What is Aperture Value in EXIF, and how does it relate to the f-number?
Asked 3/23/2020
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macOS Finder shows a photo shot at f/2.8 with an Aperture Value of 3. What does Aperture Value mean in EXIF metadata, and how is it different from the f-number or f-stop shown on the lens/camera?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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The Exif standard says that:
$$\text{ApertureValue}=2\log_2(\text{FNumber})$$
which is also:
$$\text{ApertureValue}=\frac{2\log(\text{FNumber})}{\log2}$$
There is even a nice conversion table:
ApertureValue (APEX) F-Number 0 1 1 1.4 2 2 3 2.8 4 4 5 5.6 6 8 7 11 8 16 9 22 10 32However, looking at my own photos (EOS 70D), I find that the exposure value and the FNumber are close to each other but not always equal, so this doesn't follow the table below for apertures => f/8. So I wonder if for Canon, these tags wouldn't be:
- F Number: what you (or the auto-exposure) have set the camera to
- Aperture value: actual aperture taking in account actual focal length (that changes with focus....)
Originally by user75947. Source · Licensed CC BY-SA 4.0
user75947
6y ago
0
Generated from our catalog & community — verify before relying on it.
Aperture Value (AV) in EXIF is just a logarithmic way of expressing aperture in stops relative to f/1, using the APEX system.
Formula: AV = 2 × log2(f-number)
So the common full-stop apertures map like this:
- f/1 = AV 0
- f/1.4 = AV 1
- f/2 = AV 2
- f/2.8 = AV 3
- f/4 = AV 4
- f/5.6 = AV 5
- f/8 = AV 6
- f/11 = AV 7
- f/16 = AV 8
That’s why Finder reports f/2.8 as Aperture Value 3.
The difference is mainly representation:
- f-number (or f-stop) is the familiar aperture ratio shown as f/2.8, f/4, etc.
- Aperture Value is the same aperture expressed as a stop-based numeric value.
They describe the same exposure setting, just in different scales. AV is useful because each increase of 1 means one full stop less light. Values below f/1 would produce negative AV numbers.
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