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EXIF Aperture values

  • 17-04-2011 11:13pm
    #1
    PureRGB
    Closed Accounts Posts: 34 ✭


    Hi all,

    I'm trying to extract aperture values from the EXIF data in the header of image files (for my website, purergb.com) - but I seem to be getting (what I initially thought were) weird values, such as f1.6918863/1000000, f1.970854/1000000, f1/8/1 - after a bit of searching I came across something called Additive system of Photographic EXposure (APEX) which apparently explains the odd numbers. Anyone familiar with this? and how do I convert to more conventional values?

    Many thanks!


Comments

  • #2
    DaireQuinlan
    Registered Users, Registered Users 2 Posts: 6,702 ✭✭✭DaireQuinlan


    I was looking at EXIF data a while ago, one of the libs I was looking at had the following ...
    TAG_APERTURE

    public static final int TAG_APERTURE
    The actual aperture value of lens when the image was taken. Unit is APEX. To convert this value to ordinary F-number (F-stop), calculate this value's power of root 2 (=1.4142). For example, if the ApertureValue is '5', F-number is 1.4142^5 = F5.6.


  • #3
    PureRGB
    Closed Accounts Posts: 34 ✭PureRGB


    Thanks Daire - I knew I could thust you to have a useful suggestion!

    Turns out there is quite a bit of info on wikipedia about it http://en.wikipedia.org/wiki/APEX_system - this also seems to suggest the aperture value is log2 (A^2). I guess I'll have a play around with the different options this evening


  • #4
    DaireQuinlan
    Registered Users, Registered Users 2 Posts: 6,702 ✭✭✭DaireQuinlan


    PureRGB wrote: »
    Thanks Daire - I knew I could thust you to have a useful suggestion!

    Turns out there is quite a bit of info on wikipedia about it http://en.wikipedia.org/wiki/APEX_system - this also seems to suggest the aperture value is log2 (A^2). I guess I'll have a play around with the different options this evening

    Hmm. That doesn't appear to be right at all. log2(A^2) gives 4.64..... for A=5. That -should- give you 5.56 from the common EV charts. The PDF that's referenced in the wiki article looks like a good resource,

    http://dougkerr.net/Pumpkin/articles/APEX.pdf

    That has a chart of the common EV->F/stop values.
    Aperture value (Av)|Aperture
    0|f/1.0
    1|f/1.4
    2|f/2.0
    3|f/2.8
    4|f/4.0
    5|f/5.6
    6|f/8.0
    7|f/11
    8|f/16
    9|f/22
    10|f/32


  • #5
    PureRGB
    Closed Accounts Posts: 34 ✭PureRGB


    Looking at this again, I was extracting the wrong value from the header, when I corrected this and started using (2^0.5)^value I got the value as expected.

    thanks again Daire!


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