How can the aperture value written in Exif be larger than the nominal limit of the lens?

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.

You are confused because ApertureValue and MaxApertureValue are APEX values, not f-stops.
An f-number of 1.8 is equivalent to an APEX aperture value of about 1.695994. (The formula is $$\\mathrm{A_v} = \log_2{\mathrm{A}^2},\$$ where $$\\mathrm{A_v}\$$ is ApertureValue and $$\\mathrm{A}\$$ is f-number. See the Wikipedia APEX system page for a full explanation.)
• 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. Mar 12, 2015 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. Mar 12, 2015 at 19:10