What
is IF(Internal Focus)?
Internal focusing for a TV zoom lens can simply be explained as the
application of floating to the front group of a zoom lens.
If the front group is split into two or three subgroups and the inner
subgroup is moved for focusing, the front and rear spaces of the
focusing group are changed. The difference of the influences of the two
spaces on the aberration is used to compensate the change in aberration
during focusing. Internal focusing works only when the distribution of
aberration between the fixed group and the focusing group is
appropriately designed.
Characteristics of the internal focusing system
l. Improvement in optical performance:
It is possible to improve the optical performance by making use of the
larger degree of freedom of aberrations correction. The performance has
been improved to increase the angle of view, reduce the change in
chromatic aberration during focusing, reduce distortion, etc.
2. Square hood.
A square hood can be used because the barrel of the front group is
stationary. In conventional lenses the hood had to be circular because
the front group was designed to rotate. The internal focusing lens can
eliminate ghosting and flaring more effectively than the conventional
lens by cutting unnecessary light fluxes at the square hood, and is thus
well suited for CCD cameras.
3. Reduction of the weight of the focusing lenses group.
This is an important factor in terms of operation. The focusing lens
group, which is lighter than those in a conventional lens, can be moved
quickly using little electric power, enabling smooth movement even
during manual operation.
4. The use of filters.
The hand held lens is used with various filters attached to front
barrel. However, the filters whose properties change with rotation, such
as polarising filters, cross filters, and half ND filters, were
difficult to use with a conventional lens whose front portion rotates.
These filters can be used with the internal focusing lens without any
problem because the front group does not rotate, thereby enabling the
filters to be used to their full effect.
