focal length

Could you provide a test image so we can check?

file is too big. get it frpom dropbox, please

https://www.dropbox.com/s/39sax21y4gytsu0/img2698.jpg?dl=0

for the picture attached, my hugin Pre-Release 2015.1.0.1b8093495573
says

9.7 in exif and 9.332 (multiplier 4.867) in "Lens type"
(on the "photos" tab)

JPEckmann

On Fri, 22 May 2015, tmodes wrote:

->Your question #267209 on Hugin changed:
->https://answers.launchpad.net/hugin/+question/267209
->
-> Status: Open => Needs information
->
->tmodes requested more information:
->Could you provide a test image so we can check?
->

Ok. I tested with your image and after loading it displays the correct focal length of 9.678 mm.

After the loading the image Hugin looks in its database if it founds information about the field of view it used in the past. So I assume that in your case you stitched already images from this camera and for this panorama a slightly different field of view was used and this one is loaded again automatically.

The focal length shown on the image tab is the "real" focal length as determined from the current field of view.
To see the focal length from EXIF select Display > Exif Data. There you see the focal length from the exif data.
If you want to use the focal length from the EXIF data, select Reset>Reset lens parameter from the context menu on the image.

So, if I understand correctly, you compute or use the field of view from
an earlier use of another picture from the same camera?

What is the rationale for that?
Wouldn't it be more natural that hugin uses only the information from the
exif?

Or do you consider that less "correct" than what hugin computes? (There
are vast difference like 6,2mm / 5.2mm)

So, if my analysis is correct I would suggest that oin each new start of
hugin, the "reset" shoud be automatically used.

(FYI, I am using lensfun (via hugin) to correct for distortion)

Sorry for nagging, I want to understand

On Sat, 23 May 2015, tmodes wrote:

->Your question #267209 on Hugin changed:
->https://answers.launchpad.net/hugin/+question/267209
->
-> Status: Open => Answered
->
->tmodes proposed the following answer:
->Ok. I tested with your image and after loading it displays the correct
->focal length of 9.678 mm.
->
->After the loading the image Hugin looks in its database if it founds
->information about the field of view it used in the past. So I assume
->that in your case you stitched already images from this camera and for
->this panorama a slightly different field of view was used and this one
->is loaded again automatically.
->
->The focal length shown on the image tab is the "real" focal length as determined from the current field of view.
->To see the focal length from EXIF select Display > Exif Data. There you see the focal length from the exif data.
->If you want to use the focal length from the EXIF data, select Reset>Reset lens parameter from the context menu on the image.
->
->--
->If this answers your question, please go to the following page to let us
->know that it is solved:
->https://answers.launchpad.net/hugin/+question/267209/+confirm?answer_id=2
->
->If you still need help, you can reply to this email or go to the
->following page to enter your feedback:
->https://answers.launchpad.net/hugin/+question/267209
->
->You received this question notification because you asked the question.
->

For fisheye lenses especially there can be a significant difference between the nominal focal length and the real field of view.
Also for normal lenses there is sometimes a significant difference between the printed focal length (on the lens, in exif data) and the real focal length.
So I think that using the real value as determined with Hugin is more sensible for further processing in Hugin than fully trust on the exif data.

PS: Hugin can also correct distortions. Doing this in Hugin directly has the advantage that the interpolation is only done once (instead of first lensfun + remapping in Hugin). This will result in better image quality.

thanks! just for my curiosity: so, which parameters do you use to compute
what you call the "real" focal length?

Thanks
JPEckmann

Thanks tmodes, that solved my question.