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Author Topic: Short vs Long Focal Length: What is more appropriate for airborne SfM?  (Read 283 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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Re: Short vs Long Focal Length: What is more appropriate for airborne SfM?
« Reply #2 on: October 18, 2018, 07:56:37 AM »
James,

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

S

Limpopo_River

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Re: Short vs Long Focal Length: What is more appropriate for airborne SfM?
« Reply #3 on: October 20, 2018, 05:51:53 PM »
Another benefit of using a moderately short focal length (generally the equivalent of 20-30 mm on a full-frame camera works best, but you can calculate the equivalent FL for your camera sensor) is that you can fly at a lower elevation and also increase the depth of field.  You might, however, have some problems with motion blur or distortion from a rolling shutter effect.  It's best to use a camera with a global shutter and/or reduce the speed of the UAS to avoid these problems. 

Kiesel

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Re: Short vs Long Focal Length: What is more appropriate for airborne SfM?
« Reply #4 on: October 22, 2018, 02:49:25 PM »
That answer by Geobit from 2015 may help as well:
Quote
Re: Camera lens clarification, cropped sensor / full frame
« Reply #4 on: October 13, 2015, 10:36:58 PM »

    Quote

for a given overlapping photos footprint you could choose a range of lenses. As long as focal lenght increases, the distance to object must be longer to have same coverage. As long as you get farther, the two images will become more similar one to the other.
Notice that this might carry advantage in terms of correlation: the more similar appear to be the  feature points, much more successful matches will be found for that pair... and that is great BUT at the same time the projective corresponding rays will intersect at smaller angles yielding to poor accuracy in 3D solution specially along the z axis (from object to camera). And that is BAD

On the contrary, as long as lens get wider,  ray intersections become closer to right angles. That in terms of accuracy is great BUT images become more different one to the other, this means that fewer feature point matches will be found because of their poor correlation... and that is BAD again.

The sweet point is somewhere in between wides and teles and this might be the reason to recommend normal primes.
In fact there is no exact rule but in general I'd say for aerial works you can go directly to wide angles in most cases. If there is not too much vegetation, and the terrain altitude range is not extremely long do not hesitate to use 35 or even 24mm equivalent lens.
For object scans (convergent pictures), I would recommend longer focal lenghts because the quality of the intersections can be ensured by adequate camera attitudes.

GEOBIT (Agisoft.es)