TPB Staff
01-04-2009, 01:43 PM
ASA - stands for American Standards Association, and is a system for rating film speed. A higher ASA number, such as 400, indicates a film that is more sensitive to light, and is considered a "fast" film. A lower ASA number, such as 25, means the film is less sensitive light, requiring more exposure, and is hence a "slower" film.
Aperture: the hole in the lens through which light travels to hit the film or sensor. In most instances the aperture can be controlled by adjusting its size, which is measured in f-stops.
Aperture priority: Refers to a semi-automated camera type, where the photographer has control of only the aperture size and, once adjusted for an exposure, the camera automatically adjusts the shutter speed.
Archival: in photography, this refers to the permanence of materials, generally photographic papers (inkjet and gelatin silver) as well as film. For wet darkroom photo enlarging and film development, following processing procedures according to archival standards ensures better longevity. For digital photo output, refers to inkjet or photo-printer papers as well as pigmented inks.
Back lighting: a situation where most of the available light falls in back of the subject. Can be effectively manipulated to create silhouettes, or overcome by the deliberate use of more powerful front or side lighting directed at the subject (such as a flash unit or studio lights).
Bayonet mount: A mounting system for attaching a lens to a camera, or a filter onto a lens.
Bounced light: The process of directing light from a source such as a flash or studio light so it reflects off a ceiling, wall, or reflective material back onto the subject, as opposed to directing the light straight on the subject. This has the effect of softening light.
Bracketing: A method of obtaining several exposures of the same subject, but varying the exposures slightly. The initial exposure is based from the indicated meter reading; subsequent exposures will allow greater, then less, light to reach the film or sensor. Bracketing can be done by changing only f-stops or the shutter speed.
Bulb: Seen on cameras as the "B" setting, this controls the shutter by allowing it to remain open for as long as the shutter button is held down.
Burning: used in traditonal darkrooms and also in photo editing software, this refers to the technique of darkening certain areas of a print. WIth a darkroom enlarger, more light is added, thus "burning" into the silver gelatin paper to darken the selected area.
Cable release: Refers to a flexible cord that screws onto a camera's shutter release button on one end, with a small plunger on the other end which enables the photographer to push and release the shutter (take the picture), without touching the camera. Useful for eliminating vibration in situations where the shutter is to be left open for an extended time.
Contrast: The tonal range between dark and light in a print, a negative, or a lighting situation. High contrast means a large difference between dark and light tones with very little "gray" or mid-tone area. A subject standing under a high noon sun is in a high contrast lighting situation. Low contrast means there is little to no true black/dark tones or true white/light tones, but mostly middle tones or lots of gray.
Cropping: Electing to print only part of an image as opposed to the entire frame.
DOF - Depth of field: the area from the points closest to the camera to the points farthest from the camera that in acceptable focus
Depth of field preview: a setting found on most single-lens reflex cameras that, when activated, allows previsualization of DOF at any given f-stop.
Depth of field scale: the set of numbers on a lens which indicate how much depth of field each f-stop will give
Distance scale: set of numbers that indicates how far the focused subject is from the camera.
Dodging: A technique used in darkroom printing as well as photo editing software to manipulate tonal values. In the darkroom, areas of a print are selectively covered or "held back" during exposure to prevent light from reaching the paper, thus keeping a tonal value from additional darkening.
Exposure: The combination of f-stop (aperture size) and shutter speed (or time) used to allow light to reach the film or sensor.
Extension tube: an accesssory that fits between the lens and the camera that allows for close-up focusing.
f-stop: the numerical indication of the aperture (lens opening) size. A large f-stop number, such as 32, indicates a very small opening in the lens; a small f-stop, such as 2.8, indicates a wider opened aperture.
fast lens: A lens that allows for wide-opened aperture which admits a lot of light. A lens that has a maximum aperture of f 1.4 will be a "faster" lens than one that has a maximum aperture of f 2.
filter: a piece of glass or gelatin that is attached to the front of a lens for various reasons. A clear filter, or a UV protector, is attached mainly to protect the lens' glass. Colored lenses will change the amount of light allowed to reach the film or sensor, and act to alter the light waves to create different effects. Enlarger filters used for printing with variable-contrast photo papers can control contrast.
filter factor: a number that indicates how much additional expsure is needed when various filters are used, depending upon the density of the filter.
