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liz read
 

Don't Understand Exif Info..


E.g. why do "Aperture Value" & "F Stop" show different values for same image?!
1. Example, one frame's readout shows: "AV" = 5.7; and "FS" 7.0. Huh? What? Why two different numbers?
2. Also, same image, gives these two readouts: "Aperture Value" = 1/420 sec, and "Shutter Speed" = 1/416???
3. "Brightness Value", same image, shows 837/100. What does this mean? And what's the "value" to me of this value?
Thanks!
Ciao - liz......


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5/12/2004 1:16:03 PM

 
Jon Close
BetterPhoto Member Since: 5/18/2000
  This link is a .pdf file of the JEIDA (Japan Electronic Industy Development Association) documentation of the file format standard for EXIF (Exchange Image File Format).

There is a lot of data recorded in EXIF, some of it seemingly redundant. Some are given in the familiar units while others use what is referred to as APEX (Additive Photographic EXposure system) units. For example, "F-Number" is the familiar aperture setting for the lens when the photo was taken, eg. 8 for f/8. "Aperture Value" is the same thing, but given in APEX units (= 0 for f/1, 2 for f/1.4, ...). Max Aperture Value of the lens is also recorded in APEX units.

In your example 1 - FS=7.0 means f/7 lens setting, which in APEX value is the same as AV = 5.7, ie. 5.7 stops smaller aperture than f/1.

I can't really what you've written for example 2. EXIF records Exposure Time in the familiar seconds or fractions of a second, and it records Shutter Value in APEX units (30 sec = -5, 15 sec = -4, 8 sec = -3, etc.).

Using the APEX units, Aperture Value + Shutter Speed Value = Exposure Value.

Example 3 - Brightness Value is a measure of the amount of light in the scene recorded and it is also in APEX units.


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5/13/2004 9:29:52 AM

 
liz read   Gulp...scary stuff indeed.
Jon, thanks for devoting so much time to explain stuff that boggles my simple mind. How nice it would be if the camera people delivered an EXPLANATION of exif info with cameras.
But then they would NEVER do anything so obvious!!!
Rather like their instruction manuals (I prefer to call them "obfuscation manuals") which are either simplistic (this is the STRAP of your camera) or incomprehensible; PLUS what they NEVER explain is usually of key importance to picture taking!
Phew! I feel much better after getting that snarl off my chest!
Thanks again
Liz.......


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5/15/2004 6:46:29 AM

 
liz read   Gulp...scary stuff indeed.
Jon, thanks for devoting so much time to explain stuff that boggles my simple mind. How nice it would be if the camera people delivered an EXPLANATION of exif info with cameras.
But then they would NEVER do anything so obvious!!!
Rather like their instruction manuals (I prefer to call them "obfuscation manuals") which are either simplistic (this is the STRAP of your camera) or incomprehensible; PLUS what they NEVER explain is usually of key importance to picture taking!
Phew! I feel much better after getting that snarl off my chest!
Thanks again
Liz.......


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5/15/2004 6:46:50 AM

 
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