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I have been using pyexiv2 to read exif information from JPEG files in python, and noticed that one tag in particular - ExposureTime - is not reported the same by exiv2 as with another exif library, libexif.

Any exiv2-based utility I've tried will simplify the exposuretime tag to a "rational" such as 0/1, 0, or similar. libexif based utilities (in particular, a tool "exif") will report a much more detailed "1/-21474836 sec." for the same tag, in the same image.

Firstly I'd like to understand: what can account for this difference? I'm assuming that the latter of the two is correct.

Secondly, and assuming that the more detailed tag as reported by libexif is correct, I'd like to be able to obtain this value in Python, where as far as I can see it is not possible using any EXIF tools that I have come across (pyexiv2 for example). Is there a tool or method that I am not considering?

I have stumbled across one potential solution with the use of the libexif C library in python with ctypes as noted in this previously answered question - though I could not find examples of how I could do this.

Any help is greatly appreciated. Thanks!

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j_marvin
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1 Answers1

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In case this helps, here are some hacks I recently did to set missing lens / F-Number,.. informations as I was using a manual lens plus I computed actaul absolute EV for automatic retrieval by later HDR processing tools (HDR Luminace). I commented out the "write" action for safety below. Should be pretty much self explanatory.

The top files section makes a list of files to work on in the current folder (here all *.ARW (Sony raw files)). Adjust the pattern and path as needed.

#!/usr/bin/env python
import os
import time
import array
import math

# make file list (take all *.ARW files in current folder)
files = [f for f in os.listdir(".") if f.endswith(".ARW")]
files.sort()   # just to be nice

# have a dict. of tags to work with in particular
tags = {'Aperture':10., 'Exposure Time ':1./1250, 'Shutter Speed':1./1250, 'ISO':200., 'Stops Above Base ISO':0., 'Exposure Compensation':0. }

# arbitrary chosen base EV to get final EV compensation numbers into +/-10 range
EVref = math.log (math.pow(tags['Aperture'],2.0)/tags['Shutter Speed'], 2.0) - 4
print ('EVref=', EVref)

for f in files:
    print (f)
    meta=os.popen("exiftool "+f).readlines()
    for tag in meta:
        set = str(tag).rstrip("\n").split(":")
        for t,x in tags.items():
            if str(set[0]).strip(" ") == t:
                tags[t] = float ( str(os.popen("calc -- "+set[1]).readlines()).strip("[]'~\\t\\n"))
                print (t, tags[t], set[1])

    ev = math.log (math.pow(tags['Aperture'],2.0)/tags['Shutter Speed'], 2.0)
    EV = EVref - ev + tags['Stops Above Base ISO']
    print ('EV=', EV)
#  uncomment/edit to update EXIF in place:
#    os.system('exiftool -ExposureCompensation='+str(EV)+' '+f)
#    os.system('exiftool -FNumber=10 '+f)
#    os.system('exiftool -FocalLength=1000.0 '+f)
#    os.system('exiftool -FocalLengthIn35mmFormat=1000.0 '+f)
Percy Zahl
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