A
AE - Automatic Exposure.
Alpha Channel - A saved selection/mask. Alpha channels are grayscale images that can be edited like any other image in Photoshop.
Aperture - The opening that allows light to enter the camera. The shape of the aperture opening controls the bokeh.
The effective unit of aperture is the f-number, which is the ratio of the ‘Focal Length’ to the ‘Effective Diameter’ of the aperture. A lower f-number indicates a wider opening and vice versa. Typical apertures encountered in modern DSLR lenses range from 2.4 to 22. Lenses can exceed this range but are usually more expensive.
Lenses can have apertures less than 1 but these are rare. The smaller the minimum f-number the faster the lens is said to be, because it allows more light to reach the sensor/film quicker than a slower lens in the same conditions. This makes them more responsive in lower light conditions.
A change in f-number of 1.41 is a change of 1 ‘Stop‘. A change of 1 stop doubles or halves the amount of light reaching the sensor/film.
Aperture is also the name Apple chose for its professional DAM (digital asset management) software. Aperture is now at version 3.0.3 (13/06/10). The programme is one of Apple’s range of professional software products. It provides the ability to bulk (batch) process digital images – editing both the image and data associated with the image (metadata). It can also handle other types of digital media including audio and video data. All image processing in Aperture is non-destructive.
Apple produces a cheaper alternative, iPhoto, that is supplied free with Apple computers – though subsequent upgrades are normally charged for.
APS-C – Advanced Photo Systems ‘Classic’. This refers to the size of the camera sensor. The original APS ‘Classic’ film negative size was 25.1 x 16.7 mm with an aspect ratio of 3 x 2. The film also came in APS-H (High Definition) and APS-P (Panorama) sizes.
A number of slightly different digital senors sizes are called APS-C. All APS-C sensors are significantly smaller than a 35 mm negative. As a result, a lens factor (often called the crop factor) has to be applied to get the 35 mm equivalent focal length.
For example: – Sony APS-C sensors are typically around 23.5 x 15.6 mm in size. The lens factor (crop factor) is 1.5. A 50 mm lens on a Sony APS-C senor will therefore provide the same field of view as a 50 x 1.5 or 75 mm lens.
Aspect Ratio - The ratio of length to height of an image. Keeping the same aspect ratio when changing the size of an image ensures that the content is not distorted.
B
Bit – The basic unit in computing. It can have two values 0 or 1.
An image with a 1 bit colour depth can display two colours, black or white.
An image depth of 2 bits allows four colours.
A 4 bit image can display 16 colours.
8 bits can display 256 colours.
A JPEG image with 256 tones in each of three channels can display 256 x 256 x 256 or 16.777216 million colours.
Bokeh – This is a relatively modern term. It is a westernisation of the Japenese word ‘boke’ or hazy and refers to the appearance of out of focus areas in an image with limited depth of field. Bokeh varies with the shape of aperture used in the lens and manufacturers have tried to ensure a more subjectively pleasing bokeh in certain lenses. It can also be applied post capture in the digital darkroom.
C
CCD – Charge-coupled device. This device allows charge to be moved. In combination with a photoelectric sensor it can act to digitise images. CCD devices are a mature technology that can deliver low noise images.
Circle of Confusion – More properly the Maximum Circle of Confusion Diameter, is the size at which a point becomes an observable circle. This varies with the format used in capturing the image and any subsequent magnification of the image.
CoC = AVD/DPR/AEF/25
Where
CoC = Maximum Circle of Confusion Diameter (mm)
AVD = Average viewing distance (cm)
DPR = Lines per millimetre at a viewing distance of 25cm
AEF = Anticipated enlargement Factor
For a viewing distance of 25cm, a DPR of 5 and an AEF of 5 the CoC is 25/5/5/25 = 0.04mm
CMOS - Complementary metal oxide semi-conductor. Another technology used in digitising images. In this technology several transistors amplify the charge from incident light and transmit it. Unlike CCD devices each pixel is read directly. CMOS devices tend to use less power than CCD devices and tend to be cheaper to manufacture.
