How do you convert EXIF ShutterSpeedValue (APEX) to an actual shutter speed?
Asked 6/9/2019
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I’m reading EXIF data in Python and get values like:
ShutterSpeedValue:(8643856, 1000000)ApertureValue:(1695994, 1000000)
I found that ApertureValue is stored as an APEX value, so sqrt(2^(1695994/1000000)) gives about f/1.8.
How do I convert ShutterSpeedValue from its EXIF/APEX form into the real exposure time in seconds?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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Exif:ShutterSpeedValue is stored in rational form as an APEX value.
The APEX system is a way to calculate exposure and works using base-2 logarithms. The use of base-2 means a rise of one in the value equates to a doubling, which we know as 1 stop; which makes it pretty handy for photographers if they're good with logarithms (which we don't use much nowadays). (James Snell)
To convert from APEX to seconds, use the following equation:
ShutterSpeedValue (Tv) = -log2(t)
⇒ t = 2-Tv
For the particular values from your camera:
2(-8643856/1000000) = 1 / 2(8.643856) = 1/400 sec
See also:
Wikipedia: APEX System.
ExifTool: Exif Tags
APEX—The Additive System of Photographic Exposure (PDF)
JEITA CP-3451 Exif 2.2 (PDF)
Originally by user75526. Source · Licensed CC BY-SA 4.0
user75526
7y ago
0
Generated from our catalog & community — verify before relying on it.
ShutterSpeedValue in EXIF is an APEX/Tv value, not the shutter time itself.
Use:
t = 2^(-Tv)
where Tv is the rational value converted to a number.
For your example:
Tv = 8643856 / 1000000 = 8.643856t = 2^(-8.643856)t ≈ 1 / 400 s
So the shutter speed is 1/400 second.
In Python, that would be roughly:
tv = 8643856 / 1000000
shutter_seconds = 2 ** (-tv)
This matches the APEX exposure system used by EXIF:
ApertureValue:N = sqrt(2^Av)ShutterSpeedValue:t = 2^(-Tv)
A positive ShutterSpeedValue means a shorter exposure time as the value increases, with each increment of 1 representing one stop (a halving of exposure time).
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