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A lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus on the same plane.

Ambient Light

Light in a room from a natural source or general lighting. 


A lens designed to be free of astigmatism.

Angle of Incidence

The angle of light hitting a surface. A basic rule of physics says that the angle of incidence equals the angle of reflectance, which means that the angle of the light coming towards the surface is the same as the angle of light reflected off of the surface. For example, if light hitting a surface came from a light source that was on a 45 degree angle from the surface, the light reflecting off of the surface would travel on a 45 degree angle as well. 


An optical process that samples the edges of an image to fill in the missing areas that cause a jagged appearance.


The variable opening in the lens through which light passes to the film or digital sensor. Aperture is measured in f-stops.   The aperture of a lens is like the pupil of your eye.


Apochromatic.  Apochromatic lenses bring three colors to a common focal plane.  It is more effective at reducing color aberrations than a standard achromatic lens.


APS-C stands for Advanced Photo System-type C, which is an image sensor format that is equivalent to the size of the Advanced Photo System negative (25.1 x 16.7mm).   APS-C is also referred to as sub-frame.

Aspherical Lens

A lens with a curved, non-spherical surface. Used to reduce aberrations and enable a more compact lens size.


The inability of a lens to focus on horizontal and vertical lines at the same time.  It is most noticeable at edges.

Back Focus

The distance from the tip of the lens' furthest rear surface to the film plane or sensor surface where the image is focused.

Barrel Distortion

A lens effect in which the straight lines in an image appear to be inflated or sphere shaped.   


Taking a series of images at different exposures.  Usually the first image is taken at a certain f-stop and the next two images are taken at the f-stops below and above it.  This gives you three images with three different exposures.   Shutter speeds can also be bracketed.


The quality of the out-of-focus areas in an image.   A smooth bokeh is preferred for portraiture and macro images.


The “B” setting on your camera. The shutter remains open for as long as the shutter button is pressed.


A traditional darkroom term for selectively allowing extra light to hit the paper during enlargement.   This darkens the area.  

Catadioptric Mirror

An optical system where refraction and reflection are combined via lenses and curved mirrors.


Charge Coupled Device.  A light-sensitive, integrated circuit that captures images by converting photons to electrons.  CCD's are used in Digital cameras, scanners and video cameras.

Chromatic Aberration

Chromatic aberration is a problem in lenses where colors are not focused on the same convergence point in the focal plane. As a result, the image shows fringes of different colors around the edges where bright and dark sections meet.  


A point source of light is prevented from being brought into focus, but instead appears shaped as a comet or teardrop.  Coma can be reduced by stopping down the lens.


Contrast defines the range of tonal difference between the shadows and highlights of an image.

Constant Aperture

This refers to a zoom lens that maintains the same aperture opening at all focal lengths.

Covering Power

The ability of a lens to sharply reproduce in the negative an illumiated flat field.  In other words; the lens' ability to evenly illuminate the entire negative.  

Depth Of Field

The zone of acceptable sharpness in front of and behind the subject on which the lens is focused. Depth of field varies according to focal length of the lens, chosen aperture and shooting distance.

Depth Of Field Preview

A camera feature which allows you to see what will actually be in focus by stopping down the aperture to the setting that will be used when the photo is taken.  

Diaphragm Shutter

A circular bladed shutter, similar to a leaf shutter, which also serves as an aperture.  


Optical interference that occurs when light passes through a tiny opening, such as an aperture with a small value f-number. Lens diffraction occurs when both the wavelength of light and the opening itself are roughly the same sizes.   This causes dispersion which reduces sharpness in the image.   Digital sensors are more prone to the effects of diffraction than film.


A phenomenon in which white light rays deviate by different wavelength amounts resulting in a color spectrum.


A traditional darkroom term for selectively blocking light from hitting the paper during the enlargement process.  This lightens the area.

Dynamic Range

The range of dark and light tones that a camera is capable of recording.  


The total amount of light reaching the film or sensor. 

Exposure triangle

The combination of aperture setting, ISO value, and shutter speed setting, which determines the exposure of an image.  An adjustment in one setting can be countered in another setting in order to maintain the same exposure.


A lens that has an angle of view of approximately 180 degrees.

Flatness of Field

A measure of edge-to-edge sharpness.  A flat field lens is sharp from edge to edge.

Focal Point

A point on the optical axis where light rays converge to form a sharp image.

Focusing Screen

Optical glass between the mirror and pentaprism in a single-lens-reflex camera. The mirror reflects the image from the lens upward onto the screen.  The photographer views the image passing through the screen in the viewfinder window.

FP High Speed Sync

A feature of flash units and SLR cameras that enables flash pictures to be taken at shutter speeds higher than the camera's normally synchronized speeds.


A measure of the aperture opening in the lens.   A smaller aperture number represents a larger openng while a larger number represents a smaller opening. 

Fill Light

A secondary light source used to fill in shadows.

Field Curvature

The curvature in a lens which prevents the lens from focusing evenly across the entire frame.  Field curvture majkes it impossible to photograph a flat object with even sharpness.  

