Ground glass
Encyclopedia
Ground glass is glass
whose surface has been ground to produce a flat but rough (matte
) finish.
Ground glass surfaces have many applications, ranging from mere ornamentation on windows and table glassware to scientific uses in optics and laboratory glassware
.
, a sheet of ground glass is used for the manual focusing
in some still
and motion picture cameras.
Some cameras have no separate viewfinder
, but instead use a piece of ground glass at the focal plane to focus the scene. This type of focusing and viewing system is typically used on so-called large format
cameras (cameras which use film
4×5 inches or larger). In such a camera, before the picture is taken, the ground-glass viewer is inserted in the back of the camera, and the lens opened to its widest aperture
. This projects the scene on the ground glass upside down and backwards. The photographer focuses and composes using this projected image, sometimes with the aid of a magnifying glass
(or loupe
). In order to see the image better, a dark cloth is used to block out light, whence came the image of the old-time photographer with his head stuck under a large black cloth.
A ground glass is also used in the reflex finder
of an SLR
or TLR
camera.
In motion picture cameras, the ground glass is a small, usually removable piece of transparent glass that sits between the rotary disc shutter
and the viewfinder. The ground glass usually contains precise markings to show the camera operator the boundaries of the frame or the center reticle, or any other important information. Because the ground glass is positioned between the mirror shutter and the viewfinder, it does not interfere with the image reaching the film and is therefore not recorded over the final image, but rather serves as a reference for the camera operator.
Ground glasses commonly serve as a framing reference for a desired aspect ratio
. Because most films shot with spherical lenses are shot full-frame and later masked during projection to a more widescreen
aspect ratio, it is important not only for the operator to be able to see the boundaries of that aspect ratio, but also for the ground glass to be properly aligned in the camera so that the markings are an exact representation of the boundaries of the image recorded on film.
laboratories
.
Glass flasks, stoppers, valves, funnels, and tubing are often connected together by ground glass joint
s, matching pairs of conical
or spherical surfaces that have been ground to a precise shape.
Flasks and test tube
s often have a small ground-glass label area on the side. (Pencil writing on ground glass is largely inert, rub-proof and waterproof, but can be easily erased.)
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
whose surface has been ground to produce a flat but rough (matte
Gloss (material appearance)
Gloss is an optical property, which is based on the interaction of light with physical characteristics of a surface. It is actually the ability of a surface to reflect light into the specular direction. The factors that affect gloss are the refractive index of the material, the angle of incident...
) finish.
Ground glass surfaces have many applications, ranging from mere ornamentation on windows and table glassware to scientific uses in optics and laboratory glassware
Laboratory glassware
Laboratory glassware refers to a variety of equipment, traditionally made of glass, used for scientific experiments and other work in science, especially in chemistry and biology laboratories...
.
Photography
In photographyPhotography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
, a sheet of ground glass is used for the manual focusing
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...
in some still
Camera
A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...
and motion picture cameras.
Some cameras have no separate viewfinder
Viewfinder
In photography, a viewfinder is what the photographer looks through to compose, and in many cases to focus, the picture. Most viewfinders are separate, and suffer parallax, while the single-lens reflex camera lets the viewfinder use the main optical system. Viewfinders are used in many cameras of...
, but instead use a piece of ground glass at the focal plane to focus the scene. This type of focusing and viewing system is typically used on so-called large format
Large format
Large format refers to any imaging format of 4×5 inches or larger. Large format is larger than "medium format", the 6×6 cm or 6×9 cm size of Hasselblad, Rollei, Kowa, Pentax etc cameras , and much larger than the 24×36 mm frame of 35 mm format.The main advantage...
cameras (cameras which use film
Photographic film
Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...
4×5 inches or larger). In such a camera, before the picture is taken, the ground-glass viewer is inserted in the back of the camera, and the lens opened to its widest aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
. This projects the scene on the ground glass upside down and backwards. The photographer focuses and composes using this projected image, sometimes with the aid of a magnifying glass
Magnifying glass
A magnifying glass is a convex lens that is used to produce a magnified image of an object. The lens is usually mounted in a frame with a handle ....
