Conventional lenses are designed in such way to ensure optimal correction of aberrations only at one or sometimes two the most common focus distances. That’s why despite the good correction of aberrations at these focus distances they also become more obvious and degrade the image quality at the rest of focus distances. The degree of deterioration of the image quality depends on the type of the lens and its speed – the biggest deterioration occurs with asymmetric optical schemes (f.e. retrofocus), and the smallest – with symmetric (f.e. Planar). With retrofocus lenses the less is the focal length or higher the lens speed the stronger are the aberrations.
Most of wide-angle lenses for SLR cameras are retrofocus and show less aberrations with focusing at larger distances but their field curvature becomes pronounced with focusing at smaller distances.
The floating system was introduced in order to ensure the perfect correction of aberrations (primarily, astigmatism) at the entire focusing range. The part of the optical system used to correct aberrations moves («floats») in the process of focusing. Another function of the floating system is correction of spherical aberration which becomes stronger with fast lenses at smaller focusing distances.
The floating system was patented by Minolta already in 1958 but the first mass-production photographic lens which used this technology was Nikkor-N 24mm F/2.8 Auto (1967).
Nowdays the technology of floating lens system is used by all key lens manufacturers of photographic lenses in some models of fisheye lenses, wide-angle lenses, macro lenses, moderate telephoto lenses and has the following designations:
|Nikon||CRC||Close Range Correction|
|Tokina||FE||Floating Element System|
|Zeiss||—||Floating Elements Design|
As the internal or rear focusing, the floating system has its shortcoming: the actual focal length of the lens decreases with decreasing of focus distance and the field of view becomes wider.