Tony M Feb 10, 2015:

It means basically... ...the distance passed through the air — i.e. not through the glass lens elements; as such, it is to a large extent a 'real', measurable physical distance (unlike the distances of the paths refracted through the lenses.

Murielle M Feb 10, 2015:

Bonjour, est-ce qu'on pourrait avoir de début de la phrase? ("....... air distance on the optical axis between the first lens and the second lens, and f is a focal length of the entire optical system.) Sinon je dirais aussi probablement la distance en ligne droite...

Chakib Roula Feb 9, 2015:

Distance dans le reflet peut-être.

Tony M Feb 9, 2015:

I'm pretty sure... ...it wouldn't be 'aérienne'.