D
Depth of Field (DOF) - Depth of Field is the distance over which objects appear to be sharp. DOF increases as the F-number decreases.
Near distance of acceptable sharpness is given by
Dnear = Fd(HFD-Fl)/(HFD+Fd+2Fl)
Far distance of acceptable sharpness is given by
Dfar = Fd(HFD-Fl)/(HFD-Fd)
Where: -
HFD = Hyperfocal distance (mm)
Fl = Focal length(mm)
Fd = Focal distance (mm)
DSLR – Digital Single Lens Reflex. A single lens reflex camera that captures images digitally onto electronic sensors rather than film. See SLR.
DT – Sony lenses. Digital Technology. used for lenses designed for APS-C sized sensors.
E
EF – Canon lenses. Electronic Focus. Canon standard lens mount.
EF-S – Canon lenses. Electronic Focus – Short Back Focus. The designation for Canon lenses designed for APS-C sized sensors.
EV – Exposure value. This is a base 2 logarithmic scale, where a change of 1 on the scale represents a doubling (or halving) of the exposure. This is equivalent to a 1 stop change in exposure.
EV = log base 2(Fn^2/T)
Where Fn = f-number, T = shutter speed in seconds
Zero on the EV scale is obtained with an f-number of 1.0 over 1 second or an f-number of 1.4 over 2 seconds etc. – as shown on the following table.
© Improved Images Ltd
EXIF – Exchangeable Image File Format. Not supported in GIF or PNG formats. The metadata tags include information about date/time and camera data.
Exposure – Exposure is the total amount of light that reaches the sensor. It is affected by the aperture, shutter speed and ISO.
F
Filter – Filters alter the light entering the camera in a variety of ways. Most effects can be applied in the digital darkroom.
Firewire – A high speed data communications standard initiated by Apple. The IEEE 1394a/b High Speed Serial Bus standards support devices transmitting at 400/800 Mbps but much higher speeds are encompassed in the IEEE 1394b version.
G
Gamma – Gamma is a number that describes the relationship between the varying levels of brightness a digital camera can capture. Mac OS X 10.6 now uses a gamma of 2.2 rather than the previously used value of 1.8.
Output pixel brightness (eg monitor) = input pixel brightness (eg camera) ^gamma
GIF – A lossless bitmap format that can address up to 8 bits per pixel or 256 colours. Supports animation. Best with logos and web images.
Golden Ratio - The Golden Ratio is 1.61803399. It is generated when a line is divided in two such that the ratio of the larger part to the smaller part is the same as the ratio of the whole line to the larger part. It appears in nature and is claimed to have harmonious properties.
H
Hyperfocal Distance - The focal distance which gives the maximum Depth of Field (DOF). When a lens is focussed at this distance everything between half the hyperfocal distance and infinity appears to be sharp.
HFD = 2Fl/(COC * Fn) + Fl
Where
HFD = HyperFocal Distance (mm)
COC = the Maximum Circle of Confusion Diameter (mm)
Fl = Focal Length (mm) – True (i.e. not 35mm equivalent)
Fn = F-number
Note:- Some versions of this equation omit the final focal length(Fl) element. In many practical circumstances this is not material to the result.
I
ISO - International Standards Organisation. It covers many different areas of standards. In photography it is normally used to refer to the ‘speed’ of the film. Larger ISO numbers provide ‘faster’ films that react to light more quickly, albeit with more grain. In digital cameras, increased ISO indicates increased sensitivity of the sensor. This also leads to increased noise of the image – although as modern cameras develop larger sensors they are more able to record images at progressively larger ISO’s with reduced noise. A good example is the Nikon D3 which can record up to 25,600 ISO.
iPhoto – Apple’s basic image storage and editing software programme. It ships free with every Apple computer and is updated on a fairly regular basis by Apple. It currently includes Apple’s face recognition and image location technologies.
IPTC – International Press Telecommunications Council. An internationally recognised body, based in London, that designs standards for photographic metadata amongst other data types. IPTC metadata for a photographic image typically includes administrative and copyright information.
