Download Glossary

Glossary of Photographic Terms and Abbreviations...
Aberration…Any of several failures of a lens to create perfect images. These include
chromatic errors where various wavelengths focus at different points, geometric errors
where straight lines appear curved, and field curvature errors where everything does
not focus in the same plane.
Achromatic…A lens system that has been corrected for chromatic aberration.
Acutance…An objective measure of image sharpness.
Aliasing…Low frequency artifacts that appear when signal energy above the Nyquist
frequency reaches a digital sensor. This is normally prevented by use of an AntiAliasing filter in front of the sensor.
Ambient Light…The light currently illuminating an object or space. See "Available
Light".
Anti-Aliasing Filter…An optical filter constructed of two thin birefringent plates
mounted at 90o. The thicker the plates, the greater the blurring.
See: http://en.wikipedia.org/wiki/Birefringent
AOV…"Angle of View"...see "FOV".
Aperture…The opening which varies the amount of light passing through a lens. Also
called the "iris".
Apochromat…A lens that has been corrected for chromatic aberration in 3 primary
colors. Often "APO" is an abbreviation.
APS-C…Generally the size of an APS-C film frame, ie, 25.1 x 16.7 mm, but APS-C sensors
vary widely in actual size.
Aspect Ratio…The ratio of width to height in images.
Available Light…The light currently illuminating an object or space. See "Ambient
Light".
Back-Illuminated Sensor…A recent development where the rear of a sensor is thinned
after processing. This thin Silicon back is relatively transparent allowing photons to
reach the photo-diode w/o being obstructed by the other circuit elements. This
technique is most beneficial w/ sensors that otherwise have a small fill factor [mostly
small sensors w/ high density photosites]. The thinning of the substrate after processing
makes the chip delicate, thus It is only practical on tiny sensors.
Bayer Sensor…A sensor that has 4 sets of colored photosites arranged in a 2 x 2 array.
Normally the 4 photosites have 1 red, 1 blue, and 2 green filters, but other
combinations are also used in a few sensors. Note that these 4 photosites map into four
full-color pixels, even though each is only a single color photosite, by performing Bayer
demosaicing (which involves sampling other surrounding photosites).
Bracketing…A technique whereby several identically-framed images are captured w/
different settings. Normally, the exposure time is varied, but sensitivity (ISO setting) and
white balance can also be used. This can be done manually, but some modern cameras
can do it automatically.
Bridge Camera…A camera that has characteristics of cameras with mirrors and cameras
w/mirrors. A hybrid camera. A bridge camera "bridges" the gap between the two
primary types of cameras.
Bulb…a setting which leaves the shutter open as long as the shutter button is depressed
[fully]. It’s an old terminology. Around 1894, a third setting 'B' for Beliebig [which means
“any time” in German] was added to cameras, which at the time only had two shutter
modes: M for “Moment” and Z for “Zeit” [time]. Later, in the USA, B stood for “bulb”
when shutters were commonly operated by a pneumatic bulb.
CA…See "Chromatic Aberration".
CCD…"Charge Coupled Device"…one of two primary types of sensors, the other being
CMOS.
Chimping…on cameras which have no electronic preview of the image through the
viewfinder (such as most dSLRs), this refers to looking at the image on the LCD
afterward, which requires pulling the camera down from the face and looking down at
the back of the camera. This looks similar to one primate grooming another and looking
to see if the little black speck being held (after biting it) is dead. It's a VERY degrading
description when used by users of cheap P&S cameras to describe the behavior of users
of expensive dSLRs!
Chromatic Aberration (CA)…Failure to focus all wavelengths (colors) of light at the same
spot. This causes colored borders in high contrast subjects. It's also called "PF", which
stands for "Purple Fringe", as CA is often exhibited as a purple border. "Green Fringe",
Blue Fringe", and "Yellow Fringe" are also common, but nobody uses "GF", "BF", or
"YF".
CIE Standard… A system of standards adopted by the Commission Internationale de
I'Eclairage, allowing accurate descriptions of colors.
Circle of Confusion…A blurred spot, which, due to its [small] size appears to an observer
as a sharp dot. The size of the CoC is used to calculate Depth of Field.
