Inner focusing
Internal focusing , abbreviated IF , is a type of lens construction in which the distance is not adjusted by shifting the entire lens, but only by moving one or more lenses within the lens. The other lenses, especially the front lens, keep their distance from the image capture plane ( focusing screen , film or image sensor ) so that the length of the lens does not change.
Internal focusing is widespread today in optics and photography and has also replaced the focuser on many measuring telescopes . It was developed in the 1920s by the geodesist Heinrich Wild as the first step to overcome the open construction of theodolites with today's closed construction and to make the instruments less sensitive to the effects of temperature and humidity.
Problem
Ordinary lenses focus by shifting the entire lens package in the direction of their optical axis . When focusing on close motifs, this leads to a greater overall length and an associated shift in the center of gravity. In addition, when all lenses are adjusted, a large mass has to be moved, which slows down focusing. These effects are meaningless for short focal lengths , but are considerable and a great disadvantage with telephoto lenses . That is why camera lenses from about 3 times magnification compared to the normal focal length, i.e. from about 150 mm for small images , have increasingly been built with internal focusing since the 1970s.
Lenses with internal focussing can be made more compact and lighter, which is particularly advantageous for long focal lengths . Also, the center of gravity hardly shifts when focusing, so that there is no change in the load on the tripod when taking pictures on a tripod and thus there is no annoying tilting of the camera (due to the elasticity of the tripod). However, it can be a disadvantage that the focal length generally changes with the distance setting.
Today there are also a number of lenses with short focal lengths ( normal or wide-angle ) with internal focusing, as this can also bring advantages here, namely better correction of aberrations (see floating elements ) and a constant volume of the lens, which means that no air is drawn in when focusing .
functionality
With conventional focusing, the distances between the lenses always remain the same. The lens package has an infinity setting and can be moved away from the camera starting from this. In order to image a motif sharply at a given distance, a certain distance of the lens package (more precisely: the main plane on the image side ) from the film or sensor plane must be set. This shift of the lens, called the excerpt , depends not only on the distance to the subject but also on the focal length of the lens. It increases as the focal length increases.
If you also change the distances between the lenses, as is the case with internal focusing, not only does the image-side main plane shift, but the focal length and thus the distance between the main plane and the film that is to be set also generally change. There are two ways of realizing the focus:
- The main plane is shifted forward while the focal length remains the same.
- If the position of the main plane remains the same, the focal length is reduced.
Usually you use both effects at the same time.
With internal focusing, only selected lenses in the rear area of the lens are moved. These lenses are relatively small and light and usually only have to move a little and therefore only have an insignificant effect on the focus of the lens. They can also be adjusted very quickly, both by hand and with an autofocus motor. Since the front lenses, in particular the front lens , do not belong to the adjusted lenses, the overall lens length remains unchanged. The reduced focal length and thus the enlargement at close-up is of no importance in practice. In contrast to the conventional method, however, the distortion of the lens can change with distance.
Instead of moving selected lenses, the distance can also be adjusted by changing lenses. This leads to a gradual focus and is used in Polaroid instant cameras.
Advantages of internal focusing
- Focussing moves less mass so it can be done faster.
- The focus of the lens hardly shifts, so that the camera does not tilt when taking pictures on a tripod.
- The mount of the lens can be made smaller and lighter.
- The lens is generally more robust. If the lens is hit on the front edge, the focusing mechanism is not stressed because it is completely inside.
- The front lens does not rotate (unlike the front lens / front group focusing of many zoom lenses ). This means that polarizing or graduated filters (see filter ) can be used without any problems. This is why it is also possible to use tulip-shaped lens hoods (coll. Lens hoods), which are cut into the corners of the image to avoid vignetting .
- The volume of the lens does not change when focusing, so there is no "pumping", ie. H. the lens does not draw in air that may contain dust or moisture. The lens can be sealed against dust and splash water.
Disadvantages of internal focusing
- With some zoom lenses (especially with a large focal length range), but also with some lenses with a fixed focal length, the focal length can be reduced considerably by focusing on short distances ( focus breathing or zooming effect of focus ).
Special characteristics
The rear lens focusing (engl. Rear focus , RF ) is a variety of internal focusing. The rearmost lens or lens group is shifted.
The reverse construction is the extension with a fixed rear element ( Fixed Rear Element Extension , FREE ). Only the front lens or the front elements are pulled out, the rear element remains rigid.
Individual evidence
- ↑ Tony Northrup: Tony Northrup's Buying Guide, How to Choose a Camera, Lens, Tripod, Flash, & More , Mason Press, Inc., 2016, ISBN 9780988263420 , p. 95
- ↑ Dominic Groß: Focal length information for zoom lenses - ten is sometimes only five , Spiegel online, November 8, 2009, part 1 online , part 2 online , accessed on January 2, 2020
- ↑ Nasim Mansurov: Understanding Focus Breathing - Focus breathing is a common issue on most photographic lenses , Photographylife.com, online , 2019, accessed on January 3, 2020
- ↑ Archived copy ( Memento of the original from April 28, 2010 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