(or loupe
Loupe
A loupe is a simple, small magnification device used to see small details more closely. Unlike a magnifying glass, a loupe does not have an attached handle, and its focusing lens are contained in an opaque cylinder or cone. Loupes are also called hand lenses .- Optics :Three basic types of loupes...
). In order to see the image better, a dark cloth is used to block out light, whence came the image of the old-time photographer with his head stuck under a large black cloth.
A ground glass is also used in the reflex finder
Reflex finder
A reflex finder is a viewfinder system with a mirror placed behind a lens. The light passing through the lens is reflected by the mirror to a focusing screen, usually ground glass...
of an SLR
Single-lens reflex camera
A single-lens reflex camera is a camera that typically uses a semi-automatic moving mirror system that permits the photographer to see exactly what will be captured by the film or digital imaging system, as opposed to pre-SLR cameras where the view through the viewfinder could be significantly...
or TLR
Twin-lens reflex camera
A twin-lens reflex camera is a type of camera with two objective lenses of the same focal length. One of the lenses is the photographic objective or "taking lens" , while the other is used for the viewfinder system, which is usually viewed from above at waist level...
camera.
In motion picture cameras, the ground glass is a small, usually removable piece of transparent glass that sits between the rotary disc shutter
Rotary disc shutter
A rotary disc shutter is a type of shutter. It is notably used in motion picture cameras.Rotary discs are semicircular mirrors which rotate in front of the film gate, and thus expose the film. As the mirror spins it reflects the image onto the ground glass so that it can be viewed by the camera...
and the viewfinder. The ground glass usually contains precise markings to show the camera operator the boundaries of the frame or the center reticle, or any other important information. Because the ground glass is positioned between the mirror shutter and the viewfinder, it does not interfere with the image reaching the film and is therefore not recorded over the final image, but rather serves as a reference for the camera operator.
Ground glasses commonly serve as a framing reference for a desired aspect ratio
Aspect ratio (image)
The aspect ratio of an image is the ratio of the width of the image to its height, expressed as two numbers separated by a colon. That is, for an x:y aspect ratio, no matter how big or small the image is, if the width is divided into x units of equal length and the height is measured using this...
. Because most films shot with spherical lenses are shot full-frame and later masked during projection to a more widescreen
Widescreen
Widescreen images are a variety of aspect ratios used in film, television and computer screens. In film, a widescreen film is any film image with a width-to-height aspect ratio greater than the standard 1.37:1 Academy aspect ratio provided by 35mm film....
aspect ratio, it is important not only for the operator to be able to see the boundaries of that aspect ratio, but also for the ground glass to be properly aligned in the camera so that the markings are an exact representation of the boundaries of the image recorded on film.
Lighting
Ground glass is widely used as a weather- and heat-proof light diffuser in ambient lighting, namely on glass covers or enclosuers for lamp fixtures, and sometimes on incandescent bulbs. Its functions include preventing eye damage by direct looking at lamp filament, hiding unsightly details of the lamp and fixture without blocking its light, and producing a softer illumination without sharp shadows.Chemistry
Ground glass surfaces are often found on the glass equipment of chemicalChemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
laboratories
Laboratory
A laboratory is a facility that provides controlled conditions in which scientific research, experiments, and measurement may be performed. The title of laboratory is also used for certain other facilities where the processes or equipment used are similar to those in scientific laboratories...
.
Glass flasks, stoppers, valves, funnels, and tubing are often connected together by ground glass joint
Ground glass joint
Ground glass joints are used in laboratories to quickly and easily fit leak-tight apparatus together from commonly available parts. For example, a round bottom flask, Liebig condenser, and oil bubbler with ground glass joints may be rapidly fitted together to reflux a reaction mixture...
s, matching pairs of conical
Cone (geometry)
A cone is an n-dimensional geometric shape that tapers smoothly from a base to a point called the apex or vertex. Formally, it is the solid figure formed by the locus of all straight line segments that join the apex to the base...
or spherical surfaces that have been ground to a precise shape.
Flasks and test tube
Test tube
A test tube, also known as a culture tube or sample tube, is a common piece of laboratory glassware consisting of a finger-like length of glass or clear plastic tubing, open at the top, usually with a rounded U-shaped bottom....
s often have a small ground-glass label area on the side. (Pencil writing on ground glass is largely inert, rub-proof and waterproof, but can be easily erased.)