I'm trying to calculate the EV (exposure value) for an image file based on its exif data. From the exposure value article at Wikipedia, I am using this formula for calculating EV:
EV = EV100 + LOG(S/100, 2) = LOG(N^2 / t, 2) + LOG(S/100, 2)
LOG(x, 2) is log of x in base 2 and:
N: F-Number (1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22, 32, 45, 64) t: the exposure time in seconds ("shutter speed") S: ISO (100, 200, ...)
Now, the problems that I have for calculating EV based on exif data (the values shown are from a sample image I took with my Canon 6D, the camera was set on Av mode, with Aperture set to 11 and ISO auto):
- I'm not sure which exif tag to use for each of the needed parameters:
- Time: So far I have one candidate
ExposureTimebut I might be missing something
- On top of that, I can see two tags in exif data;
MeasuredEV2: 9.75. I believe the camera has calculated these internally but first, why there are two of them? And secondly, why when I calculate the EV using the formula does not match any of these two values (based on the formula, I have
14.27for the different values provided above).
Can someone please help me calculate the actual EV for an image?
I'm trying to use this formula to smooth the exposure for my time-lapses. Below you can see the flow of change for 600 pictures that I took as a time-lapse. The chart shows
CalcEV which I calculated using the provided formula and parameters:
As you can see, while the two
MeasuredEV2 are pretty close to each other, the
CalcEV diverges from them after a while. Which means I have no idea what's going on!
Per @Michael C's request (which I believe he is onto something), here you can see the changes for ISO over the 600 images I took:
And for the sake of completeness, the corresponding time (shutter speed):
And as I said before, the aperture was fixed on 11 for the whole set.