Camera lens clarification, cropped sensor / full frame

[Kadrasko]

Hey everybody, I have been looking into photogrammetry for the last few weeks and tested it out with a Nikon d800 + 50mm f/1.8G. I am looking into buying several Nikon d5300 and prime lenses for general / head rig. Here comes my question: Agisoft recommends a 50mm lens. But for what type of sensor? In my head it is either d5300 + 35mm and d800 + 50mm OR d5300 + 50mm and d800 + 70mm? Hope it makes sense, and thanks for any tips

[Arie]

Hi Kadrasko,
with Nikon DX cameras (APS-C sensor size, such as D5300) you have a crop factor of 1,5. This means, you have to multiply this number by the focal length to get the equivalent field-of-view when used on a full-frame (FX camera).

Therefore a 35mm lens on a DX camera (D5300) would be the same field-of-view as a 52mm lens on an FX camera (D800).
Cheers.

[Kadrasko]

Ok, thanks! So what it says it use a 50mm for a full frame camera, and for the others whatever fits closer to the 50mm sweet spot after compensating for the cropped sensor?

[Arie]

To be honest, I don't know exactly why the Agisoft team recommends a 50mm lens... Probably because it suffers less from distortions and abberations than wide angle lenses.

Maybe Alexeji might chime in and clarify why this focal length is recommended.
Cheers.

p.s. you might want to ask over at the body-scan section. They got lots more experience regarding the requirements for body models.

[JMR]

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)