Astrophotography Definitions

The following are a few general photographic definitions that relate to astrophotography.

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  • Aperture - Lens opening. The opening in a camera lens through which light passes to expose the film. The size of aperture is either fixed or adjustable. Aperture size is usually calibrated in f-numbers-the larger the number, the smaller the lens opening.

  • Aperture Priority - An exposure mode on an automatic or auto focus camera that lets you set the aperture while the camera sets the shutter speed for proper exposure. If you change the aperture, or the light level changes, the shutter speed changes automatically.

  • ASA - American Standards Association. Denotes a speed system in which manufacturers rate their film in terms of of its sensitivity to light. Now superseded by ISO.

  • Autofocus (AF) - System by which the camera lens automatically focuses the image of a selected part of the picture subject.

  • Automatic Camera - A camera with a built-in exposure meter that automatically adjusts the lens opening, shutter speed, or both for proper exposure.

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  • B (Bulb) Setting - A shutter-speed setting on an adjustable camera that allows for time exposures. When set on B, the shutter will stay open as long as the shutter release button remains depressed.

  • Background - The part of the scene the appears behind the principal subject of the picture.

  • Bracketing - Taking a number of additional pictures of the subject through a range of exposures-both lighter and darker-when unsure of the correct exposure.

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  • Cable Release - Flexible cable used for firing the shutter. Useful for slow shutter speeds and time exposures, when touching the camera may cause shakes.

  • Color Balance - How a color film reproduces the colors of a scene. Color films are made to be exposed by light of a certain color quality such as daylight or tungsten. Color balance also refers to the reproduction of colors in color prints, which can be altered during the printing process.

  • Contrast - The range of difference in the light to dark areas of a negative, print, or slide (also called density); the brightness range of a subject or the scene lighting.

  • Cropping - Removing unwanted areas of an image, usually for a more pleasing composition. May also refer to the framing of the scene in the viewfinder.

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  • Double Exposure - Two pictures taken on one frame of film, or two images printed on one piece of photographic paper.

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  • Emulsion - Micro-thin layers of gelatin on film in which light-sensitive ingredients are suspended; triggered by light to create a chemical reaction resulting in a photographic image

  • Existing Light - Available light. Strictly speaking, existing light covers all natural lighting from moonlight to sunshine. For photographic purposes, existing light is the light that is already on the scene or project and includes room lamps, fluorescent lamps, spotlights, neon signs, candles, daylight through windows, outdoor scenes at twilight or in moonlight, and scenes artificially illuminated after dark.

  • Exposure - The product of the intensity of light that reaches the film (controlled by the aperture) and the length of time this intensity of light is allowed to act (controlled by the shutter speed).

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    Fast Film - Film which has an emulsion that is very sensitive to light. Such film have a high ASA rating.

    Fast Lens- Lens with a wide maximum aperture (low f number).

  • Film - A photographic emulsion coated on a flexible, transparent base that records images or scenes.

  • Film Speed - The sensitivity of a given film to light, indicated by a number such as ISO 200. The higher the number, the more sensitive or faster the film. Note: ISO stands for International Standards Organization.

  • Filter - A colored piece of glass or other transparent material used over the lens to emphasize, eliminate, or change the color or density of the entire scene or certain areas within a scene.

  • Fixed-Focus - Describes a non-adjustable camera lens, set for a fixed subject distance.

  • Fixed-Focus Lens - A lens that has been focused in a fixed position by the manufacturer. The user does not have to adjust the focus of this lens.

  • f-Number - A number that indicates the size of the lens opening on an adjustable camera. The common f-numbers are f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, and f/22. The larger the f-number, the smaller the lens opening. In this series, f/1.4 is the largest lens opening and f/22 is the smallest. Also called f-stops, they work in conjunction with shutter speeds to indicate exposure settings.

  • Focal Length - The distance between the film and the optical center of the lens when the lens is focused on infinity. The focal length of the lens on most adjustable cameras is marked in millimeters on the lens mount.

  • Focus - Adjustment of the distance setting on a lens to define the subject sharply.

  • Foreground - The area between the camera and the principal subject.

  • Format - Size or shape of negative, slide, printing paper, or camera viewing area.

  • Frame - One individual picture on a roll of film. Also, tree branch, arch, etc., that frames a subject.

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  • Graininess - The sand-like or granular appearance of a negative, print, or slide. Graininess becomes more pronounced with faster film and the degree of enlargement.

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  • Highlights - The brightest areas of the subject.

  • Hue - The name of a color (example - red, blue, yellow).

