Internal focusing

Inner focusing system, abbreviated IF, is a type of construction of lenses, in which the distance adjustment is not performed by a shift of the whole lens, but only by one or a plurality of lenses inside the lens. The other lenses, especially the front lens, retain their distance from the image capture plane ( reticle, film or image sensor), thus increasing the overall length of the lens does not change.

The inner focusing system is now widely used in optics and photography, and has replaced the focuser with many measuring telescopes. It was developed in the 1920s by the geodesics Heinrich Wild as a first step to overcome the open design of theodolites by today's closed design and make the instruments less sensitive to temperature and humidity effects.

Problem

Ordinary lenses focus by the fact that the entire lens package moves in the direction of its optical axis. This results in setting on close subjects to a greater length and an associated shift in focus. In addition, a large mass to move with the readjustment of all lenses that slows down the focusing. These effects are insignificant for short focal length with telephoto lenses but considerably and a great disadvantage. Why are camera lenses from about 3x magnification compared to the normal focal length, for small image that is about 150 mm, built since the 1970s increasingly with internal focusing.

Lenses with internal focusing can be more compact and easier to build, which at long focal lengths is particularly favorable. Even when focusing the focus hardly displaced, so that when using a tripod no change in the tripod load and thus no disturbing tilting the camera ( due to the elasticity of the tripod ) takes place. However, a disadvantage may be that, in general, changes the focal length by the distance setting.

Operation

In conventional focus distances of the lenses remain always equal to each other. The lens package has a infinity and can be from the camera of this move away. To focus depict a subject in a given distance, a certain distance of the lens package (more precisely, the image-side principal plane ) set of the film or Sensoerebene. This excerpt said displacement of the lens depends not only on the subject distance also depends on the focal length of the lens. It increases with increasing focal length.

When changing the distances of the lenses from one another, as is the case with the inner focusing system, not only the image-side principal plane shift, but it changes, in general, the focal length, and thus the adjusted distance between the principal plane of the film. Thus, there are two ways to realize the focus:

• The main level is moved forward while maintaining the same focal length.
• At a constant position of the main plane, the focal length is decreased.

Usually one uses both effects simultaneously.

When internal focusing lens in the rear area, only selected of the lens to be moved. These lenses are relatively small and light and have to move very much usually, and affect the lens focus as a result only slightly. Also, they can easily be adjusted very quickly, both by hand and by an autofocus motor. Since the front lens, in particular, the front lens does not belong to the reclined lenses, the lens overall length remains unchanged. The reduced focal length and thus magnification at close range has no meaning in practice. In contrast to the conventional method, but may change the distortion of the lens with a distance.

Rather than move selected lens, the distance can be concealed even by you switch out lenses. This leads to a gradual focusing and found in Polaroid instant cameras application ( see Polaroid ).

Advantages of internal focusing

• When focusing less mass is moved so that it can be done faster.
• The weight shift is much smaller, so that the tripod load hardly changes and the camera is not prone.
• The version of the lens can be designed smaller and lighter.
• The lens is more robust in general. If the lens would collect a blow to the front edge, the focusing mechanism is not overburdened as it lies completely inside.
• The front lens does not rotate (as opposed to Frontlinsen-/Frontgruppen-Fokussierung many zoom lenses ). This can pole or graduated filter (see Filters ) can be used without problems. Therefore, the use of flower-shaped lens hood (ugs. lens hoods ) is possible, which are cut into the corners to avoid vignetting.

Disadvantages of Internal Focusing

• With some zoom lenses (especially with a large focal length range ) is significantly reduced by focusing on short distances the maximum focal length to some extent.

Special forms

The rear focus (English rear focus, RF) is a variety of internal focusing. The rearmost lens or lens group is moved.

The reverse design is the extract extension with fixed rear element ( engl. Fixed Rear Element Extension, FREE). Only the front lens or the front elements is pulled, the rear element remains rigid.