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A photo shot with an 1:1.8 lens contains the following exif data:

ApertureValue: 1695994/1000000  
MaxApertureValue: 16/10

How is this possible? Both values are <1.8.

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What is writing that value? EXIF data is just metadata appended to an image file, and is editable by many programs and devices. –  drewbenn Jan 13 '12 at 17:46
    
OT, but I was curious so I just looked it up: I now believe that the proper capitalization is "Exif" which is how it is used throughout the standard. –  drewbenn Jan 13 '12 at 18:29

1 Answer 1

up vote 5 down vote accepted

You are confused because ApertureValue and MaxApertureValue are APEX values, not F-stops.

An F-stop of 1.8 is equivalent to an APEX aperture value of about 1.695994. (The formula is Av=log₂A², where Av is ApertureValue and A is f-number. See the Wikipedia APEX system page for a full explanation.)

Some exif readers (like ExifTool) convert these to F-stops for the user, but others do not.

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Woah! I can't believe I didn't realize that. That's crazy but indeed seems to be correct. See EXIF standard here. –  mattdm Mar 12 at 18:52
    
According to the ExifTool docs, FNumber and ExposureTime correspond to the values we're used to working with, and ApertureValue and ShutterSpeedValue are the APEX values. –  mattdm Mar 12 at 19:00
    
FWIW, my Pentax camera doesn't seem to set the APEX tags at all. My Nexus phone does, however. Both ExifTool and Exiv2 convert ApertueValue to f/number for display (in this case, f/2.4), but if I look at the raw values, Fnumber is set to 240/100 and ApertureValue to 252/100 — the corresponding APEX value. Hmmm. I'm not sure if these tools are making it more or less confusing by converting. –  mattdm Mar 12 at 19:10

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