Advertisement
If you have a new account but are having problems posting or verifying your account, please email us on hello@boards.ie for help. Thanks :)
Hello all! Please ensure that you are posting a new thread or question in the appropriate forum. The Feedback forum is overwhelmed with questions that are having to be moved elsewhere. If you need help to verify your account contact hello@boards.ie
Hi there,
There is an issue with role permissions that is being worked on at the moment.
If you are having trouble with access or permissions on regional forums please post here to get access: https://www.boards.ie/discussion/2058365403/you-do-not-have-permission-for-that#latest

EXIF Aperture values

  • 17-04-2011 10:13pm
    #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: 6,713 ✭✭✭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: 6,713 ✭✭✭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!


Advertisement