IS – Canon lenses – Image Stabiliser. This feature in a lens compensates for movement by moving a lens element horizontally and/or vertically and is controlled by two gyroscopic sensors.
J
JPEG - Joint Photographic Experts Group. This group provides a standard for compressing images. The file format is called JPEG or JPG and is a lossy compression method. This type of format is not well suited to use on images with many lines or text or icons where sharp contrasts are common because it leads to compression artifacts.
K
Kelvin – The scale in which colour temperature is measured. The Kelvin scale is named after the famous Scottish physicist, Lord Kelvin, who devised the scale in 1848. 5500K or 5500Kelvin is roughly the temperature of the noonday sunlight in a temperate climate.
L
L – Canon lenses – Abbreviation for Canon’s professional range of camera lenses. The lenses are generally white with a red band on the barrel.
Lab – A colour space that is device independent. It consist of three channels: L – luminance, A – Green to Red, B – Blue to Yellow.
Lossless compression – In these methods no image data is thrown away during the compression process.This leads to larger file sizes and tends to be used where the image quality needs to be maintained. The TIFF and PNG formats provide lossless compression.
Lossy compression – This refers to methods of compression that involve some loss of image information, such as that involved in JPEG images(in most cases). Typically used where small image sizes are needed. Too much compression can however lead to artifacts appearing in the image. Lossless compression formats such as area available with the TIFF image format avoid this problem.
M
Macro photography – Close-up photography providing images at 1:1 magnification or larger.
Metadata – Data about data. In relation to images, this typically includes details about the camera, exposure, aperture and flash settings and/or copyright and administrative information etc. See also IPTC and EXIF.
N
NEF - Nikon Electronic File. This is Nikon’s RAW file format.
Neutral Density – A type of filter with no colour effect that blocks a proportion of light falling on the sensor. They are often used to allow a slower shutter speed and in graduated form to reduce the dynamic range between a brightly lit sky and the foreground. They come in a range of strengths measured in stops or transmittance.
Nikon – A Japanese manufacturer of a wide range of optical/electronic equipment. Its name is an abbreviation of the company’s original name, Nippon Kogaku or Japan Optical and was first used in 1946. It is currently one of the companies in the Mitsubishi Group.
Nikkor – The name for Nikon’s range of lenses. The brand was first introduced in the 1930′s and was originally reserved for the highest quality lenses.
O
OS – Sigma lenses. Optical Stabilisation. Sigma’s variation of in-lens optical path stabilisation to reduce image blurring.
P
Parallax – The offset that occurs when an object is viewed from different positions. This is avoided in SLR and DSLR cameras because the photographer views the scene through the same lens that is used to take the image.
Photoshop – A raster image processing programme (package of programmes). An Adobe product, it is part of the Creative Suite (CS) suite of software. It comes in Standard and Extended forms.
Many would claim Photoshop is the raster image manipulation programme and the industry standard. It offers a very wide range of processing options, as well as a hefty price tag.
Photoshop tends to have major updates on an approximately 18 month cycle. It is currently on version CS5 (13/06/10). The CS in the title of the latest version signifies that Photoshop is part of the Creative Suite of professional programmes that cover everything from web page development to cartoon production. Photoshop’s Extended version contains 3D functionality, that used to be part of the standard version of the programme.
Adobe also produces Photoshop Elements, the foreshortened brother of Photoshop for a much reduced price.
How is Photoshop different from Photoshop Elements, Aperture and Lightroom?
These programmes actually fall into two discrete groups with Photoshop Elements in one and Aperture and Lightroom in the other. Photoshop Elements is aimed at an enthusiast customer base with shallower pockets than the professionals customer and is therefore offered hundreds of pounds cheaper. Elements is very much a junior sidekick to the main Photoshop programme and is now in version 8. It is much cheaper than Photoshop and often has sufficient functionality for many non-professional photographers.
PNG – Portable Network Graphics format. This format replaced the GIF format as a free, open source successor. It uses lossless compression and supports 16.7 million colours.
Polariser - A filter that polarises light entering the camera. It comes in radial or circular forms and tends to have maximum impact at right angles to the sun, where polarising effects are largest, tending to darken and enrich the sky colour.
