Library module, metadata question (F-stop vs Aperture labels)

[kurtholz]

While changing my displayed fields in the Library module under Metadata, I noticed that there seems to be a labeling choice that I dont quite understand.

Simply put, we have the choice of displaying F-Stop"and/or Aperture in two dispict and seperate fields. While I've always considered these two items basically the same (or very closely related) as far as functional photography, I now feel that I may be missing the point somewhere.

Would the information in these two fields ever be different from each other?

Since both fields use/display the same info (f/?), is this just Adobe's effort to give us a choice of what we want to "label" essentually the same info? Or is there truely a difference between these two fields and they may diverge at some point?

Thanks!  Kurt

[johnrellis · Accepted solution]

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I've vaguely wondered the same thing over the years, and your post finally motivated me to do some research.

Shorter answer:

The F-Stop and Aperture displayed in the Metadata panel are always the same.

I believe originally F-Stop and Aperture represented two different EXIF fields in the photo, FNumber and ApertureValue.  The EXIF standard says that those two fields both represent the size of the lens opening, stored in different units (f-stops for FNumber and APEX units for ApertureValue).  

But in practice, many cameras record FNumber or ApertureValue but not both, and some cameras record different aperture measurements in the two fields.  So the early LR would often display different values for F-Stop (FNumber) and Aperture (ApertureValue converted to f-stop units). 

I'm guessing that a product manager saw this and said screw it, that's too confusing for the pea brains of us photographers. So they changed the Metadata panel F-Stop and Aperture to always show the same value: FNumber if it is present or ApertureValue otherwise (converted to f-stops).

They retained Aperture for backward compatibility with plugins that created custom metadata panels and compatibility with the habits of photographers used to seeing two separate fields in panels.

(My surmise about early LR history is just that, an educated guess.)

Very Long Nerd Answer

The Exif standard defines two fields, FNumber and ApertureValue:

 

FNumber is the traditional f-stop, e.g. 2, 2.8, 4, 5.6, 8, etc.

ApertureValue is the f-stop expressed in APEX units.  Annex C of the standard explains APEX is the Additive System of Photographic Exposure, in which:

ApertureValue = 2 * log2 (FNumber)

(Given two f-stops, the difference in their ApertureValues is the number of stops between the two. For example, f/8 is four stops away from f/2 (f/2, f/2.8, f/4, f/5.6, f/8), and the difference in ApertureValues of f/2 and f/8 is 4.)

So according to the standard, FNumber and ApertureValue should always represent the same measurement of the lens opening, just in different units. Both those numbers are represented as "rationals" (a 32-bit numerator divided by a 32-bit denominator), more than enough to provide at least 16 decimal digits of precision when converting from one unit to the other.

But in practice, it appears that many cameras record different equivalent values for the two fields.  I've got a test catalog with 714 raws from 22 camera makes and 260 models. Using the Any Filter plugin's Sort command, I exported a spreadsheet of file name, camera make and model, EXIF:FNumber and EXIF:ApertureValue, the latter arithmetically converted to f-stop units:

f-stop = (2^ApertureValue) ^ 1/2

I've attached that spreadsheet here. (Open it in Excel or Google Docs.)

Many cameras record FNumber but not ApertureValue including models from DJI, Haselblad, Leica, Nikon, Olympus, Om Digital Solutions, Panasonic, Ricoh, Sigma, Sony.  For these photos, LR's Metadata panel displays FNumber for F-Stop and Aperture.

A few cameras record ApertureValue but not FNumber, including five models from Leica and one from Sealife. For these photos, LR's Metadata panel displays ApertureValue converted to f-top units for F-Stop and Aperture.

Of the 347 photos that record both fields (out of a total 714), 92 record different aperture measurements, e.g.

These differences aren't caused by the different units used to store the values internally in the EXIF -- as mentioned above, 64 bits are used to represent each value, more than enough to get many more than 3 digits of accuracy in the conversion.

For all 92 photos recording different values for FNumber and ApertureValue, the Metadata panel displays FNumber for both F-Stop and Aperture, ignoring the different ApertureValue.