Shutter Speed vs Exposure Time

That’s just the difference between accepted, ‘photographical’ values and real, measured values.

Just keep in mind you are converting from analogue values of apertures and shutter speeds to digital for processing and then back to analogue in the program that displays EXIF data.
It really doesn't matter in most real world applications.

OK - I need to tweak this a bit having read the EXIF specification document.

xarqi wrote in post #4237639
By my reading, shutter speed and exposure time begin the same, but because of the way the information is stored, there is a slight variation. In one case, a log function is invoked, and this is then truncated; in the other, the value is stored as a real number directly (also of finite precision). Conversion of these possibly slightly different stored values to the conventional form 1/x, (especially when x will be rounded to an integer) may result in different values being displayed.

It turns out that EXIF stores these values as pairs of 48 bit integers, representing a signed (in the case of shutter speed) or unsigned (in the case of exposure time) rational number: e.g. if the stored values are 1 and 2, the number is 1/2; if they are -3 and 8, the number is -3/8.

The APEX shutter speed value is defined as -log(2) of the exposure time (s), so both numbers have the same ultimate source. Any difference is in precision, arithmetic accuracy, and representation.

e.g. exposure time (that you set) of 1/4 gives APEX value of - 2; 1/8 -> -3; 1/16 -> -4, etc.

But, there is a problem, and it is not clear from the EXIF spec how it is handled (as far as I can see). In the table in appendix C, it shows 1/30s -> -5, and this is incorrect. It is 1/32s that gives -5; 1/30 gives -4.9068905.... What is not clear is how the derived values are converted to the signed rational values for storage - they could be rounded or truncated, or approximated to the nearest rational pair (within the precision available), e.g. 1/30 s, when converted to the APEX shutter speed value could be stored as -490689 / 100000, or as some other combination that approximates the true value better.

End of the day - both numbers have the same origin, but are processed differently - for mere mortals, the actual difference makes no practical difference.

Thanks guys fully understand now.

If the shutter speed is 1/250 then one full stop either way is either 1/125 or 1/500 - so why would you bother about it saying 1/256 in stead of 1/250??

I was simply wondering why but i know now so i am fine.

Riverlander wrote in post #4238020
If the shutter speed is 1/250 then one full stop either way is either 1/125 or 1/500 - so why would you bother about it saying 1/256 in stead of 1/250??

APEX value stored for shutter speed was equal to -8. 2 to the power of -8 = 1/256.
Looks like APEX values may be rounded.