  For the current generation of Imaging Atmospheric Cherenkov Telescopes
(IACTs), with their large mirrors and their cameras with fine segmentation of
photodetectors, the focusing capability is a relevant issue. The optical system
of an IACT has a limited depth of field. Therefore, focusing the telescopes
close to the shower maximum in the atmosphere has a significant impact on the
data acquisition and analysis. As the distance of the shower maximum to the
telescope depends (among others) on the zenith angle, an adjustable focus would
be desirable. The fifth Cherenkov telescope of the H.E.S.S. II array is
equipped with a focus system which allows to adjust the position of the camera
along the optical axis, possibly during data taking. This impact has been
studied on gamma-ray Monte Carlo simulations, and the results in terms of
gamma-ray trigger rate, energy reconstruction and gamma-ray effective area will
be shown.
