Photos taken with portrait orientation. Panorama Settings FOV seems very wrong.

[Richard Birket]

Hi all.

I have been using PTGui for several years now. Mostly, it has been very reliable, but an issue that keeps coming up is about photo orientation and the FOV of the resultant panorama.

I create architectural visualisations for architects. They give me their plans. I create a 3D model. Then I shoot photography of the site and render the 3D model from the same relative position that the photos were taken from, using a 'virtual' camera (in 3D Studio Max) with exactly the same focal length. The rendered image is then superimposed onto the photographic background. Simples.

However, often I am supplied with photos of the site by the architect. Sometimes (and even I do this) the camera has been held in a vertical (portrait) orientation to give me a bit more sky and foreground. I shoot 3 or 4 photos with a view to stitching them back together in PTgui. The CRITICAL thing I need to know about the stitched panorama is the effective focal length of the camera that would have been used to create such a panorama in one shot. This is found under 'Panorama Settings' in PTgui (I am on version 10.0.7). 

The value I use is the horizontal field of view. I plug this value into my 3D Studio Max camera and I know that there will be a good match.

However, often, that FOV value doesn't seem to make very much sense. 

A panorama I am just constructing is made of 5 vertically oriented photos, each taken with the Canon 400D with the kit lens, set to 55mm (in 35mm equivalent terms). There is a good degree of overlap between them. The control points are automatically created and the stitch seems reasonably good. The problem is that the horizontal FOV reads 16.49 degrees (vertical FOV is 7.15 degrees). A 16.49 FOV translates to a 124mm lens (in 35mm terms)!!! That is far too telephoto for the resultant panorama.

What am I doing wrong?

[PTGui Support]

Hi Richard,

Most likely PTGui has optimized the lens focal length to something
larger than 55mm. This also leads to a longer focal length for the
resulting panorama. If this is not desirable, go to the Optimizer tab
and disable optimization of the horizontal field of view. Of course you
need to reset the lens focal length to 55mm before running the optimizer
again.

Why did PTGui optimize the lens focal length? Because it gave lower
control point errors. For normal and tele lenses the lens projection
does not give much clues to what the true focal length is. PTGui will
use the 55mm just as a starting point, and optimize until the control
point errors are minimized.

Kind regards,

New House Internet Services BV
Joost Nieuwenhuijse

-----------------------------------------------
PTGui - Photo Stitching Software

www.ptgui.com
For support see: http://www.ptgui.com/faq/
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[Erik Krause]

Am 15.10.2014 18:52, schrieb Richard Birket:
> A panorama I am just constructing is made of 5 vertically oriented photos,
> each taken with the Canon 400D with the kit lens, set to 55mm (in 35mm
> equivalent terms). There is a good degree of overlap between them. The
> control points are automatically created and the stitch seems reasonably
> good. The problem is that the horizontal FOV reads 16.49 degrees (vertical
> FOV is 7.15 degrees). A 16.49 FOV translates to a 124mm lens (in 35mm
> terms)!!! That is far too telephoto for the resultant panorama.

Do you allow FoV to be optimized? If yes, PTGui changes the source image
FoV such, that the photos fit best, which - in case of partial (non
360°) panoramas - can even lead to very small output FoV. This happens
likely if you shot handheld or your panoramic tripod head wasn't set up
correctly. For a correct result uncheck FoV under "Optimize globally" on
Optimizer tab (advanced interface) before the first optimizer run or
"Align Images".

Please note that the FoV is an assumption: EXIF data doesn't always
record the exact FoV since the lens doesn't encode it exactly, and if
PTGui determines FoV (f.e. in a 360° panorama) it can differ slightly
from the real value due to lens distortion.

If you want to overlay real world images to a computer generated
panorama it's best to do exactly this. For details see
http://www.ptgui.com/support.html#5_33

--
Erik Krause
http://www.erik-krause.de