flat: a term used to describe a very low contrast print or film negative. Can also describe a low contrast lighting situation.
focal length: the distance between the film/sensor plane of the camera and the optical center of the lens when the lens is focused on infinity. Ask Brad to expand on this one for a different description.
grain: the silver halide crystals embedded in a film negative. When a negative is enlarged or scanned, the crystals appear as visible tiny particles.
gray card: a piece of cardboard that has been manufactured to be 18% gray and is used to determine exposure.
highlight areas: the brightest areas on a print, or a film negative. Highlight areas on a print are susceptible to becoming "blown out" on a print from with digital sensors. In a film negative, highlight areas appear as the dark areas.
hot shoe: the slotted area on a camera body that holds an electric charge which enables a flash unit to synchronize with the shutter, without having to use a cord.
incident light: light that falls onto the subject, as opposed to light that is reflected by it.
infrared: a part of the light spectrum that is invisible to the human eye.
ISO: International Standard Organization, the international standard for rating film speed (light sensitivity). The number ratings are identical to those of ASA.
interchangable lens: a lens that can be removed from a camera body and exchanged for a different one.
latent image: the invisible (undeveloped) image created by light striking film or other photosensitive surfaces. The image will become visible when developed.
leaf shutter: A shutter located in the read of a camera lens, which consists of overlapping blades.
lens opening: aperture
lens shade: An accessory that attaches to the front of a lens to keep extraneous light from entering the lens and causing flare.
light meter: unit used to measure light, and transfer those measurements into the correct combination of f-stop and shutter speed, for the indicated light conditions. Some meters are built into the camera, and some are separate, hand-held instruments.
loupe: a smallish, compact magnifying device used to inspect negatives, transparencies or contact sheets.
macro lens: A lens made specially to focus close up.
monopod: refers to any model of one-legged, adjustable stands upon which a camera can be attached for additional stability.
motor drive: a device or accessory that can be attached to a film camera body and rapidly advances the film or automatically rewinds it. Many cameras have them built in.
mounting: fixing a print to a support, such as foam core or cardboard, for display purposes.
negative: Refers to an exposed strip of film containing a reversed image. The dark areas on a negative will be the light areas on an enlarged photographic print.
neutral density filter: a filter whose purpose is only to decrease the amount of light able to reach film or sensor. It does not alter the image in any other way.
orthochromatic: materials that are not sensitive to red light.
overexposure: exposing film or sensor to more light than the meter indicates is acceptable to obtain a good image.
panchromatic: materiala that are sensitive to all colors.
panning: Moving the camera in the same direction as a moving subject, with the intent to keep the main subject in sharp focus and the surrounding parts of the image in blur.
parallax: the difference between what the eye sees and what the lens sees, occuring when the viewer and the lens are separate, such with rangefinder or twin lens reflex cameras.
PC cord: also called sync cord, both terms refer to an electrical cord connecting between a flash unit and the camera body, allowing for synchronization of shutter and flash so the flash goes off at the correct time of exposure.
photoflood: Bright light bulb used as a source of artificial light, generally combined with a metal reflector.
photogram: an image made without a camera by positioning one or more objects between a light source and photosensitive material.
pinhole camera: any enclosed, light tight container with a fixed aperture from a small hole punctured or drilled in the side. Can be made from a wide variety of materials.
pixel: a contraction of picture and element, refers to the small elements that, when tightly together, constitute an image on a computer or television screen, or the detecting elements of a charge-coupled device used as an optical sensor in digital cameras. Each one has a specific color and is contiguous to the next, to form a color image.
polarizing filter: a filter used in front of a lens to reduce or eliminate glare or reflections of light.
positive: an image rendered normally, where the light/dark areas in the image correspond to the light/dark areas of the subject. Also refers to slide film, transparency or color reversal.
preview setting: see depth of field (DOF) preview.
post-processing: refers to image processing with any graphics or photo processing software, after exposure with digital cameras.
rangefinder: A focusing system using two superimposed images, which when brought together to form one image means the subject is in focus.
reciprocity failure: refers to the eventual breakdown of the reciprocal relationship between f-stop and shutter speed at either very slow exposures (1 second or longer) or very fast exposures.
recycling time: the time a flash unit needs after firing before being fully recharged.
reflected light: light that bounces off or is reflected off the subject.
reflector: a board or tool used to redirect light, generally made of metal or other shiny material, white cloth or pressboard.
replenisher: chemical added to film developers to boost strength and extend life.
resin-coated (RC) paper: photographic paper that is coated with clear plastic and contains a much thinner layer of silver halide emulsion that fiber-based (FB) papers. RC papers require less processing times, shorter washes and tend to dry very flat.
reticulation: tiny cracks in film emulsion, generally from temperature variations during development.
