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What is the Brightness Value in Lightroom?
- Thread starter zalle
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JS Burnie
Senior Member
I think it is an average luminescence value. See this old thread: https://www.dpreview.com/forums/post/55461535
Also see the exif standard: http://www.cipa.jp/std/documents/e/DC-008-2012_E.pdf
I can't pretend to follow all of this.
Also see the exif standard: http://www.cipa.jp/std/documents/e/DC-008-2012_E.pdf
I can't pretend to follow all of this.
Doug Pardee
Veteran Member
tl;dr: Bv is the brightness of the scene as metered by the camera.
Welcome to the complex world of lighting, metering, and exposure. It's not nearly as simple as many photographers try to make it.
Let's start here: many photographers refer to lighting conditions in terms of "Ev." As a common example, "full sunlight is approximately 15 Ev." There are many problems with this, including the teeny-tiny detail that "Ev" is purely a product of shutter speed and f-number; it doesn't include sensitivity (ISO). So when people say that full sunlight is approximately 15 Ev, you need to automatically add "at ISO 100." Still, no matter how flaky it is, this terminology isn't going away.
FIrst complication: your camera doesn't know about lighting conditions. All it knows is how much light is coming into the lens, which will be very different between a pile of coal, a snow drift, or a (switched on) light bulb in the same lighting conditions. The only way to measure the actual lighting conditions is to use an incident light meter, which few people bother with.
Next complication: if you have filters on your lens, they'll cut the light that the camera's meter sees.
Another complication: if you use flash, that will add light that your camera's meter doesn't see.
So, what your camera's meter is measuring isn't lighting conditions at all but rather the reflected and direct light reaching the metering sensor (or main sensor, for mirrorless like Fuji).
In the old days, we assumed a default average scene reflectance of 18% or 12% or so. In any metering mode other than Multi, your Fujifilm camera assumes 18%. In Multi mode, the camera tries to figure out what kind of scene you're pointing at, and makes a guess as to the average reflectance for that kind of scene.
That measured scene brightness -- perhaps modified by the "smarts" of the Multi metering mode -- is Bv.
Multi mode not only assumes that the scene isn't necessarily 18% gray, it also assumes you don't even want 18% gray to come out at 18% gray because it looks "too dark" to most people. Consequently, the Bv produced by Multi metering will be about one less than Bv produced by other metering modes (in Fujifilm); that will trigger about one stop greater exposure and/or higher ISO.
Back to the main discussion...
The Bv value from the metering system sets the goal for the auto-exposure and auto-ISO systems. The formula in the old APEX system is simply Av+Tv-Sv = Bv. Av and Tv are log2 numbers for aperture and shutter speed, respectively, with higher numbers being lower exposure (brighter scene). Sv is a log2 number for sensitivity, with higher numbers being more sensitive (dimmer scene). Av=0 is f/1.0, Tv=0 is 1 second, and Sv=0 is ISO 3.125 (yup, that's an oddball).
By plugging our ears and singing La-la-la-la loudly enough, thus ignoring many unpleasant complications, we can pretend that if we add 5 to the Bv value (5 being the Sv for ISO 100), we'll get the "Ev at ISO 100" value for lighting conditions. So, assuming an average scene reflectivity of 18%, no flash used, no filter mounted, minimal light loss in the lens, and using a non-Multi metering mode, the lighting conditions for that picture would've been 5.47 Ev (at ISO 100). That would be on the order of a well-let home interior at night.
There are a few cases where the EXIF specification doesn't say how the Bv should be determined:
Welcome to the complex world of lighting, metering, and exposure. It's not nearly as simple as many photographers try to make it.
Let's start here: many photographers refer to lighting conditions in terms of "Ev." As a common example, "full sunlight is approximately 15 Ev." There are many problems with this, including the teeny-tiny detail that "Ev" is purely a product of shutter speed and f-number; it doesn't include sensitivity (ISO). So when people say that full sunlight is approximately 15 Ev, you need to automatically add "at ISO 100." Still, no matter how flaky it is, this terminology isn't going away.
FIrst complication: your camera doesn't know about lighting conditions. All it knows is how much light is coming into the lens, which will be very different between a pile of coal, a snow drift, or a (switched on) light bulb in the same lighting conditions. The only way to measure the actual lighting conditions is to use an incident light meter, which few people bother with.
Next complication: if you have filters on your lens, they'll cut the light that the camera's meter sees.
Another complication: if you use flash, that will add light that your camera's meter doesn't see.
So, what your camera's meter is measuring isn't lighting conditions at all but rather the reflected and direct light reaching the metering sensor (or main sensor, for mirrorless like Fuji).
In the old days, we assumed a default average scene reflectance of 18% or 12% or so. In any metering mode other than Multi, your Fujifilm camera assumes 18%. In Multi mode, the camera tries to figure out what kind of scene you're pointing at, and makes a guess as to the average reflectance for that kind of scene.
That measured scene brightness -- perhaps modified by the "smarts" of the Multi metering mode -- is Bv.
Multi mode not only assumes that the scene isn't necessarily 18% gray, it also assumes you don't even want 18% gray to come out at 18% gray because it looks "too dark" to most people. Consequently, the Bv produced by Multi metering will be about one less than Bv produced by other metering modes (in Fujifilm); that will trigger about one stop greater exposure and/or higher ISO.
Back to the main discussion...
The Bv value from the metering system sets the goal for the auto-exposure and auto-ISO systems. The formula in the old APEX system is simply Av+Tv-Sv = Bv. Av and Tv are log2 numbers for aperture and shutter speed, respectively, with higher numbers being lower exposure (brighter scene). Sv is a log2 number for sensitivity, with higher numbers being more sensitive (dimmer scene). Av=0 is f/1.0, Tv=0 is 1 second, and Sv=0 is ISO 3.125 (yup, that's an oddball).
By plugging our ears and singing La-la-la-la loudly enough, thus ignoring many unpleasant complications, we can pretend that if we add 5 to the Bv value (5 being the Sv for ISO 100), we'll get the "Ev at ISO 100" value for lighting conditions. So, assuming an average scene reflectivity of 18%, no flash used, no filter mounted, minimal light loss in the lens, and using a non-Multi metering mode, the lighting conditions for that picture would've been 5.47 Ev (at ISO 100). That would be on the order of a well-let home interior at night.
There are a few cases where the EXIF specification doesn't say how the Bv should be determined:
- If you use exposure compensation, does the Bv reflect the metered Bv or the Bv as adjusted by EC?
- If you do manual exposure, does the Bv reflect the metered Bv, or the Bv as set by the photographer?
- If you use, say, an adapted lens where the camera doesn't know the aperture that's used during metering, how is Bv to be calculated?
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