CMOS…"Complimentary Metal-Oxide Semiconductor"….one of two basic types of
sensors, the other being CCD.
Compact Camera…A camera that is about the size and shape of a deck of cards.
CMYK…An abbreviation for the cyan, magenta, yellow and black color space. They are
the “subtractive” colors used in a four color printing process. They can also be used in
computer files of photos or drawings.
Color Temperature…An objective description of the color of a light source...expressed as
the temperature of a "black body" in degrees Kelvin (K). Works well for the sun and
incandescent lights which have a continuous spectrum.
Curvature-of-Field…Some lenses, especially WA ones, focus at different distances off-axis
vs on axis. This causes the image to be slightly OOF at the edges when the subject and
image planes are flat.
DC…"Digicam"...any digital camera, but normally a small one.
Depth-of-Field (DOF)…The range of object distances at which the image appears sharply
focused. Highly subjective! Although there are "formulas" to calculate DOF, they are
based on empirical data.
Diffraction Limited…Reduced resolution of a lens, due to a small aperture.
Diffraction…An effect that produces fringes/rings when light passes through a small
hole.
Digicam…A Digital Camera of any sort, but it generally means, a cheap one (ie, dSLRs are
not normally referred to as "digicams").
Diopter…A unit used to express the power of a lens. It is the reciprocal of the focal
length expressed in meters.
DR…"Dynamic Range": The difference between the brightest and dimmest light levels
that can be captured. DR is normally expressed in F-stops.
Dragging the Shutter…A flash exposure technique using a slower than necessary
exposure time, to increase the background brightness at the expense of some
blurring. Can be artistic.
dSLR…"digital Single Lens Reflex". Also DSLR, D-SLR, and d-SLR.
Entry-level dSLR…The cheapest dSLR that a manufacturer makes, targeted at new users.
ETTR…"Expose to the Right"...the concept that if you watch the right side of the
histogram and adjust the exposure until the brightest parts of the image just touch (or
come just shy of touching) the right edge, the exposure is "right".
EV…"Exposure Value" describes all combinations of exposure time and f/number that
give a specific exposure. 1 EV is defined as a 1 second exposure at f/1. See also "LV".
Exposure… The product of the intensity of light projected on the sensor and the time the
light is allowed to act on the sensor, thus it is the combination of scene illumination,
exposure time, and aperture used to create a photographic image. Some people choose
to ignore illumination, but there are instances when the photographer HAS to control
the illumination. Others include sensor sensitivity as a part of the exposure.
FF…See "Full Frame".
FL…See "Focal Length".
Flare…Light reflected off the lens surfaces, internal camera surfaces, or lens parts. This
often appears as circles or arcs on the image, but can also appear as a haze over the
entire image which reduces the contrast. Scattering can be greatly reduced by coating
lens surfaces with special coatings, called anti-reflection coatings.
Flash…A device to illuminate the scene to be photographed. Most commonly the light is
produced from a small quartz (glass) tube filled with a mixture of gasses (mostly Xenon)
that, when ionized by an electrical discharge, glow brightly at a color temperature of
about 5600 degrees Kelvin, which is the color temperature of noon sunlight. People call
these many names; Flash, FlashGun, FlashHead, SpeedLight, SpeedLite, etc...but not
FlashLight...it's reserved for something else. 
Flat Field…When a lens renders a flat subject as a flat image (on the sensor), it's said to
have a "flat field". Many WA lenses don't have a flat field. See also "Curvature-of-field".
F/number…A number that represents the facility of a lens to capture light. Normally
described as f/n or f:n (the "n" is the "F/number"). The "F/#"or "F:#" form implies focal
length divided by the aperture diameter. See "Aperture" and "F-stop".
Focal Length (FL)…The distance from the image (on the sensor) to the lens when the lens
is focused on an object at infinity. Technically, the point on the lens used for this
measurement is the "exit pupil".
Focal Plane…A plane perpendicular to the optical axis at which the image is focused.
