Importing images, I saw that IE build automatically some differents camera devices (i.e. device, device_2, device_3, ...) instead of only one, with differents parameters of focal, and that it was impossible to assign the default device to all images. Could anyone explain  its logic?

Hi ,
 
IM creates different camera devices only if the resolutions (in pixels) of your images are different. For example if you load the following images :
image1 (1600x1200) , image2 (1600x1200) , image3 (2048x1536) , image4 (1024x768)
IM will create 3 devices, one for image1 and image2, one for image3 and one for image4.
That means you can share the focal length only for image1 and image2 ...
 
If the focal length values are different that means probably that the Ratios of your images are different.
 
Hope it helps...
 
Stef