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Author Topic: Short vs Long Focal Length: What is more appropriate for airborne SfM?  (Read 73 times)

smescarzaga

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Hello all,

I've been trying to search through academic papers to get to some critical or baseline publications that discuss and resolve the question posed in the subject line. I haven't much luck beyond the off-handed suggestions that long focal lengths are less desirable for 3D reconstruction; but no explanation as to why. Can anyone chime in on this questions and, if possible, suggest any relevant literature?

Thanks,
S

James

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Re: Short vs Long Focal Length: What is more appropriate for airborne SfM?
« Reply #1 on: October 15, 2018, 02:55:14 PM »
This is how i think about it:

http://www.agisoft.com/forum/index.php?topic=2478.msg13468#msg13468

With a short focal length you get a wider field of view, and so intersecting 'rays' from overlapping images can meet at shallower angles to the ground which gives better depth resolution.

The black shaded areas in my diagram in the linked post indicate the amount of depth reconstruction uncertainty.

Longer focal length = narrower field of view = all intersecting rays meeting almost perpendicular to the ground which you can see in the image gives rise to great uncertainty in depth resolution.

I can't remember if i read this somewhere, or just figured it out, so can't point you to anything more scientific!

smescarzaga

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James,

Thanks for your contribution. This certainly helps in my understanding!

S