FOV…"Field of View". The angle subtended by the scene. It can be expressed as the
horizontal angle, the vertical angle, or the diagonal angle, but normally the horizontal
FOV is the default. Some people try to confuse everyone with "Angle of View". AOV and
FOV are synonyms. A problem arises because FOV is sometimes stated as "abc ft at xyz
yards", like for binoculars. But it is stated as an angular measure by educated people.
F-stop…A full "F-stop" is defined as a “standard” aperture value that allows half as much
light through as the previous stop and twice as much light as the next stop. The
"standard" full "F-stops" are: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, etc. Many cameras
also allow half "F-stops" to be set. They are: 1.2, 1.8, 2.4, 3.5, 4.8, 6.8, etc. A few cameras
even use 1/3-stops. The "definition" of F:1 is that the diameter of the glass opening in
the lens [aperture or iris] is equal to the focal length.
Full-frame…Same size as a 35mm film frame, ie, 36mm x 24mm. Yes, there are other
"full-frame" sizes, but when this term is used, it's referring to the "standard" 35mm still
picture frame.
Gamma…A gamma value is used to quantify contrast. It is the slope of an input–output
curve in log–log space. The center slider in a Photoshop histogram is the Gamma slider.
Gamma encoding (compression) is used in most still and video formats.
Geometric Distortion…A characteristic of lenses that make straight lines curved. If they
bow out, it's called "barrel distortion". If they bow in, it's called "pin-cushion distortion".
A WA lens with terrible geometric distortion is labeled a "fish-eye" lens in order to get
customers to buy it. ;-)
Guide Number…A measure of flash range. It is the product of distance and f/number,
expressed in either feet or meters.
HDR…"High Dynamic Range"...a technique wherein several images w/ different EVs are
combined to increase the DR.
High Key…A photo with mostly light tones and few middle tones/shadows.
Hot Shoe…A fitting on a camera (usually the top) that accepts accessories, such as a
flash. It contains electrical contacts.
ISO…International Standards Organization…they write the rules for measuring all types
of things. For cameras, the most relevant parameter is "ISO Sensitivity", a measure of the
ability of the sensor to capture photons and the subsequent gain of
associated amplifiers. Be aware that the ISO Standard that governs measuring and
specifying digital sensor sensitivity is broad…it allows 5 different ways of performing
these sensitivity measurements. These 5 procedures are roughly equivalent, but they can
be used by a manufacturer to gain a marketing advantage (with customers who think
that bigger numbers always mean better products). Independent labs that test camera
sensitivity using a single method, find a wide variability!
JPEG…An acronym for the "Joint Photographic Experts Group", which created the
standard. A lossy, compressed image storage format. The degree of compression can be
adjusted. It is the most common digital image format.
LCD…"Liquid Crystal Display" The little screen on the back of most digital cameras. It is
NOT normally called a "Viewfinder"…it's called an "LCD Screen".
LAS…"Lens Acquisition Syndrome" is a terminal disease that renders the owners of
removable-lens-cameras broke financially.
LED…"Light Emitting Diode".
Low Key…A photo with mostly dark tones and which has few highlights.
LV…"Light Value" refers to a "light level" for either incident or reflected light…a synonym
for "EV at ISO 100 sensitivity".
Macro…A term denoting a magnification ratio close to 1:1. Often used loosely to mean
"close-focusing”.
Magnification…The size of the image relative to the size of the subject. Also, the ratio of
subject-to-lens distance to the image-to-lens distance.
MLU…"Mirror Lock-Up" is a feature of mirror-type dSLRs that eliminates one source of
vibration.
MOS…"Metal Oxide Semiconductor". There are two types of MOS…"NMOS" and
"PMOS"…both types are used in a "CMOS" process. Olympus uses an NMOS sensor, but
calls it "Live MOS" for marketing reasons.
MTF…"Modulation Transfer Function". Indicates the contrast of a B/W pattern at a given
[high] spatial frequency relative to the contrast at low spatial frequencies. It's used to
measure and quantify the resolution of a lens.
ND…"Neutral Density" A material (usually a filter) that blocks some light w/o imparting
a color cast.
Nodal Point…A location on the optical axis (center-line) of a compound lens system
where all rays of light appear to originate or terminate.