RGB: red/green/blue, the three colors to which digital sensors and other devices are sensitive.
Sabattier effect: An effect that occurs when re-exposing film or paper light during developments. Not to be confused with solarization.
safelight: a light with a colored filter used to illuminate darkrooms. Safelight filters are of various colors to screen out specific light to which photosensitive materials could be adversely affected.
screw mount: a way of attaching a lens to a camera or a filter to a lens, threaded mounts that allow for attaching and screwing one into the other.
selective focus: Generally refers to choosing a shallow depth of field, so the resulting image is partly in focus, with areas in foreground or background staying blurred.
sheet film: for larger film format cameras, film that comes as single sheets or pieces, rather than in a long roll.
shoe: the slotted coupling located on the body of the camera where a flash unit can be attached.
shutter: The barrier in a lens or camera that keeps light from reaching the film or digital sensor, shutter speed is controlled to determine how long the shutter will remain open at the time of exposure to admit light. Shutters can be in blade, curtain, or other movable type of cover.
shutter priority: refers to a semi-automatic camera type, where the photographer can select the shutter speed and the camera will automatically set a corresponding aperture (f-stop).
shutter speed: the length of time the shutter stays open at the time of exposure. With most cameras, this is adjustable.
silver halide: refers to the silver combined with halogen elements (such as bromide or chloride) embedded into the emulsion of black and white film, and photographic (silver gelatin) papers.
SLR: single-lens reflex, refers to the type of camera that has one lens through which the picture is viewed, focused, and exposure is taken. SLRs accept interchangable lenses of various fixed focal lengths, or zoom lenses with variable focal lengths.
soft: refers to both a negative or print that is not sharply in focus, or has low contrast.
solarization: a reversed effect that occurs when film is grossly overexposed in camera. The term is commonly used to refer to the Sabattier effect.
spot meter: a type of metering where only a small area of the desired subject is selected. Can be a dedicated metering unit or an available selection on some hand held meters.
spotting: a processing technique where photo dyes or pencils are used to fill in scracthes, dust spots or other unwanted artifacts on negatives or dried, finished prints.
stop bath: refers to a weak, usually acid-based, solution used to stop further development of film or paper by neutralizing the developer. Plain water can also be used.
strobe: refers to electronic flash.
synchronization: the coordination between the shutter opening and the flash going off for correct exposure.
telephoto lens: lenses with long focal lengths, a narrow field of view, and shorter depth of field, which combine to make the subject appear larger in the viewfinder - and closer to the camera - than do lenses with a more normal focal length or wider field of view.
test strip: a small section of photographic paper on which several different exposures from the same negative are taken, used to determine a basic starting exposure.
through the lens (TTL): generally refers to cameras which have built-in meters which read the light through the lens when attached. Most modern SLRs have TTL capacity.
toning: the addition of color to a B&W image. With B&W film, once a print is made, this is achieved with a chemical process with dyes or the manipulation or replacement of the silver halides in the emulsion; with digital images, achieved by manipulation with photo processing software.
transparency: refers mainly to slide film, or any positive image on film through which light can be shown.
tripod: refers to any model of three-legged, adjustable stand on which a camera can be mounted for extra stability.
twin-lens reflex (TLR): a type of camera with two fixed lenses, one on top for composing and focusing, and one underneath for taking the picture.
underexposed: refers to an exposure in which too little light was allowed to reach the film or sensor. Can sometimes be improved in the darkroom or with digital post-processing.
UV: or UV filter, refers to ultraviolet light.
variable contrast paper: photographic paper which has been specially coated to render a variety of contrast ranges during a single exposure session.
view camera: refers to a camera type that has an enlarged viewing screen of ground glass, focuses with bellows and uses mainly larger format sheet film rather than roll film.
viewfinder: the piece of glass or sometimes plastic on a camera body through which the image is viewed and composed.
wide-angle lens: lenses with short focal lengths and a wide field of view, which subsequently makes the subject appear smaller and further away from the camera.
zoom lens: any lens with adjustable focal lengths.
