Floating System
Nowadays, optical technology has advanced to the point where high-quality images can be produced from any shooting distance. In general, regardless of the shooting distance, using an RF or EF lens is unlikely to result in any significant difference in imaging performance. This is because most modern lenses include elements that correct for various types of aberration based on the shooting distances at which the lens is most commonly used. However, when the subject is outside of this "standard range" (especially when it is closer than the distance for which the lens was designed), some form of aberration is more likely to occur.
The degree of performance degradation will vary depending on the aperture setting and the type of optical system used. Aberration is more likely to occur when the focal length is short and the aperture is large. Wide-angle lenses, in particular, exhibit higher image curvature at close distances. If the centre of the image is sharp, there will be some blurring at the periphery, whereas a sharp focus at the periphery will cause blurring in the centre. To address this issue, Canon developed floating mechanism technology, which provides optimal aberration correction across all shooting distances.
This reduces aberration across all shooting distances, resulting in excellent imaging performance from close range to infinity. This floating system is used in the EF24mm f/1.4L II USM and other large-aperture wide-angle lenses, as well as the EF180mm f/3.5L Macro USM, to achieve crisp imaging performance at close distances. Large aperture lenses, even those with a near-symmetrical lens structure, exhibit spherical aberration when used at close range. As a result, a floating mechanism is used in EF lenses like the EF50mm f/1.2L USM and EF85mm f/1.2L II USM, as well as RF lenses like the RF70-200mm f/2.8 L IS USM and RF100-500mm f/4.5-7.1L IS USM. Unlike the mechanism used in wide-angle lenses, the rearmost lens element of these lenses is fixed in place, whereas the floating mechanism allows the other elements to move. This helps to ensure excellent imaging performance and minimal lens flare at all shooting distances.
RF70-200mm f/2.8L IS USM
