What does a focal length value like (3680, 1000) mean in image metadata?
Asked 7/7/2017
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2 answers
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I extracted image metadata and saw this value for focal length:
FocalLength: (3680, 1000)
How should this be interpreted? Does it represent a normal focal length value in Exif data?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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Values in Exif metadata can be stored as various data types, including ASCII text strings, "short" or "long" integers, or "rationals". A short integer is stored in two bytes; a long integer takes four bytes. A "rational" is:
Two LONGs. The first LONG is the numerator and the second LONG expresses the denominator.
That is,FocalLength: (3680, 1000) means 3680 / 1000, or 3.68. (Elsewhere in the spec, this is defined to be in millimeters.) For whatever reason, whatever program you are looking at this with is just showing you the two "long" values rather than doing the math for you.
It's also incredibly important to note that hundredths of a millimeter is almost certainly excess precision. For practical use, call this 3.7mm or 4mm.
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
9y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. In Exif, some fields are stored as a rational value: two integers representing a fraction.
So:
- numerator = 3680
- denominator = 1000
That means the focal length is 3680 / 1000 = 3.68 mm.
Some software displays the raw rational pair instead of converting it to a decimal value for you. In Exif, focal length is defined in millimeters.
For practical purposes, this would usually be treated as about 3.7 mm, or even rounded to 4 mm depending on context, since that level of precision is more than typically useful.
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