Brightness level and fl value in A700 EXIF.

[lock]

I found a brightness index in the EXIF that seems to be logarithm of the fl-value. FL seems to stand for FLUX LUMINANCE: amount of light on a square inch:
[Camera]
Exposure time: 1/20"
F number: F5.6
Exposure program: Aperture priority
ISO speed rating: 6400/39°
Exif version: Version 2.21
Date and time of original data generation: 2008-09-23 21:13:09
Date and time of digital data generation: 2008-09-23 21:13:09
Meaning of each component: YCbCr
Image compression mode: 8 bpp
Brightness: -1.5 (1/2.8 fL)
Exposure bias: ±0 EV
Maximum lens aperture: 4.97 Av (F5.6)
Metering mode: Center-weighted average
Light source: Tungsten (incandescent light)

It's linked to this picture just to get an idea:



Does anyone know more about this value ?
I'd like to know if it's specific to the area covered by the metering ( spot, center weight) or that it is simply the total amount of light (probably based on matrix). Also I would like to know if the value is somehow convertable to EV (or LV) values. any suggestions ?

[Javelin]

I was tytring to work out what that value ment to. but I have images where flash was used on a dim... but lit with low ambient light that show a negative or even 0

[01af]

The formula for the brightness value is: BV = AV + TV - SV (also often written as: EV = AV + TV = BV + SV).

The respective formulas for AV, TV, and SV are:

AV = 2 * ld(k)
TV = -ld(t)
SV = ld(ASA / 3.125) = (DIN / 3) - 2

where ld(x) is the logarithmus dualis (i. e. logarithm to the base of two) of x, k is the aperture number, t is the shutter speed in seconds, and ASA and DIN are the film or sensor speed in, well, ASA or DIN.

In the example above, k = 5.6, t = 1/20, ASA = 6,400, and DIN = 39. So AV = 5; TV = 4.5; SV = 11 ... and thus, BV = 5 + 4.5 - 11 = -1.5.

-- Olaf

[lock]

Yep. Found similar formulas after some tips elsewhere.
But if the brightness value presented in Exif are simply recalculations of the exposureparameters, the values are indeed influenced by the metering mode. So with spot metering the brightness value is specific to the spot area. Am I right ?

Also Ev=log2 ((F value*Fvalue)/shutter speed in sec.) but this is not independent of the iso 6400. At a standard iso 100 this would have been Ev 3.3 ( 6 stops less).

lock

[01af]

But if the brightness values presented in EXIF are simply recalculations of the exposure parameters ...

That's what they are indeed.

... the values are indeed influenced by the metering mode. So with spot metering the brightness value is specific to the spot area. Am I right?

Yes, you are.

Also Ev = log2((F value * F value) / shutter speed in sec.) but this is not independent of the ISO 6400/39°.

EV is independent from the ISO value. It depends solely on aperture and shutter speed, and nothing else.

At a standard ISO 100/21° this would have been Ev 3.3 (6 stops less).

That's right. EV gives no indication of the brightness level of the scene because the film/sensor speed is not taken into consideration. In your example, EV was approx. 9.5. But does that mean the subject was dark? Or brightly illuminated? No-one can tell unless the information about the ISO setting is supplied. From EV and SV you can calculate BV which does give an idea about actual brightness. EV alone does not.

-- Olaf

[lock]

EV is independent from the ISO value. It depends solely on aperture and shutter speed, and nothing else.

-- Olaf

Hi Olaf. Thanks for the input btw. Much appreciated.
What I meant to say is that the actually calculated value -being 9.3- is influenced by the fact that I did use iso 6400. That gave me the increased shutter speed (as I was in Av mode). But any camera reporting the Ev value would probably recalculate it to iso 100 equivalent, like in all my low light pictures with Pentax cameras showing very low values of Ev (from 4 to 1 Ev mainly)despite using iso 1600. As Iso 6400 is 6 stops faster than iso 100, you can simple take the 9.3-6 stops (Ev's), bringing the 3.3 at an equivalent iso 100 level.

Btw, the reason why I want to have these values is because I want to know what kind of lenses I have to buy to get the pictures I want. I've got loads of pictures with gear that was not up to the task (too slow, not good with open aperture hence shutterspeeds to high), but the pictures I took with them did provide valuable data about the light circumstances I was in. To make the pictures 1:1 comparable with the Sony gear in terms of info about the light, I need to convert them. That's why I'm going through all this...