Exposure will not be affected.
85mm is the limit the most any brand flashes can reach, by moving the flash tube (=the l&) behind-ahead only, inside the head.
To go to longer ones, i.e.: 135mm or 200mm, it would need very heavier fresnel-lenses on its front, like big theatre-projectors do.
Settings likewise 85mm, compress the sent lightning, (it beeing like a cone shaped) from its border toward its center; so, lightening less space/area but as more strongly.
This way, allows the sent light to go to longer distances according with Lens' Focal-length, then avoiding to lighten what stays outside(around) the Lens' sight.
It means increasing the Flash's useful-power onto the long distances getting tele-photos; where, otherwise the most light would go as dispersed around.
If You get a 135mm Lens, just some light will go as "wasted" around, on things outside the picture; but the whole scene will be lightened at all, however.
And CAMERA PROPERLY EVALUES THOSE LIGHT WHICH COME BACK TO IT FROM THE SCENE's area only; as "Through The Lens" (TTL).
If the situation does not overtake Flash's power-limit, picture gets no reason to work out as under-exposed.
Instead, problems may start whether You use a widest Wide-Lens, more wide than Flash do.
I.e.: Flash sets up to 24mm, with a 15mm Lens.
Anton