Aperture: the hole in the lens through which light travels to hit the film or sensor. In most instances the aperture can be controlled by adjusting its size, which is measured in f-stops.
Aperture priority: Refers to a semi-automated camera type, where the photographer has control of only the aperture size and, once adjusted for an exposure, the camera automatically adjusts the shutter speed.
Archival: in photography, this refers to the permanence of materials, generally photographic papers (inkjet and gelatin silver) as well as film. For wet darkroom photo enlarging and film development, following processing procedures according to archival standards ensures better longevity. For digital photo output, refers to inkjet or photo-printer papers as well as pigmented inks.
Back lighting: a situation where most of the available light falls in back of the subject. Can be effectively manipulated to create silhouettes, or overcome by the deliberate use of more powerful front or side lighting directed at the subject (such as a flash unit or studio lights).
Bayonet mount: A mounting system for attaching a lens to a camera, or a filter onto a lens.
Bounced light: The process of directing light from a source such as a flash or studio light so it reflects off a ceiling, wall, or reflective material back onto the subject, as opposed to directing the light straight on the subject. This has the effect of softening light.
Bracketing: A method of obtaining several exposures of the same subject, but varying the exposures slightly. The initial exposure is based from the indicated meter reading; subsequent exposures will allow greater, then less, light to reach the film or sensor. Bracketing can be done by changing only f-stops or the shutter speed.
Bulb: Seen on cameras as the "B" setting, this controls the shutter by allowing it to remain open for as long as the shutter button is held down.
Burning: used in traditonal darkrooms and also in photo editing software, this refers to the technique of darkening certain areas of a print. WIth a darkroom enlarger, more light is added, thus "burning" into the silver gelatin paper to darken the selected area.
Cable release: Refers to a flexible cord that screws onto a camera's shutter release button on one end, with a small plunger on the other end which enables the photographer to push and release the shutter (take the picture), without touching the camera. Useful for eliminating vibration in situations where the shutter is to be left open for an extended time.
Contrast: The tonal range between dark and light in a print, a negative, or a lighting situation. High contrast means a large difference between dark and light tones with very little "gray" or mid-tone area. A subject standing under a high noon sun is in a high contrast lighting situation. Low contrast means there is little to no true black/dark tones or true white/light tones, but mostly middle tones or lots of gray.
Cropping: Electing to print only part of an image as opposed to the entire frame.
DOF - Depth of field: the area from the points closest to the camera to the points farthest from the camera that in acceptable focus
Depth of field preview: a setting found on most single-lens reflex cameras that, when activated, allows previsualization of DOF at any given f-stop.
Depth of field scale: the set of numbers on a lens which indicate how much depth of field each f-stop will give
Distance scale: set of numbers that indicates how far the focused subject is from the camera.
Dodging: A technique used in darkroom printing as well as photo editing software to manipulate tonal values. In the darkroom, areas of a print are selectively covered or "held back" during exposure to prevent light from reaching the paper, thus keeping a tonal value from additional darkening.
Exposure: The combination of f-stop (aperture size) and shutter speed (or time) used to allow light to reach the film or sensor.
Extension tube: an accesssory that fits between the lens and the camera that allows for close-up focusing.
f-stop: the numerical indication of the aperture (lens opening) size. A large f-stop number, such as 32, indicates a very small opening in the lens; a small f-stop, such as 2.8, indicates a wider opened aperture.
fast lens: A lens that allows for wide-opened aperture which admits a lot of light. A lens that has a maximum aperture of f 1.4 will be a "faster" lens than one that has a maximum aperture of f 2.
filter: a piece of glass or gelatin that is attached to the front of a lens for various reasons. A clear filter, or a UV protector, is attached mainly to protect the lens' glass. Colored lenses will change the amount of light allowed to reach the film or sensor, and act to alter the light waves to create different effects. Enlarger filters used for printing with variable-contrast photo papers can control contrast.
filter factor: a number that indicates how much additional expsure is needed when various filters are used, depending upon the density of the filter.
flat: a term used to describe a very low contrast print or film negative. Can also describe a low contrast lighting situation.
