# What's the difference between aperture value and f-number?

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?

## 2 Answers

The Exif standard says that:

ApertureValue=2*log₂(FNumber)


which is also:

ApertureValue=2*log(FNumber)/log(2)


There is even a nice conversion table:

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....)
• 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. Mar 25, 2020 at 0:34
• @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. Mar 25, 2020 at 1:27
• 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. Mar 25, 2020 at 19:57

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.