Should I use 0.976 as the Canon 5D mkII Crop Factor / Focal Length Multiplier?

Question

When I import images from my Canon 5D mkII to Hugin I've noticed that it auto inserts the value of 0.976 as the Focal Length Multiplier. Should I change this to 1?

The images meta-data read with exiftool confirms the figure:

Scale Factor To 35 mm Equivalent: 0.975938688184433

I checked the canon uk specs page, and it lists the sensor as 36x 24 mm CMOS, so does that confirm that the Focal Length Multiplier should be 1?

If the crop factor on this camera should be 1, does anyone have any information about why it's recording strange metadata to it's images that suggests otherwise?

Update: I've tried playing with the Focal Length Multiplier value in Hugin to see how it affects the alignment statistics of a panorama, and obtained the following numbers:

Focal Length Multiplier | control points found | mean error | max error
        0.95                       92               5.0         19.4
        0.976                      93               4.9         19.3
        0.988                      93               4.8         19.0
        0.994                      93               4.7         19.0
        0.997                      93               4.7         19.0
        1                          93               4.7         18.9
        1.003                      107              5.5         32.1
        1.006                      107              5.5         32.0
        1.024                      104              5.2         31.4
        1.12                       105              5.3         29.6
        1.25                       105              5.8         27.5

I feel like these figures point towards 1 as being the best choice for low alignment error, not sure if that makes it the 'correct' figure for the camera though, and no idea why Canon would store an 'incorrect' value in the image meta :S

Answer 1

I think you should, in fact, use the slightly-smaller value. That's not because I've measured, but because I can resolve the apparent contradiction from exiftool: it's showing you a rounded value. Try giving it the -n flag, to disable what exiftool calls "print conversion":

 $ exiftool -n -ScaleFactor35efl sample.jpg
 Scale Factor To 35 mm Equivalent: 0.975938688184433

Which, of course, is ridiculous over precision, but there's the raw value. I agree with Philip's comment that the difference probably has little practical effect, but it seems likely that if you're going for "most correct", the metadata is right.

I suspect that the small difference is due to the edge of the frame being masked out and used for setting black level and similar. (Some of it is for demosaicing edge pixels, but 2% seems excessive for that.)

And I can confirm that this lines up with Canon's specs, too: the spec sheet lists

Effective Pixels Approx 21.1M
Total Pixels     Approx 22.0M

and, a-ha:

22 × 0.975938688184433² ≈ 20.95403911

which is pretty close to the effective number — close enough that I'm going to call the remaining difference rounding error. (This works out to a strip of about 100 pixels around each edge, by the way.)