"Light Value" in Exiftool?

[heidi.h]

Hi Phil,
How is the "LightValue" output calculated?
Is it calculated by the camera, or by the Exiftool application?

[Phil Harvey]

This is a Composite tag calculated by ExifTool using the formula:

LV = 2 * log2(Aperture) - log2(ShutterSpeed) - log2(ISO/100)

- Phil

[kaarigar]

How to interpret the Light Value tag? Is higher / lower better / worse?

[Phil Harvey]

Googling turns up this explanation.

- Phil

[Neal Krawetz]

There are a couple of values for judging the lighting (may or may not be present in the metadata):

EXIF "Brightness" is the amount of lighting detected by the camera prior to capturing the photo.  Some cameras average the entire image, others spot-check specific areas. The "EXIF: Metering Mode" vaguely identifies where it looked.

As Phil noted, the Light Value is derived from the aperture, shutter speed, and ISO value. It identifies how much light should be allowed onto the sensor. Intuitively, the numbers seem reversed to me. A bigger value (e.g., +11) means less light comes into the sensor. A low value (e.g., -5) means more light hits the sensor.

If brightness is high, then light value should be high for an average picture.
High brightness with low light value will over-expose the image. (Lots of available light and lots of light hitting the sensor.)
Low brightness with high light value will under-expose the image.

The caveat here is that the "Brightness" is kind of arbitrary.  If the camera works like a light meter, then it only measures reflectivity and not ambient light. (Light meters can always be wrong!) If the camera just evaluates the brightness of the room, then it measures ambient and not necessarily reflectivity.