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For a manual lens (*Rokinon 8mm f/3.5 Fisheye) I want to add the focal length information to the EXIF metadata in the resulting pictures with exiftool. I use the lens on a Canon 60D body.

Obviously I need to write the tag

FocalLength

For the Rokinon I set it to 8mm. Additionally based on my camera with an APS-C sensor I want to set the 35mm equivalent focal length as well. What is the right tag for this? One candidate I discovered is

FocalLengthIn35mmFormat

However for another lens, the electronically connected Canon EF-S 15-85mm f/3.5-5.6 IS USM this tag is not set!!! The derived (composite) tag FocalLength35efl is calculated, so the information must come from somewhere. According to details on the exiftool website it might come from

 FocalPlaneXSize / FocalPlanYSize

For a manual lens, which tag should I write to reveal the 35mm equivalent info? So should I rather write FocalPlaneXSize than FocalLengthIn35mmFormat?

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    If you set the real focal length, does Exiftool not compute the composite tag from there? (Does it set the focal plane size tags?) – mattdm Jan 12 '15 at 13:37
  • @mattdm, FocalPlane???? are calculated and write in camera – Romeo Ninov Jan 12 '15 at 15:14
  • @RomeoNinov Right, so, hence the question — if those are being written, isn't setting just FocalLength sufficient (and then FocalLengthIn35mmFormat will be calculated). I haven't experimented, but the question strongly implies that it isn't being written. – mattdm Jan 12 '15 at 15:27
  • @mattdm, the software use "Scale Factor To 35 mm Equivalent" to calculate from FocalLength to FocalLengthIn35mmFormat – Romeo Ninov Jan 12 '15 at 15:35
  • @RomeoNinov ScaleFactor35efl itself is calculated from the FocalPlaneXResolution et al tags. – halloleo Jan 13 '15 at 2:57
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Good practice is adding

0x920a Focal length
0x9205 Max Aperture Value
0xa405 Focal Length In 35mm Format (60D sensor active area has 26.82mm diagonal)
0xa432 Lens Info
0xa433 Lens Make
0xa434 Lens Model
  • ExifTool docs seem to indicate that the "Focal Length In 35mm Format" tag is computed on the fly, not something written. Can you go into a bit more detail here? – mattdm Jan 12 '15 at 16:52
  • It should be computed if the tag is not present. The tag is listed in JEITA CP-3451. – Iliah Borg Jan 12 '15 at 17:25
  • If it's computed, is it still best practice to set it? What is it computed from? Why not just set those things directly? – mattdm Jan 12 '15 at 17:49
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    If one uses the exact diagonal, it is worthwhile to include the correct value, and to follow the standard IMHO is also a good thing. – Iliah Borg Jan 12 '15 at 17:52
  • The comments to this answer shed light on my question! I will look in regards to FocalPlaneDiagonal et al at the EXIF data of pictures taken with the electronic lens and report back here. – halloleo Jan 12 '15 at 20:44
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After thinking about the situation, I suggest not to write FocalLengthIn35mmFormat at all.

As mentioned in the question, metadata of pictures taken with an electronically connected lens (the Canon EF-S 15-85mm f/3.5-5.6 IS USM) do not provide the FocalLengthIn35mmFormat tag.

exiftool calculates the composite tag FocalLength35efl from other information (as mentioned in @RomeoNinov's answer) and tests on pictures of the manual lens revealed that it does so even if you only set FocalLength (as @mattdm had pointed out in a comment). The reason is that the Canon 60D body can and does set the EXIF tags FocalPlaneXResolution, FocalPlaneXResolution, FocalPlaneResolutionUnit for all lenses - electronic or manual.

So, in order to stay consistent with the EXIF metadata of electronically connected lenses on the Canon 60D body, do not write the FocalLengthIn35mmFormat tag.

  • @mattdm I know you pointed this out in a comment, but I think this answers the question, so I put it in an answer. – halloleo Jan 13 '15 at 2:51
  • @RomeoNinov Thanks for the calculation and your comments, they helped me to get a better understanding! – halloleo Jan 13 '15 at 2:55
  • If all you need is exiftool, you don't need this tag. EXIF standards, however, suggest otherwise. If you will be using some processing software, no guarantee it will calculate 35mm eq. same way as exiftool, or will calculate it at all. IMHO adding this tag is safe route. – Iliah Borg Jan 13 '15 at 3:14
  • @halloleo, FocalPlane?Resolution tags are characteristic of the camera, not of the lens and this is the reason they are in RAW file all the time :) – Romeo Ninov Jan 13 '15 at 5:12
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The tag you search is named "Focal Length"

Here is what I see with 70D, Sigma 24-105/F4

Focal Length : 80.0 mm (35 mm equivalent: 126.1 mm)

FocalPlane???? tags depend on digital zoom

P.S. FocalLengthIn35mmFormat is calculated from FocalLength multiplied by "Scale Factor To 35 mm Equivalent"

P.P.S. Here are the formulas to calculate scale factor (because this is calculated value)

FocalPlaneXSize = SensorWidth / (FocalPlaneXResolution / 25.4) (if size is in inch, otherwise use 10)
FocalPlaneYSize = SensorHeight / (FocalPlaneYResolution / 25.4) (if size is in inch, otherwise use 10)
FocalPlaneDiagonal = sqrt(FocalPlaneXSize^2 + FocalPlaneYSize^2)
Crop Factor (ScaleFactorTo35mmEquivalent) = Full Frame diagonal / FocalPlaneDiagonal

(if someone need them)

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    This is just a restatement of things already said in the question. – mattdm Jan 12 '15 at 15:47
  • I write above which tag should be used and get negative vote. Then explain how other tags are calculated (so not need to be in EXIF info). I really can't understand what OP expect as answer – Romeo Ninov Jan 12 '15 at 15:56
  • The question says that halloleo is already setting that tag, and that, as you repeat, FocalLengthIn35mmFormat is a derived tag. Presumably, that is not sufficient in practice, or else the question wouldn't need to have been asked. That's why I asked for clarification. – mattdm Jan 12 '15 at 15:58
  • Hope this time with exact formulas will be happy. And will clarify his question :) – Romeo Ninov Jan 12 '15 at 20:01
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    I still am not sure it addresses the actual problem. I guess we'll need to see what the original poster thinks! – mattdm Jan 12 '15 at 20:41

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