Distance in EXIF

yehudakgtbnet wrote:

Hi,

I'm using 7D with 500+1.4. In viewing the EXIF data of my photos the Distance line is empty or doesn't exist at all (depending on which EXIF reader I'm using).

Is this normal?

Thank you,

Yehuda

Focus Distance as an EXIF data item is almost meaningless. And in fact Canon does not record this in the metadata. Instead they (for lenses that support it) record data to the fields: Focus Distance Upper (FDU) and Focus Distance Lower (FDL). This data gives you an approximate range for where the lens reports to the camera it's focused. If you use ExifTool to examine a .CR2 file or a JPEG created by DPP those are the only metadata fields you'll find that refer to focus distance. On the other hand, if you check a tiff or JPEG created by Adobe from a .CR2, you'll find an additional field: Approximate Focus Distance (AFD). This is bogus data that Adobe cooks up somehow that typically falls between FDU and FDL (i.e., FDL < AFD < FDU), but not always!

A few other focus distance EXIF facts based on the Canon and Sigma lenses I tested: 70-200/4 IS, 100/2.8 IS Macro, Sigma 35/1.4:

1. FDL and FDU do not vary freely. They are reported for fixed "focus zones," for lack of a better phrase.
2. Focus zones differ by lens but there appears to be a limited number of zones that a lens reports or that the camera will record. It seems an unlikely coincidence that all three of the tested lenses each have 31 focus zones given that they are very different types of lenses from different manufacturers. These tests were all done on a 7D, so other cameras may differ. I recently bought a 5D3 and I'll eventually repeat these tests with it whenever I find the time.
3. For a particular focus zone, AFD is always the same value even when the focus has clearly shifted between shots.