Capturing Photos with zoom lenses

[Bibi]

The Agiosft Tutorial recommends to set focal length of a used zoom lens either to maximum or minimum value -what is the reason behind this recommandation?

[Alexey Pasumansky]

Hello Bibi,

When zoom-lens is set to the intermediate focal length position, it may change slightly due to the camera shake, but the EXIF metadata will contain the same values due to the rounding to 0.1 mm or even to 1 mm. It means that practically two images are captured with the different focal length, but would be put to the same calibration group according to the EXIF information.

In case zoom-lens is used it may be reasonable to split the camera calibration group, if the alignment result is inaccurate.

[Smallpoly]

Adding onto that, you can also help prevent the focal length from changing by securing it with some painter's tape.