focal length: the distance between the film/sensor plane of the camera and the optical center of the lens when the lens is focused on infinity. Ask Brad to expand on this one for a different description.
grain: the silver halide crystals embedded in a film negative. When a negative is enlarged or scanned, the crystals appear as visible tiny particles.
gray card: a piece of cardboard that has been manufactured to be 18% gray and is used to determine exposure.
highlight areas: the brightest areas on a print, or a film negative. Highlight areas on a print are susceptible to becoming "blown out" on a print from with digital sensors. In a film negative, highlight areas appear as the dark areas.
hot shoe: the slotted area on a camera body that holds an electric charge which enables a flash unit to synchronize with the shutter, without having to use a cord.
incident light: light that falls onto the subject, as opposed to light that is reflected by it.
infrared: a part of the light spectrum that is invisible to the human eye.
ISO: International Standard Organization, the international standard for rating film speed (light sensitivity). The number ratings are identical to those of ASA.
interchangable lens: a lens that can be removed from a camera body and exchanged for a different one.
latent image: the invisible (undeveloped) image created by light striking film or other photosensitive surfaces. The image will become visible when developed.
leaf shutter: A shutter located in the read of a camera lens, which consists of overlapping blades.
lens opening: aperture
lens shade: An accessory that attaches to the front of a lens to keep extraneous light from entering the lens and causing flare.
light meter: unit used to measure light, and transfer those measurements into the correct combination of f-stop and shutter speed, for the indicated light conditions. Some meters are built into the camera, and some are separate, hand-held instruments.
loupe: a smallish, compact magnifying device used to inspect negatives, transparencies or contact sheets.
macro lens: A lens made specially to focus close up.
monopod: refers to any model of one-legged, adjustable stands upon which a camera can be attached for additional stability.
motor drive: a device or accessory that can be attached to a film camera body and rapidly advances the film or automatically rewinds it. Many cameras have them built in.
mounting: fixing a print to a support, such as foam core or cardboard, for display purposes.
negative: Refers to an exposed strip of film containing a reversed image. The dark areas on a negative will be the light areas on an enlarged photographic print.
neutral density filter: a filter whose purpose is only to decrease the amount of light able to reach film or sensor. It does not alter the image in any other way.
orthochromatic: materials that are not sensitive to red light.
overexposure: exposing film or sensor to more light than the meter indicates is acceptable to obtain a good image.
panchromatic: materiala that are sensitive to all colors.
panning: Moving the camera in the same direction as a moving subject, with the intent to keep the main subject in sharp focus and the surrounding parts of the image in blur.
parallax: the difference between what the eye sees and what the lens sees, occuring when the viewer and the lens are separate, such with rangefinder or twin lens reflex cameras.
PC cord: also called sync cord, both terms refer to an electrical cord connecting between a flash unit and the camera body, allowing for synchronization of shutter and flash so the flash goes off at the correct time of exposure.
photoflood: Bright light bulb used as a source of artificial light, generally combined with a metal reflector.
photogram: an image made without a camera by positioning one or more objects between a light source and photosensitive material.
pinhole camera: any enclosed, light tight container with a fixed aperture from a small hole punctured or drilled in the side. Can be made from a wide variety of materials.
pixel: a contraction of picture and element, refers to the small elements that, when tightly together, constitute an image on a computer or television screen, or the detecting elements of a charge-coupled device used as an optical sensor in digital cameras. Each one has a specific color and is contiguous to the next, to form a color image.
polarizing filter: a filter used in front of a lens to reduce or eliminate glare or reflections of light.
positive: an image rendered normally, where the light/dark areas in the image correspond to the light/dark areas of the subject. Also refers to slide film, transparency or color reversal.
preview setting: see depth of field (DOF) preview.
post-processing: refers to image processing with any graphics or photo processing software, after exposure with digital cameras.
rangefinder: A focusing system using two superimposed images, which when brought together to form one image means the subject is in focus.
reciprocity failure: refers to the eventual breakdown of the reciprocal relationship between f-stop and shutter speed at either very slow exposures (1 second or longer) or very fast exposures.
recycling time: the time a flash unit needs after firing before being fully recharged.
reflected light: light that bounces off or is reflected off the subject.
reflector: a board or tool used to redirect light, generally made of metal or other shiny material, white cloth or pressboard.
replenisher: chemical added to film developers to boost strength and extend life.
resin-coated (RC) paper: photographic paper that is coated with clear plastic and contains a much thinner layer of silver halide emulsion that fiber-based (FB) papers. RC papers require less processing times, shorter washes and tend to dry very flat.
reticulation: tiny cracks in film emulsion, generally from temperature variations during development.
