Focusing screen

The shim ( also: ground glass; international, English: focusing screen) is made ​​of a translucent material disc that is used is used in professional cameras, SLR cameras and film cameras, and serves as a projection of the design consideration and the manual focus position of the lens.

Operation

The focusing screen is made of glass or transparent plastic with a smooth and an etched side. The " rough " side is visually exactly in the same plane as the film or digital sensor of the camera used. The camera lens designs on the frosted surface a real image so that the photographer can adjust the sharpness and the neckline and also assess the sharpness of the image curve. In professional cameras, the dial can be inserted into the rear standard of the camera, making all desired settings, and then replaced the focusing screen for recording against the sheet film cassette. With SLR cameras, the dial is built mostly fixed, but many system cameras also allow the exchange of different variants for various purposes.

Basically, image effect, sharpness and sharpness course can assess all the better, the more the disk is frosted. However, increases with the matting of the loss of light by scattering, so that the viewfinder can be very dark. The technical design of the dial is therefore always full of compromises. It finely polished, etched or equipped with micro lenses honeycomb wheels are used for special purposes such as astrophotography also completely transparent slices. Completely clear windows provide a very bright image, but do not allow manual focus, they are dependent on other focusing aids or be used in auto focus cameras.

If the focus is not detected in the image plane of the photographic recording, but with a dial in an auxiliary level, there may be errors in the focusing distance setting, which then is reflected in more or less blurred shots.

Focusing Aids

Matt slices of cameras without auto-focus (MF ) cameras have in the middle often an ingrained or imprinted optical focusing aid in the form of a sectional image indicator ( also cut image wedge sectional image rangefinder or measured wedge called ), which is surrounded mostly by a microprism collar (formerly also Monoplan grid ring or Stipple called ). There are also versions which have exclusively a microprism spot. In the picture alongside the matte side you can see the first e of the word " focal length " in the split image of the focusing screen.

For autofocus cameras ( AF) is omitted aids such as a rule, here the area of ​​the central auto focus area is often marked on the focusing screen. For some autofocus system cameras also focusing screens are available with these optical adjustment aids. By this, however, it may lead to incorrect exposures on cameras with above the focusing screen arranged light meter. This can be compensated by specially arranged lobes on the focusing screens, press the switch inside the camera to switch the calibration of the meter. Alternatively, can be recalibrated with newer cameras and most of the camera's exposure meter through the Service or the end user.

With SLR cameras, the dial is complemented by a converging lens, which serves as a condenser. For uniform illumination of the viewfinder image the underside of the dial is additionally ground as a Fresnel lens.

Technical Development

Legacy cameras can be seen often on her dark viewfinder image. During the development of the ground glass of SLR cameras were becoming brighter and more brilliant. Beginning of the eighties the so-called developed Minolta Acute- Matte focusing screens with spherical microlenses, which provided a "jumping " sharpness at very high brightness which have been adapted by Hasselblad. The microlenses used in these and similar focusing screens amplify light from the central region of the beam path so that caused by edge -ray image blurring can not be seen on the screen. How can degrade the readability of the image sharpness with increasing brightness. There are also special focusing screens, which have not been optimized for maximum brightness impression, but in the best possible manual focusing capability out such focusing screens (eg on the basis of super- spherical microlenses as 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 gradient.

In many system cameras, the focusing screens are interchangeable. For them there is a range of special washers, for example, with a lattice structure (called reticle ) to the architecture and repro photography or scaling for the macro range. Also, there are shims that are not "matt": so-called plain or aerial image slices for Astro- and microphotography. These provide an extremely bright and detailed aerial photo, on which one can not focus directly. The focus is then done when you can see at the same time the cross hairs in the center of the image and the aerial image sharp with relaxed eye and without refocusing of the eye and a slight lateral displacement of the eye a "jump " of the subject in relation to the crosshair causes ( parallax method).

For autofocus cameras partially the display of grid lines, guide lines for precise alignment on the horizon and the highlighting of the active autofocus metering field is possible.

In twin-lens reflex cameras often helps a swing- motion when setting details on the waist-level viewfinder.

Species

Common ground glass types are:

• Matt discs with focusing marks for auto or manual focus cameras
• Grid focusing screen with a grid pattern as a composition aid
• Ground glass screen without any visual assistance
• Matt discs with cross line

• Camera technology