Flash Sync

Matching a shutter speed duration with the duration of a flash unit's pulse.  Using a shutter speed that is too fast will result in photos with a black bar in them.

Field of View

The width of view that a lens can see.   


The effect produced by he reflection of light rays from the lens surfaces forming secondary images and a fogged appearance.  

Focal Length

The distance between the optical center of a camera lens and the film or sensor.   

Focal Plane Shutter

Located in the camera, a focal plane shutter opens horizontally or vertically and can be made of metal or cloth.  

Golden Hour

Golden hour is the period right before sunset and after sunrise.  The sun's light is warmer during these times.


A term used to describe an image that contains multiple shades of gray, as well as black and white.


Guide Number.   Indicates the power of a flash in relation to ISO film speed.   GN = aperture times distance.  Example:   f4 x 10 feet = GN40

Hard Light

Light that is harsh and contrasty.


High Dymanic Range.  A technique that uses the combination of 3 or more bracketed images to produce a single image with a wide dynamic range.  


Histogram is the visual representation of the luminance of an image. The left side of the graph represent the shadows, while the right side belongs to the highlights.

Hot Shoe

A mounting device built onto the top of a camera that enables a flash unit to be fired.   

Hyperfocal Distance

A focal point in a landscape image that is chosen to ensure that all parts of the image are in focus.  Thehyperfocal point in an image is dependent upon the f-stop chosen and the field of view of the lens.

Incident Light

Light that falls on the subject from the sun or general lighting.


The sensitivity of the film or sensor.   The higher the ISO the more sensitive the recording medium is to light.  


Kelvin is the absolute thermodynamic unit used to measure color temperature.. The scale goes from 1,000K  to 10,000K and is tightly related to white balance.   Daylight is 5400K.   Lower numbers are warmer while higher numbers are cooler.

Leaf Shutter

A mechanism with one or more pivoting metal leaves which does not allow light through the lens onto the film.   When triggered,  the shutter opens, allowing light to expose the film.  Leaf shutters are located very close the lens or inside it.


Linear refers to the way a digital sensor sees the value of light.   Unlike film and the human eye, digital sensors see an images dark and light areas with the same value.  This produces a dark and low contrast image compared to real life.    In real life light areas are of higher value than dark areas.    Linear basically means flat.

Low Pass Filter

A filter fitted in front of the image sensor to reduce moiré by filtering ultraviolet and infrared light.


Macro is the term given to close up photography.  A 1:1 macro ratio means that the size of the subject is the same on the film as real life.   A 1:2 ratio mens the size of the subject on the film is 1/2 lifesize.   

Mustache Distortion

A combination of barrel and pincushion distortions.


The difference between image coverage in the viewfinder and the image taken by the lens.  Parallax occurs when the optical axis of the viewfinder is in a different position than that of the camera lens.


Optical distortion in which straight lines along the edge of an image curve and bend toward the center.

Prime Lens

'Normal Lens".  A lens with a single fixed focal length.


Digital images that have not undergone processing by the camera.   RAW images need post processing to be finished.   RAW images have less noise than jpegs.

Rangefinder Camera

A camera that uses a viewfinder independent of the taking lens to focus.  Rangefinder focusing requires the aligning of two images in the viewfinder to achieve proper focus and compensate for parallax errors.

Reflective Light

Light that bounces off an object.   A reflective meter reads the light bouncing off the subject.

Reverse Adapter

An adapter that attaches to the lns filter ring and allows the lens to be mounted on a camera in reverse.   This changes the lens into a macro lens.


Saturation refers to the color intensity of an image.  


Sliding a lens element to one side while keeping it parallel to the film plane.   This action corrects for converging lines in images where the camera is not parallel to the subject.   Example:   When you take picture of a building from the street the building will get narrower toward the top.   Shifting the lens corrects for this narrowing.

Shutter Speed

Shutter speed is the length that a shutter stays open to allow light to hit the film or sensor.


Single Lens Reflex.   A camera that uses a mirror to let the photographer view th actual image the film will record.  

Soft Light

Diffused, low contrast light which does not produse harsh shadows,   

Telephoto Lens

'A magnifying lens.  In 35mm photography a 100mm lens is twice hte magnication of a 50mm lens.  Telephoto lenses have a narrow field of view..  Telephoto lenses achieve magnification without increasing the length of the lens to its equivalent focal length.  A 200mm telephoto is not 200mm long.


Twin Lens Reflex.  A camera that uses a mirror and viewing lens that is separate from the taking lens.  The taking lens does not use a mirror and has a buit-in leaf shutter.


Twin Lens Reflex.  A camera that uses a mirror and viewing lens that is separate from the taking lens.  The taking lens does not use a mirror and has a buit-in leaf shutter.

Variable Aperture

This refers to a zoom lens which changes aperture as the focal length is changed.  


The darkening of the edges of an image as compared to the bfightness of the center.  

Wide Angle Lens

A lens that sees a wide field of view. 

White Balance

The process of removing unrealistic color casts so that objects which appear white in person are rendered white in your photo.   Digital cameras use white as a reference point for rendering accurate colors.

Zoom Lens

A lens with variable focal lengths.

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