How do I calculate exposure value (EV) from EXIF aperture, shutter speed, and ISO?
Asked 6/12/2019
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2 answers
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I want to compute the exposure value for a photo using EXIF metadata. I’m using aperture, shutter speed, and ISO from the file, but I’m unsure which EXIF fields are the correct ones to use and why my result doesn’t match camera-reported EV fields.
For example, EXIF may include values like FNumber vs ApertureValue, ISO vs BaseISO, and ExposureTime, plus maker-note fields such as MeasuredEV.
What is the correct formula for EV from EXIF, and which metadata values should be used? Also, why might a camera’s internal MeasuredEV values differ from a simple calculation?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
1
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.
Originally by user71772. Source · Licensed CC BY-SA 4.0
user71772
5y ago
0
Generated from our catalog & community — verify before relying on it.
Use the actual exposure settings used to make the shot: FNumber, ExposureTime, and ISO. The standard relation is:
EV = log2(N² / t) - log2(S / 100)
where N is f-number, t is shutter time in seconds, and S is ISO. Equivalently, EV100 = log2(N² / t) and EV = EV100 - log2(S/100).
So the sign on the ISO term is minus, not plus. That is why your calculated value is too high.
Which EXIF fields to prefer:
- FNumber: use this for the actual lens aperture.
- ExposureTime: use this for shutter speed.
- ISO: use this for sensitivity.
Fields like ApertureValue are often stored in APEX units, not plain f-number, so they may not match directly. BaseISO is not the same as the exposure ISO you should use here.
As for MeasuredEV / MeasuredEV2: those are likely camera maker-note metering values, not standardized EXIF fields. They can reflect the camera’s internal metering/processing and may not match a simple EV calculation from the recorded exposure settings exactly.
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