Focus Systems

Floating Elements

On of the most underestimated effect on image quality is the performance deterioration towards closer focus distances. Curvature of field becomes significantly pronounced here and reduce the sharpness towards the image borders. The same goes for large aperture lenses that suffer from increased spherical aberrations here. Normal lenses shift just a single group of elements for focusing. Additional floating elements (see picture below) can improve the close-focus performance significantly. However, the vast majority of all lenses does not feature this mechanism. Even worse, many so-called "macro" lenses (e.g. 70-300macro, 400/5.6macro) don't have a floating system so don't expect good results at critical magnifications (e.g. 1:4 or closer)! Many ultra-wide and virtually all true macro lenses (e.g. 50/2.8macro, 100/2.8macro, 200/4macro) have FEs though.

Technology - Technology
  • IF (internal/inner focusing)

  • One of the middle lens groups in front of the diaphragm moves during focusing resulting in fast AF and light manual focusing. No change in physical length of the lens body which also remains quite small. Shorter minimum focus distances but often some decrease in focal length towards closer focus distances. Non rotating front element.
  • RF (rear focusing)

  • Only one or more rear lens groups behind the diaphragm move during focusing. Due to the small size and weight of the group it enables faster AF. There may be a change in physical length when zooming. A lens with a constant length and RF is in fact called an IF lens. Non-rotating front element..
  • Extension system

  • All groups or the front group(s) are shifted for focusing. The physical length changes usually during focusing and zooming often combined with a rotating front element. Relatively slow AF with long and heavy lenses. Relatively small max. magnifications (larger min. focusing distance). In the case of an all-group shifting mechanism there's only a relatively small increase of aberrations (=minor decrease of image quality).
  • Film back focusing

  • So far this type of focusing is used exclusively in the Contax AX. In contrast to the other focusing systems there's no need to shift heavy groups of glass but the film plane in the camera itself is moved for focusing. The shifting distance is relatively small so the focusing speed is potentially fast. The mechanical construction is quite complicated (=expensive) and there're a couple of positive and negative side effects. The most positive effect is the "increased" quality at closer focus distances. All (manual) lenses can be set to infinity focus distance (point of max. potential image quality). Furthermore the focusing system can be used as an optional "extension tube" with increased close focus capabilities. A certain drawback is the need to "pre-focus" long tele lenses (300mm & up) off infinity because the shifting distance of the film back does not reach out to provide focusing over the whole "normal" focusing range.


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