Aperture is the size of the lens opening for the shot, given as an f-number: f/2.8 lets in twice as much light as f/4. EXIF records the aperture twice, in two different units, and nearly every question about these tags comes from that.
Two tags, two units
FNumber (0x829D) is the f-number itself, stored as an unsigned rational such as 14/5 or 28/10 for f/2.8. This is the value the camera displayed.
ApertureValue (0x9202) holds the same setting in APEX units, from the Additive System of Photographic Exposure, where each full stop adds 1. The conversion is Av = 2 × log2(N), and back again N = 2^(Av/2):
| f-number | APEX Av |
|---|---|
| f/1.4 | 0.97 |
| f/2 | 2 |
| f/2.8 | 2.97 |
| f/4 | 4 |
| f/5.6 | 4.97 |
| f/8 | 6 |
| f/16 | 8 |
MaxApertureValue (0x9205) uses the same APEX scale for the widest aperture the lens can open to, which is useful for telling an f/1.8 prime from an f/4 zoom when the lens model is missing.
What ExifTool shows you
ExifTool converts APEX to an f-number before printing, and it does so even with -n, so both tags look identical in normal output. The stored APEX number only appears in verbose mode:
$ exiftool -G1 -s -FNumber -ApertureValue -MaxApertureValue photo.jpg
[ExifIFD] FNumber : 2.8
[ExifIFD] ApertureValue : 2.8
[ExifIFD] MaxApertureValue : 1.8
$ exiftool -v1 photo.jpg | grep -E "Aperture|FNumber"
| | 0) FNumber = 2.8 (14/5)
| | 3) ApertureValue = 2.970853659 (24361/8200)
| | 4) MaxApertureValue = 1.695993817 (13166/7763)Some cameras write neat APEX values instead. A Canon EOS RP JPEG shot at f/16 stores ApertureValue as 524288/65536, exactly 8.
When the two disagree, believe FNumber. ApertureValue is redundant and the f-stop labels are rounded: store an exact APEX 5 for a nominal f/5.6 and ExifTool prints f/5.7, because 2^(5/2) is 5.657. Most phones have a fixed aperture per camera module, so FNumber only changes when the phone switches lenses.
Editing aperture values
The EXIF editor writes FNumber, ApertureValue and MaxApertureValue for JPEG and TIFF files. Type the f-number (2.8) in all three; ExifTool does the APEX conversion. For the wider picture of exposure tags in a JPEG file, see what EXIF data is.