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  • ISO Speed - The emulsion speed (sensitivity) of the film as determined by the standards of the International Standards Organization. In these standards, both arithmetic (ASA) and logarithmic (DIN) speed values are expressed in a single ISO term. For example, a film with a speed of ISO 100/21 would have a speed of ASA 100 or 21 DIN.

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  • Lens - One or more pieces of optical glass or similar material designed to collect and focus rays of light to form a sharp image on the film, paper, or projection screen.

  • Lens Shade - A collar or hood at the front of a lens that keeps unwanted light from striking the lens and causing image flare. May be attached or detachable, and should be sized to the particular lens to avoid vignetting.

  • Lens-Shutter Camera - A camera with the shutter built into the lens; the viewfinder and picture-taking lens are separate.

  • Lens Speed - The largest lens opening (smallest f-number) at which a lens can be set. A fast lens transmits more light and has a larger opening than a slow lens.

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  • Negative - The developed film that contains a reversed tone image of the original scene.

  • Neutral Density Filter - A gray filter that reduces the amount of light entering the camera when aperture and speed settings cannot be altered. This filter will not affect the color content of the light.

  • Normal Lens or (Standard lens) - Lens with a focal length approximately equal to the diagonal of the picture format. On 35mm cameras this is usually 50mm (SLRs).

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  • Object - General term for an element in a scene. Often interchangeable with subject.

  • Overexposure - A condition in which too much light reaches the film, producing a dense negative or a very light print or slide.

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  • Panorama - A broad view, usually scenic.

  • Polarizing Screen (Filter) - A filter that transmits light traveling in one plane while absorbing light traveling in other planes. When placed on a camera lens or on light sources, it can eliminate undesirable reflections from a subject such as water, glass, or other objects with shiny surfaces. This filter also darkens blue sky.

  • Print - A positive picture, usually on paper, and usually produced from a negative.

  • Processing - Developing, fixing, and washing exposed photographic film or paper to produce either a negative image or a positive image.

  • Program Exposure - An exposure mode on an automatic or autofocus camera that automatically sets both the aperture and the shutter speed for proper exposure.

  • Push Processing - Increasing the development time of a film to increase its effective speed (raising the ISO number for initial exposure ) for low-light situations; forced development.

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  • Reciprocity - Most films are designed to be exposed within a certain range of exposure times-usually between 1/15 second to 1/1000 second. When exposure times fall outside of this range-becoming either significantly longer or shorter-a film's characteristics may change. Loss of effective film speed, contrast changes, and (with color films) color shifts are the three common results. These changes are called reciprocity effect.

  • Retouching - Altering a print or negative after development by use of dyes or pencils to alter tones of highlights, shadows, and other details, or to remove blemishes.

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  • Shutter - Blades, a curtain, plate, or some other movable cover in a camera that controls the time during which light reaches the film.

  • Shutter-Priority Cameras - Semi-automatic exposure system whereby the user selects the shutter speed, and the camera meter then sets the lens aperture.

  • Shutter Speed - The time (normally set in fractions of a second) the shutter mechanism effectively allows the film to receive light.

  • Single-Lens-Reflex (SLR) Camera - A camera in which you view the scene through the same lens that takes the picture.

  • Slide - A photographic transparency (positive) mounted for projection.

  • Slow Film - Film having an emulsion with a low sensitivity to light. Such film have a low ASA rating

  • Standard Lens - See Normal Lens.

  • Stopping Down - Changing the lens aperture to a smaller opening; for example, from f/8 to f/11.

  • Subject - The person or thing being photographed.

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  • Telephoto Lens - A lens that makes a subject appear larger on film than does a normal lens at the same camera-to-subject distance. A telephoto lens has a longer focal length and narrower field of view than a normal lens.

  • Time Exposure - General term for an exposure longer than can be set using the camera's fixed shutter speeds.

  • Tint - Shades of white in a finished print, controlled by the color of the paper, varying from white to buff.

  • Tone - The degree of lightness or darkness in any given area of a print; also referred to as value. Cold tones (bluish) and warm tones (reddish) refer to the color of the image in both black-and-white and color photographs.

  • Tripod - A three-legged supporting stand used to hold the camera steady. Especially useful when using slow shutter speeds and/or telephoto lenses.

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  • Underexposure - A condition in which too little light reaches the film, producing a thin negative, a dark slide, or a muddy-looking print.

  • Unipod - A one-legged support used to hold the camera steady.

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  • Viewfinder - System for viewing the subject, showing the field of view of the cameras lens

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  • Wide-Angle Lens - 35mm camera lens having a focal length appreciably shorter than 50mm. Gives a wide angle view and considerable depth of field.

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  • Zoom Lens - Lens that is constructed to allow continuously variable focal length within a specific range.

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    Copyright © 1999 By Keith Cooley