Shim
The shim (also: focusing screen ; international, English : focusing screen ) is made of transparent material disc with multiple cameras , SLR -Fotoapparaten and - video cameras is used and as a projection of the design consideration and the manual focus position of the lens is used.
functionality
The focusing screen is made of glass or transparent plastic with a smooth and a matt side. The “rough” side is optically exactly in the same plane as the film or the digital sensor of the camera used. The camera's lens creates a real image on the matted surface so that the photographer can adjust the focus and the section and also assess the sharpness of the image. With view cameras, the focusing screen is pushed into the rear standard of the camera, all the required settings are made and then the focusing screen is exchanged for the flat film cassette for the recording . In SLR cameras, the focusing screen is usually permanently installed, but numerous system cameras also allow different versions to be exchanged for a wide variety of purposes.
Basically, the effect of the image, sharpness and sharpness progression can be assessed better the more matted the pane. However, with increasing matting, the loss of light due to scattering increases, so that the viewfinder can appear very dark. The technical design of the adjusting disk is therefore always subject to compromises. Finely ground, etched or equipped with micro-honeycomb lenses are used, for special purposes such as astrophotography also completely transparent panes. Completely clear panes provide a very bright image, but do not allow manual focusing, they are dependent on other focusing aids or are used in autofocus cameras.
If the focus is not determined in the image plane of the photograph, but with a focusing screen in an auxiliary plane, focusing errors can occur when setting the distance , which is then reflected in more or less blurred images.
Adjustment aids
Matting screens of cameras without autofocus ( MF cameras ) often have a ground or embossed optical focusing aid in the middle in the form of a sectional image indicator (also called sectional image wedge , sectional image rangefinder or measuring wedge ), which is usually surrounded by a micro prism ring (formerly also called a monoplan grid ring or grain grid) . There are also versions that only have a microprismatic spot. In the adjacent picture of the matt side you can see the first e of the word “focal length” in the cross-sectional image indicator on the focusing screen.
In the case of autofocus cameras (AF), such aids are generally dispensed with; the area of the central autofocus measuring field is often marked on the focusing screen. For some autofocus system cameras , focusing screens with these optical adjustment aids are also available. However, this can lead to incorrect exposures in cameras with an exposure meter arranged above the focusing screen . These can be compensated for by specially arranged cams on the setting disks, which operate the switches inside the camera to switch the calibration of the exposure meter. Alternatively, with newer cameras, the camera's exposure meter can usually be recalibrated by the service or the end user.
In the case of single-lens reflex cameras, the focusing screen is supplemented by a converging lens that serves as a condenser . For more even illumination of the viewfinder image, the underside of the focusing screen is also ground as a Fresnel lens .
Technical development
Older camera models can often be recognized by their dark viewfinder image. In the course of development, the focusing screens of single-lens reflex cameras became brighter and more brilliant. At the beginning of the 1980s, Minolta developed the so-called Acute-Matte focusing screens with spherical microlenses that provided a “jumping” impression of sharpness with very high brightness, which Hasselblad also adapted. The microlenses used in these and comparable focusing screens amplify light from the central area of the beam path, so that image blurring caused by edge rays cannot be seen on the focusing screen. The legibility of the image sharpness can deteriorate with increasing brightness. There are also special matting screens that have not been optimized for maximum brightness impression, but for the best possible manual focusability; such matting screens (e.g. based on super-spherical microlenses as in the Minolta M series) are not quite as bright as AF-optimized focusing screens, but still significantly brighter than old focusing screens. They also allow an assessment of the depth of field.
With many system cameras, the focusing screens are interchangeable. There is a range of special panes for them, for example with a grid structure (so-called reticle ) for architectural and repro photography or scaling for the macro area. There are also focusing screens that are not “matt”: so-called clear or aerial image screens for astrophotography and microphotography . These provide an extremely bright and detailed aerial image that cannot be focused on directly. The focus has been achieved when, with the eye relaxed and without refocusing the eye, the crosshair in the center of the image and the aerial image can be seen in focus and a slight lateral shift of the eye does not cause the subject to "jump" in relation to the crosshair (parallax Method).
With autofocus cameras, grid lines, auxiliary lines for an exact alignment on the horizon and the highlighting of the active autofocus measuring field are sometimes possible.
With two-lens reflex cameras, a swiveling magnifying glass often helps to adjust details on the light shaft finder .
species
Common types of focusing screens are:
- Matting screens with focus markings for car or manual focus cameras
- Grid matting disks with grid pattern as composition aid
- Full matt screen without visual assistance
- Matting screens with crossed lines