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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)

where 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):

  1. I'm not sure which exif tag to use for each of the needed parameters:
  • F-Number: FNumber: 11 or ApertureValue: 11.3?
  • ISO: ISO: 500 or BaseISO: 519?
  • Time: So far I have one candidate ExposureTime but I might be missing something
  1. On top of that, I can see two tags in exif data; MeasuredEV: 10.38 and 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 14.14 and 14.27 for the different values provided above).

Can someone please help me calculate the actual EV for an image?

[UPDATE]

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 MeasuredEV, MeasuredEV2, and CalcEV which I calculated using the provided formula and parameters: ISO, FNumber, and ExposureTime.

enter image description here

As you can see, while the two MeasuredEV and 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!

[UPDATE]

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:

enter image description here

And for the sake of completeness, the corresponding time (shutter speed):

enter image description here

And as I said before, the aperture was fixed on 11 for the whole set.

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    \$\begingroup\$ What was the ISO value from frames 1-220 or so? What were the ISO values between frames 220 or so and 370? 370- the end? \$\endgroup\$
    – Michael C
    Jun 13, 2019 at 7:08
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    \$\begingroup\$ There's a lot going on in this question! This site works best when you narrow it down to just one thing. For example, the question about FNumber vs. ApertureValue is answered here, but that doesn't cover the rest of your question. \$\endgroup\$
    – mattdm
    Jun 13, 2019 at 12:14
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    \$\begingroup\$ I'm voting to close this question as off-topic because it is way too broad. Just the sub-question about ExposureValue vs ExposureValue2 EXIF tags is suitable for its own question. The basic question of "what is exposure value (EV)" is its own question (which has been asked and answered here before). And the question of BaseISO tag, and why it's such a large and abnormal value, also deserves its own question. \$\endgroup\$
    – scottbb ♦
    Jun 14, 2019 at 19:16

1 Answer 1

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The formula should be :

EV = LOG(N^2 / t, 2) - LOG(S/100, 2).

If a cloud passes and the EV diminishes by half, you can keep the same N and t but need to double S (ISO). EV of a scene is decreasing with S needed to take the picture.

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    \$\begingroup\$ An EV calculator can be found on an archived DPReview article on exposure. This supports Hugues' formula. \$\endgroup\$
    – qrk
    Jan 26, 2022 at 5:12

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