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. 5. Optical accessories. Optical accessories, such as a wide converter or tele-side converter attached to the barrel of the focusing operation. A handy mattebox can be attached directly to the barrel of the front group.