Normal Lens…A lens that "sees" a view similar to what the human eye sees [this doesn’t
make much sense, because the human eye “scans” a scene with its high-resolution
central area [cones] and stores the outlying picture; this is vastly different than what a
camera does!]. In technical terms, it has a horizontal field of view about 40 degrees.
With a 35mm film camera, this meant a 50mm lens, but modern, smaller sensor
cameras require a shorter focal length. For example, a 35mm lens is often described as a
"normal" lens for APS-C sized cameras. For compact cameras, the "normal" FL is much
smaller yet. The FL of a "normal" lens is approximately equal to the diagonal size of
the sensor.
NR…"Noise Reduction". Specifically, it's a computer algorithm that examines an area of
an image, decides if there is a specific noise pattern present, and tries to remove that
pattern w/o affecting the sharpness of detailed [non-noise] elements in that area of the
image.
Nyquist Frequency…Half the sampling frequency of a discrete signal processing system.
It is an absolute lower bound for the sample rate for alias-free signal sampling. It applies
to both temporal and spatial realms.
OOF…"Out Of Focus". Also “OoF”.
Pellicle Mirror…A fixed mirror, as opposed to a moving mirror, used in some mirrorreflex cameras. A pellicle mirror is only partly reflective, so that both the reflected and
transmitted components occur simultaneously. A camera w/ a pellicle mirror is less
sensitive than a camera w/ a moving mirror.
Pentaprism…A 5-sided prism which reverses a scene, so that it can be viewed as it
normally appears. It is used in high-quality SLR and dSLR camera viewfinders.
Perspective Distortion…Making parallel lines appear to merge. Perspective distortion is
a characteristic of all lenses. It's NOT necessarily a defect! It's caused by a photographer
pointing the camera up or down at an object. Sometimes this is unavoidable, as when
photographing a tall building. Other times it's controllable by the photographer, as
when photographing a room (the camera needs to be ½ way between the floor and the
ceiling).
PF…"Purple Fringing" is a form of Chromatic Aberration (CA). It is normally caused by
poor lenses, but it can be also caused or exacerbated by the micro-lenses on the face of
a sensor.
Phase Detection…an autofocus system used in “reflex” cameras [although its use is
expanding to include some “mirrorless” cameras]. Its advantage is speed, since it is
generally an open-loop control process [measure mis-focus, compute lens position to
bring into focus, tell lens to go to this position, & check focus]. The “other” autofocus
system is Contrast Detection, which is a closed loop control process whereby the
camera adjusts the focus until the perceived contrast in the selected focus area is a
maximum; it is accurate, but can be slow. Some recent cameras use both systems. 
Photosite…The actual photodiode that generates current proportional to the photon
density impinging on it. A photosite is smaller than the pixel-pitch would imply because
there are other components, like transistors, capacitors, resistors, circuit traces, etc. that
occupy space around a photosite. The difference between the area defined by the pixelpitch and the area of the photosite is called the “Fill Factor”. Often, micro-lenses are
positioned above the photosites to somewhat mitigate the negative effect of a small fill
factor.
Pixel…The smallest complete element in a digital picture. "Pixel" stands for "picture
element". Note that a RAW image does not have any “pixels”, but rather the data from
masked photosites. It must be “rendered” into an RGB form via Bayer interpolation
algorithms.
Pixel Density…A measure of how tightly packed photosites are. Approximately the
inverse of photosite area [assuming a large “fill factor”]. Some people believe that
sensors with large photosites [and thus low pixel densities] have better IQ: Some
misguided souls don't. In general, a low pixel density is better [unless you take it to a
silly extreme]. Currently, there are no digital cameras with too few pixels, but arguably
there are some with too many.
Point & Shoot…An automatic camera that doesn’t require the operator to think. Also
"P&S" and "PnS". Can be a term of derision, implying that the owner is not
smart enough to use a "real" camera with manual controls. Point & Shoot is also a lifestyle decision and a perfectly valid way of taking pictures…with any camera (even a dSLR
can be use in a point & shoot style)!
Post Processing…Editing a photo after it has been removed from the camera.
PP…see "Post Processing’.
Primary Colors…The primary, additive colors; red, green, and blue. The primary
subtractive colors are cyan, magenta, and yellow.
