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In macOS Finder it says a photo taken at f/2.8 has an aperture value of 3. What is an aperture value, and how is it different from f-stop?

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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 32

However, 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....)
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  • \$\begingroup\$ What are you using to view the EXIF info? It may be converting the hex value for the Aperture Value field to f-number (actual as opposed to rounded off setting) rather than to actual AV. Irfanview seems to do this. \$\endgroup\$
    – Michael C
    Commented Mar 25, 2020 at 0:34
  • \$\begingroup\$ @MichaelC Both Gwenview and Exiftool report the same values. These two items are defined as RATIONAL in the standard and seem to be encoded the same way. \$\endgroup\$
    – xenoid
    Commented Mar 25, 2020 at 1:27
  • \$\begingroup\$ Considering that they are always the same number for the same f-number regardless of focus position or zoom position for zoom lenses across various different lenses (for example f/3.2 always shows as AV "F 3.22" in irfanview with lenses as varied as the EF 70-200mm f/2.8 L IS II and the EF 50mm f/1.4), I doubt it is doing what your last point suggests. Instead it seems to be showing the theoretical actual figured number to two decimal places for our rounded f-number scale that only uses a single decimal place. \$\endgroup\$
    – Michael C
    Commented Mar 25, 2020 at 19:57
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The AV (Aperture Value) is an expression that tells how many stops away from f/1 an aperture is.

Since f/1 is zero distance from f/1, it has an AV of zero (0)
Since f/1.4 is one stop slower than f/1, it has an AV of one (1)
Since f/2 is two stops away from f/1, it has an AV of two (2)
Since f/2.8 is three stops from f/1, its AV is three (3)
... and so on.

For instance, f/16 is eight stops slower than f/1, it has an AV of eight (8).

Apertures faster than f/1 have negative AV. For example, f/0.5 is two stops faster than f/1, and has an AV of -2.

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