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Lightroom shows me a "brightness value" for each of my photos:

enter image description here

I notice that it's negative for night photography and positive for daytime photos, but I don't know how it's computed. Is it the average or median pixel value in the image? In other words, an indication of where the photo falls in the spectrum between high- and low-key images?

Can I use the brightness value to compare images, or make them equally bright perceptually? Are there other uses for this value?

  • Did you check if it is in the EXIF? Maybe it's not from Lightroom, just shown there. – Itai Oct 18 '15 at 14:57
  • Good question. I opened the ARW in Sony's Image Data Converter, but the properties did not have a field for this. – Vaddadi Kartick Oct 19 '15 at 2:35
  • In other words, it seems to be synthesized by LR. – Vaddadi Kartick Oct 24 '15 at 1:59
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    Sony's software may not expose all EXIF fields. Use EXIF Tool for that, it is definitely not created by Lightroom. – John Cavan Oct 24 '15 at 2:06
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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.

  • Thanks. I don't understand this equation, though. Shouldn't it be exposure = aperture * time * ISO, so taking logarithms, you'll end up with Ev = Av + Tv + Sv. – Vaddadi Kartick Oct 24 '15 at 1:59
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    @KartickVaddadi - Exposure value has a meaning, which isn't 'exposure' as you're using it. See: en.wikipedia.org/wiki/Exposure_value for some details. – John Cavan Oct 24 '15 at 2:03

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