Prime Lens – A lens with a fixed focal length. This has a number of benefits including lighter weight and less complex optics. See also Zoom lens.
Q
R
Raster images - Raster images are made up of pixels, which become visible when the image is magnified sufficiently. They differ from vector images, which retain sharpness at any magnification.
Raw - This is the name of files generated straight from digital camera sensors. It is not an acronym. Unlike JPEG images generated by cameras no processing has been applied to the basic data. Capturing images in this format rather than JPEG results in larger file sizes but an enhanced ability to improve the images in the digital darkroom.
Rear Sync – A flash mode where the flash takes place near the end of the shutter passage rather than at the start. This mode ensures a trail of light is left behind, rather than in front of a vehicle photographed in low light.
Red eye – An optical effect where the pupils of subjects’ eyes look red when flash is used. It can be mitigated and corrected in a number of ways, including using a pre-flash that causes subjects to reduce the size of their pupils. It can also be corrected post capture in the digital darkroom.
RF – Rear focusing. A form of internal focusing in which only the rear elements move and the length of the barrel does not change. Using rear elements uses tends to use less energy and helps to speed up focusing.
RGB – Red Green Blue. A colour model based on the three additive primary colours. All three colours combine to produce white and
RPS – Royal Photographic Society. The Royal Photographic Society was founded in 1853 ‘to promote the Art and Science of Photography’.
The society is based in Bath and has Regions and Groups all over the world. Their website is http://www.rps.org
S
Shutter - The mechanism that exposes the sensor/film to the light. The speed of opening controls the amount of light that enters. Typical DSLRs can remain open from 30 seconds to 1/8000 sec.
Slow sync – A shooting mode that combines a slow shutter speed with flash in low light situations in order to balance exposure in both the foreground and background.
sRGB – Standard Red Green Blue colour space. This colour space was designed by HP and Microsoft for displaying images on the Internet. The gamut of the colour space is much smaller than Adobe RGB.
SLR – Single Lens Reflex. A type of film camera that employs a moving mirror to allow the phototographer to view the scene through the lens that will take the image. This helps avoid parallax alignment issues.
SLT – Single Lens Translucent. Sony’s range of digital translucent mirror cameras, released under the Alpha brand. The range started with the Alpha 33 and Alpha 55 models, released in late 2010.
T
TIFF - Tagged Image File Format. A format that can provide lossless compression of images images. This format can contain much more information than JPEGs, such as Photoshop layer information and as such, tend to be much larger.
U
Unsharp mask - This term comes from the days of film processing and is a method of giving the appearance of sharpening in the digital darkroom. Typically shortened to USM.
USB – Universal Serial Bus. A method of communicating between a computer and peripheral devices, such as cameras. Miniature forms of the connectors are often used on digital cameras for space saving reasons.
USM – Ultrasonic Motor – Canon lenses. Fast and silent focusing motor.
V
VC - Vibration Compensation – Tamron lenses. Tamron’s version of in-lens shake compensation.
Vector images - Vector images are defined mathematically from a series of individual points. Vector images can be scaled virtually infinitely with no loss of definition – unlike raster images.
Vignette - Darkening seen around the edges of an image. It is caused by the shadow of filters or lenses, especially at small focal lengths. It can be generated in-camera and removed or added in the digital darkroom for specific effects, such as drawing attention to the central area of an image.
VR – Vibration Reduction – Nikon lenses. This is Nikon’s in-lens approach to reduce blurring due to camera shake.
W
White Balance – The correct representation of white in an image. The human eye is very flexible and can correct for colour casts generated when the colour of the ambient light changes. This keeps a white object looking white. Most current camera sensors cannot do this as well as the eye and it is often necessary to apply a correction in the digital darkroom in order to properly represent white in the final image.
Workflow - The order of acquisition and/or processing steps used on an image to achieve a specific outcome.
X
Y
Z
Zoom Lens – A lens which can continuously change its focal length. This allows the photographer to enlarge the subject without moving his own position.
