So I recently realized that I got burned by how MaxApertureValue is defined, set and printed. I had assumed that it took f-stop, but it really is APEX value. To protect well-meaning lunkheads like me, it would be nice to have a warning when setting this to common f-stop values (1.4, 2.8, etc). Perhaps allow for -MaxApertureValue= to accept strings like f/1.4 or f/2 to have these set as expected? A second part would be in printing to add the units, so instead of printing "Max Aperture Value : 1.4", print "Max Aperture Value : 1.4 Ev (f/1.65)". This would have indicated to me that something was not as I expected. (Similar things could be done for ShutterSpeed value, etc.)
Anyway, it seems that other software also presents this data in different ways, which only adds to the confusion!
The ExifTool EXIF Tag Name documentation (https://exiftool.org/TagNames/EXIF.html) says this about MaxApertureValue: (displayed as an F number, but stored as an APEX value)
So you aren't a lunkhead. These values are converted to equivalent F-number by ExifTool, and written in the same way.
- Phil
What makes it hard is that, while it's in the documentation, it's not clear in the output. Outputting as "f/2.0" instead of just 2.0 would make it clear what units are being used. Also, using the -n or -MaxApertureValue# form doesn't disable this conversion -- exiftool still prints the converted to equivalent f-number and not the unconverted APEX value.
It doesn't help that the APEX values and f-numbers are so close (f/2.0 = 2 Av & f/4.0 = 4, plus f/1.8 = 1.7 Av and f/3.5 = 3.6 Av, etc). It was easy for me to get confused. It also doesn't help that the camera and software makers seem to get these confused as well.
Quote from: sfmurph on July 26, 2014, 02:11:38 PM
What makes it hard is that, while it's in the documentation, it's not clear in the output. Outputting as "f/2.0" instead of just 2.0 would make it clear what units are being used.
I see. But I can't change this now or I break more than 10 years of backward compatibility (plus, I don't recall anyone ever suggesting this before).
QuoteAlso, using the -n or -MaxApertureValue# form doesn't disable this conversion -- exiftool still prints the converted to equivalent f-number and not the unconverted APEX value.
Correct. This just disables the rounding to 1 decimal point.
QuoteIt doesn't help that the APEX values and f-numbers are so close (f/2.0 = 2 Av & f/4.0 = 4, plus f/1.8 = 1.7 Av and f/3.5 = 3.6 Av, etc). It was easy for me to get confused. It also doesn't help that the camera and software makers seem to get these confused as well.
I agree. If this were 10 years ago, I would probably implement your suggestion.
- Phil
Ah yes, backwards compatibility can be tough to maintain!
What burned me is the read-write round trip. Do this to see what the MaxApertureValue is:
$ exiftool -overwrite_original_in_place file.JPG -MaxApertureValue
Max Aperture Value : 1.4
And the un-rounded version
$ exiftool -overwrite_original_in_place file.JPG -MaxApertureValue#
Max Aperture Value : 1.4142135623731
But then setting with that value actually sets it to something else (but close)
$ exiftool -overwrite_original_in_place file.JPG -MaxApertureValue=1.4
then
$ exiftool -overwrite_original_in_place file.JPG -MaxApertureValue#
Max Aperture Value : 1.40000000201821
The problem is that the rounded value is what the APEX value is calculated from, so instead of having stored "1", it's 0.9708536585 (7961/8200).
The workaround is to use precise, mathematical f-numbers and not the convenient stings 1.4, 2.8, etc.
Quote from: sfmurph on July 27, 2014, 04:40:34 PM
But then setting with that value actually sets it to something else (but close)
$ exiftool -overwrite_original_in_place file.JPG -MaxApertureValue=1.4
then
$ exiftool -overwrite_original_in_place file.JPG -MaxApertureValue#
Max Aperture Value : 1.40000000201821
The problem is that the rounded value is what the APEX value is calculated from, so instead of having stored "1", it's 0.9708536585 (7961/8200).
This rounding phenomenon is discussed in FAQ number 4 (https://exiftool.org/faq.html#Q4). To get an APEX value of 1, you must set the MaxApertureValue f-number to the square root of 2, not to 1.4:
> exiftool a.jpg -maxaperturevalue=1.414213562373095
1 image files updated
> exiftool a.jpg -maxaperturevalue#
Max Aperture Value : 1.4142135623731
> exiftool a.jpg -v | grep Max
| | 3) MaxApertureValue = 1 (1/1)The last command with the
-v option shows the APEX value that was stored in the file.
- Phil