What does Lightroom’s “brightness value” mean?
Asked 10/18/2015
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Lightroom shows a “brightness value” in the metadata for each photo. I’ve noticed it tends to be negative for night shots and positive for daytime images. What exactly is this number, and how is it calculated?
Is it based on the image contents, such as an average or median pixel brightness, or is it an exposure-related EXIF value from the camera? Can it be used to compare photos or to judge whether two images will appear equally bright?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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At a high level, this is an EXIF value that represents the APEX average luminance value of a scene. Luminance, in this case, is basically the intensity of light travelling travelling towards the camera. This is why it's typically positive for a daytime shot, because there is much more light present, reflecting off of the surface of the subjects.
The EXIF standard (in Annex C) explains the usage in reference to exposure value.
Ev = Av + Tv = Bv + Sv
Where Ev is "Exposure Value", Av is "Aperture Value", Tv = "Time Value" (shutter speed), Bv is "Brightness Value", and Sv is "Sensitivity Value" (film speed). So, its basically a way of calculating exposure and is recorded in the image EXIF for reference.
Originally by user472. Source · Licensed CC BY-SA 4.0
user472
10y ago
0
Generated from our catalog & community — verify before relying on it.
Lightroom’s “brightness value” is generally not a Lightroom-computed average of the pixels. It’s an EXIF/APEX metadata value describing the scene luminance the camera recorded for exposure purposes.
In APEX notation:
EV = AV + TV = BV + SV
where EV is exposure value, AV aperture value, TV shutter/time value, BV brightness value, and SV sensitivity (ISO/film speed).
So BV is basically a scene-brightness reference used in exposure calculations. That’s why daytime scenes are usually positive and dark/night scenes can be negative.
It should not be treated as a perceptual measure of how bright the finished photo looks, nor as a reliable way to make different images “equally bright.” It’s tied to exposure metadata and scene luminance, not to the edited rendering, tone curve, display brightness, or the image histogram after processing.
Use it mainly as a technical metadata field for understanding exposure conditions, not as a visual matching tool.
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