RGB: red/green/blue, the three colors to which digital sensors and other devices are sensitive.
Sabattier effect: An effect that occurs when re-exposing film or paper light during developments. Not to be confused with solarization.
safelight: a light with a colored filter used to illuminate darkrooms. Safelight filters are of various colors to screen out specific light to which photosensitive materials could be adversely affected.
screw mount: a way of attaching a lens to a camera or a filter to a lens, threaded mounts that allow for attaching and screwing one into the other.
selective focus: Generally refers to choosing a shallow depth of field, so the resulting image is partly in focus, with areas in foreground or background staying blurred.
sheet film: for larger film format cameras, film that comes as single sheets or pieces, rather than in a long roll.
shoe: the slotted coupling located on the body of the camera where a flash unit can be attached.
shutter: The barrier in a lens or camera that keeps light from reaching the film or digital sensor, shutter speed is controlled to determine how long the shutter will remain open at the time of exposure to admit light. Shutters can be in blade, curtain, or other movable type of cover.
shutter priority: refers to a semi-automatic camera type, where the photographer can select the shutter speed and the camera will automatically set a corresponding aperture (f-stop).
shutter speed: the length of time the shutter stays open at the time of exposure. With most cameras, this is adjustable.
silver halide: refers to the silver combined with halogen elements (such as bromide or chloride) embedded into the emulsion of black and white film, and photographic (silver gelatin) papers.
SLR: single-lens reflex, refers to the type of camera that has one lens through which the picture is viewed, focused, and exposure is taken. SLRs accept interchangable lenses of various fixed focal lengths, or zoom lenses with variable focal lengths.
soft: refers to both a negative or print that is not sharply in focus, or has low contrast.
solarization: a reversed effect that occurs when film is grossly overexposed in camera. The term is commonly used to refer to the Sabattier effect.
spot meter: a type of metering where only a small area of the desired subject is selected. Can be a dedicated metering unit or an available selection on some hand held meters.
spotting: a processing technique where photo dyes or pencils are used to fill in scracthes, dust spots or other unwanted artifacts on negatives or dried, finished prints.
stop bath: refers to a weak, usually acid-based, solution used to stop further development of film or paper by neutralizing the developer. Plain water can also be used.
strobe: refers to electronic flash.
synchronization: the coordination between the shutter opening and the flash going off for correct exposure.
telephoto lens: lenses with long focal lengths, a narrow field of view, and shorter depth of field, which combine to make the subject appear larger in the viewfinder - and closer to the camera - than do lenses with a more normal focal length or wider field of view.
test strip: a small section of photographic paper on which several different exposures from the same negative are taken, used to determine a basic starting exposure.
through the lens (TTL): generally refers to cameras which have built-in meters which read the light through the lens when attached. Most modern SLRs have TTL capacity.
toning: the addition of color to a B&W image. With B&W film, once a print is made, this is achieved with a chemical process with dyes or the manipulation or replacement of the silver halides in the emulsion; with digital images, achieved by manipulation with photo processing software.
transparency: refers mainly to slide film, or any positive image on film through which light can be shown.
tripod: refers to any model of three-legged, adjustable stand on which a camera can be mounted for extra stability.
twin-lens reflex (TLR): a type of camera with two fixed lenses, one on top for composing and focusing, and one underneath for taking the picture.
underexposed: refers to an exposure in which too little light was allowed to reach the film or sensor. Can sometimes be improved in the darkroom or with digital post-processing.
UV: or UV filter, refers to ultraviolet light.
variable contrast paper: photographic paper which has been specially coated to render a variety of contrast ranges during a single exposure session.
view camera: refers to a camera type that has an enlarged viewing screen of ground glass, focuses with bellows and uses mainly larger format sheet film rather than roll film.
viewfinder: the piece of glass or sometimes plastic on a camera body through which the image is viewed and composed.
wide-angle lens: lenses with short focal lengths and a wide field of view, which subsequently makes the subject appear smaller and further away from the camera.
zoom lens: any lens with adjustable focal lengths.