Floating system

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.

Retrofocus scheme is a type of optical scheme designed and patented by Pierre Angenieux in the early 1950s. Also known as «inverted telephoto» lens. The scheme allows to design wide-angle lenses with increased back focal distance for SLR cameras. The rear element of retrofocus lens is placed at longer distance from the plane of film or digital sensor in comparison with non-retrofocus lenses therefore such lenses show less vignetting but suffer from barrel distortion.

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 optical scheme of popular Sigma 35mm F/1.4 DG HSM | A lens with floating system. Copyright © Sigma Corporation

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:

Manufacturer Desigation Meaning
Canon Floating System
Nikon CRC Close Range Correction
Sigma Floating System
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.

Your comment

Being unregistered user or not logged in user you won't get any email notifications about replies on this comment.

Copy this code

and paste it here *