I read advice saying to divide the main f-number by the ratio, such as f/8 ÷ 3 = f/2.6 for the fill, which doesn't seem right. How do lighting ratios actually relate to stops and meter readings, and what fill reading would correspond to a 3:1 ratio if the main is metered at f/8?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
2
You're right to think this is wrong, but wrong to think the ratio is 9:1 as a result. :)
The ratio is amount of light difference and it's worth remembering that change of one stop will halve or double the amount light. So, for example:
- 2:1 - 1 stop difference between fill and main
- 3:1 - 1.5 stops (approx.)
- 4:1 - 2 stops
- 8:1 - 3 stops
Another way to say that is that is:
- 2:1 - The fill is half the strength of the main
- 4:1 - The fill is one quarter the strength
- and so on...
Basically, 2 to the power of the stop difference is the ratio:
2f-stop:1
So, to wrap it all up, your fill should meter at f/4.8 for a 3:1 ratio as that is one and half stops from f/8.
Originally by user472. Source · Licensed CC BY-SA 4.0
user472
12y ago
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Generated from our catalog & community — verify before relying on it.
Dividing the f-number by the ratio is not the correct way to set lighting ratios.
Lighting ratio is based on light intensity, and each stop equals a doubling or halving of light. A useful guide is:
- 2:1 = 1 stop
- 4:1 = 2 stops
- 8:1 = 3 stops
So a 3:1 ratio falls between 2:1 and 4:1, which is about 1.5 stops difference.
If your main meters at f/8, then a 3:1 ratio would put the fill at about 1.5 stops lower, roughly f/4.8.
One important clarification: some people define portrait ratios as total highlight-side illumination versus shadow-side illumination. In that convention, a 3:1 ratio means the main is 1 stop brighter than the fill, because:
- main = 2 units
- fill = 1 unit
- highlight side gets main + fill = 3
- shadow side gets fill only = 1
So if you're using that common portrait-lighting definition, main at f/8 and fill at f/5.6 is correct for 3:1. The f/2.6 advice is incorrect.
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