Principal Point…The point from which the FL is measured. All cameras have compound
lenses. Compound lenses have 2 principal points. The rear PP is used for measuring the
FL.
Prosumer Camera…A small camera with manual controls. Also used to refer to a dSLR
that is targeted at beginners.
PS…Photoshop, the granddaddy of photo editors. There are 3 “flavors” of PS: CSn,
Lightroom, and Elements.
RAW Image Format…This format simply records all the data from the sensor w/o any
Bayer demosaicing. This information is not [yet] an image, because it has no RGB data. It
is large for two reasons: 1) the data is typically 16-bits deep, 2) the data is not
compressed [but it can be, w/o loss of fidelity]. Even when it’s losslessly compressed, it’s
larger than a JPEG file. No camera settings are applied to this data, which allows the
photographer to make decisions on what are normally “camera” choices at the time of
post processing.
Rendering…The process of creating an RGB image that can be viewed from raw sensor
data. There are many RGB formats, such as JPEG, TIFF, GIF, PNG, DNG, etc. The data that
comes from a sensor is not in an RGB format, so it must be processed, ether by the
small processor in the camera or by a large processor in a home computer.
Retrofocus…a lens design with negative (diverging) lens elements positioned in front of
the diaphragm and positive (converging) lens elements positioned at the rear of the
diaphragm. This makes the distance from the rear of the lens to the focal plane longer
than the lens focal length. Retrofocus design has been adopted in wide angle lenses
designed for SLR cameras so the rear of the lens does not impede the movement of the
mirror. It has a negative effect on image quality, because it increases the number of lens
elements. Very high quality WA lenses protrude into the mirror-box and require the
mirror to be locked up [and a separate viewfinder to be used!].
RGB… An abbreviation for the red, green and blue color space. They are the “additive”
colors used in computer monitors and TV screens. They can also be used in computer
files of photos or drawings.
Sensor…The piece of processed Silicon, [pronounced SIL-i-con, not SIL-e-cone; silicone,
not capitalized, is used in breast implants] that senses images. A Silicon breast implant
would be painful! 
Shutter Speed…Shutters are quite fast. The first Leica focal-plane shutters traveled at 2
meters-per-second [m/s]; they traveled horizontally. In 1965, Copal invented the
modern, multi-blade metal shutter which traveled at 3.4 m/s. Nikon later increased the
velocity to 7.3 m/s. The fastest focal plane shutter was the Minolta Dynax 9xi at 13.3
m/s. Note that “shutter speed” is quite distinct from “exposure time” and it’s usage in
that way should be avoided!
SLR…"Single Lens Reflex". A camera with a mirror that reflects the image from the main
lens to a viewing screen. That image is then viewed by an optical viewfinder.
SLR-like…Looks like an SLR, but isn't (ie, it doesn't have a mirror). It can sound like an
SLR if the designers added the electronic sound of a mirror going flip-flop.
Stitching…The technique of combining several overlapping images [w/ a computer
program] to make one larger image.
Super-zoom Camera…A small camera with an 8X (or greater) zoom lens.
Telecentric Lens…A lens that has parallel light rays that strike perpendicular to the
sensor. See: http://en.wikipedia.org/wiki/Telecentric_lens. Olympus claims to have
"near" telecentric lenses...they are lenses that approximate a "real" telecentric
lens...these lenses require large elements, to minimize the angle-of-incidence
issues caused by the color mask and/or micro-lens array. A "real" telecentric lens would
have its exit pupil infinitely far in front of the lens and would NOT change the size of the
image when the lens was focused. A true telecentric lens is big and expensive!
TTL…"Through-The-Lens". Applies both to measuring exposure and computing flash
power by looking through the primary lens.
USM…"Un-Sharp Mask". A legacy term from the film photography days. In spite of the
strange name, it's an algorithm used (among other things) to sharpen digital
photographs.
Vignette…The reduction in exposure in an image at the sides and corners; it is caused by
several things, such as a lens hood or filter and also the natural off-axis light falloff of
most lenses.
WA…"Wide Angle"…a lens that captures a big (angular) scene.