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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

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    +1 for an interesting question - but I suspect that practically, a 2.5% difference in focal length one way or the other isn't going to make the difference between a good photo and a bad one so I wouldn't worry about it too much. – Philip Kendall Nov 18 '14 at 20:03
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    I'm curious if the alignment statistics are consistently that way with different lenses, or even the same lens at different focal lengths. – Please Read Profile Nov 19 '14 at 0:06
  • @mattdm Yeah I'm pretty curious about that too, the figures above were with a fixed 50mm lens. I'll probably do further testing when I can. – a_henderson Nov 19 '14 at 0:17
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    One possibility is that the focal length isn't being set precisely enough and that the 50mm lens is actually closer to 48.8mm. – Please Read Profile Nov 19 '14 at 0:21
  • @PhilipKendall this question is about panorama stitching, so 2.5% difference in focal length could resolve some stitching errors and potentially noticeably improve the output from hugin, or at least make things easier. – Ryan Nov 19 '14 at 7:08
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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.)

  • +1 for the exiftool resolution, I've updated the question. – a_henderson Nov 18 '14 at 22:35
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    btw, the scale factor is a composite tag, computed from FocalLength and FocalLengthIn35mmFormat. – ths Nov 19 '14 at 10:29
  • @ths which means the difference may well be accounted for by the actual vs, rounded focal length of the lens. – Michael C Feb 12 '16 at 19:52

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