A glossary of useful photo terms

Aperture
The aperture is the hole inside the lens that allows light through. Aperture is measured in f numbers - a ratio of the diameter of the hole and the focal length of the lens. The size of this hole can be adjusted: a larger hole allows more light in, a smaller hole less. An important effect of this is the depth of field. A smaller hole (higher f number) produces a lot of depth of field. The reverse happens with a larger hole (smaller f number). The focal length divided by the f number gives the effective aperture diameter.

Blown highlights
Blown highlights are areas of a photo that are so bright they are pure white. A usual digital camera can record 256 shades of brightness, from "0" (pure black) to "255" (pure white). Some cameras have much greater of tonal range with many more shades. Every pixel captured is assigned a brightness value. When it comes to editing, software can add to, or subtract from this value to make a photo brighter or darker. If a pixel already has the value of "255", it can go no higher. No detail can be found in these areas of a photo and they end up pure white. These areas are called blown highlights.

Bokeh
Bokeh is a photography term that refers to the way a lens blurs an image. Bokeh refers to how evenly and pleasingly the out of focus (blurred) area looks.

CCD
Stands for Charge-Coupled Device. A type of sensor.

CMOS
Stands for Complementary Metal Oxide Semiconductor. A type of sensor.

Colour space
An image file can be associated with one of various spaces:rgb is the most common and is for screen use and most digital printing cmyk is the four colour space when the file is to be printed by lithography for a book or magazine for examplegrayscale is just normal black and whiteindex colour is a few selected colours often used for graphics rather than photoshalftone is just black and white - no grey

Colour temperature
On digital cameras, electronic compensation for the colour temperature associated with a given set of lighting conditions, ensuring that white light is registered as such on the imaging chip and therefore that the colours in the frame will appear natural.

Compact Flash (CF) memory card
A popular type of memory card. Used in digital photography to store images. The amount of memory per card varies. Popular memory capacities range from 256mb to 32GB. The larger capacity cards are really of use to professionals only. CF cards are the largest popular format memory card. These days they are used mostly in larger digital cameras.

Cross Processing
Cross processing is an old film term. It refers to the practice of deliberately developing film in chemicals intended for a different type of film. For example, developing a slide film using print film chemicals. The result is unnatural colours and contrasts. In digital photography the effect can be achieved (and is easier to control) using software.

Depth of field
Depth of field is a measure of how much of a scene (from the front to the back of the image) will be in focus.

Differential focus
Focus on an object while blurring background and/or foreground.

Diffuser
A piece of opaque plastic that sits over the top of a flashgun. Its job is to soften the light from the flashgun, and therefore reduces harsh shadows caused by a 'bare' flash.

DPI - dots per inch
The number of pixels in an image for one row an inch long.

DSLR
Stands for Digital Single Lens Reflex. This is the digital equivalent of a SLR camera. The photograph is recorded on a digital image sensor and saved onto a memory card, not film

File format
In film photography photos are recorded onto light sensitive film. This is developed with special chemicals and a negative is produced. In digital photography photos are stored as a digital file. You need to decode the file in order to view it (see this section of the history of digital photography for more). There are different types of file formats used in digital photography. Some are lossy and some are lossless (see separate entry). The most common lossy file format used in digital photography is JPEG. The most common lossless file format used is TIFF. Increasingly becoming common are lossless RAW files.

Film Grain
Film grain is not really digital photography terminology - the digital photography equivalent is noise. Film grain is the small speckles that show up on photographs. Film contains tiny granules of light sensitive material. The more sensitive the film is, the larger these granules are. When a film negative (especially from sensitive film) is enlarged onto photographic paper (in order to make a print), the tiny grains get enlarged too, and become visible. The effect of film grain is often considered to be artistic, and it is sometimes put back into digital images (using software) for artistic purposes.

Focal length
The distance between the rear nodal point of a lens and the point at which the image is focused, when the lens is focused at infinity. At one particular distance - its focal length - the image will be rendered sharply, but at others it will not. Lenses are sorted into categories depending on their focal length in relation to the format with which the are used. A 50mm lens on a full-frame body is classed as 'standard', though on an APS-C body it strays into a telephoto. Meanwhile, on a large format camera it will fall under the term wideangle. Strictly speaking, a 'standard' lens for a particular format is one whose focal length is roughly equivalent to the diagonal of the imaging element.

Format - Camera
The camera format is based on the size of the digital image sensor. Most of the time the format is determined by how it relates to traditional 35mm size. Larger than 35mm and you have medium or larger format cameras and smaller than 35mm and you have a crop camera.
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Format - Output
Images files can be saved in a variety of formats and sizes depending on use:
small low resolution for web and screen use such as highly compressed jpg or png photos at 72 or 96 dpi
high resolution for print and display use such as tiff, jpg and png usually at 300dpi
These can be prepared in various colour spaces depending on what they are to be used for - rgb, cmyk or grayscale are the more common

Foveon Sensor
A type of sensor where the colour recording layers are stacked on top of each other. This means that every "pixel" can record the level of red, green and blue light hitting it. This differs from the bayer sensor, where each pixel can record only one colour of light.

High resolution
An image file that is primarily for printing and is usually large and stored at 300dpi but can be anything above this.

Hot Shoe
A hot shoe is the special slot on the top of a camera that can accept camera accessories, usually a flashgun. They are uncommon on compact digital cameras and are usually found on digital SLRs (DSLRs).

Image editing software
Image editing software is used to manipulate a photo once it has been taken. Digital cameras store images as files on memory cards. Once downloaded to a computer these files can be edited using image editing software. Editing can be used to improve a photo (for example, improving the contrast and then sharpening the image), or can be more extreme (for example, cutting a person out of one photo and pasting them into a different photo). Most digital photos can be improved by a little editing.

ISO
In film photography the ISO was a measure of how sensitive film was to light. It was called film speed. The higher the number, the more sensitive it was. In digital photography the number (still called "ISO") refers to how sensitive the image sensor is to light. By making the sensor more sensitive to light, photos can be shot with higher shutter speeds and/or in lower light. The downside of making the image sensor more sensitive is that it introduces noise into the picture. Noise is the digital equivalent of film grain.

JPEG
JPEG is a type of lossy file format. Stands for Joint Photographic Experts Group. It is the most common file format used in digital photography. When the image is saved, the camera strips out data (parts of the photo) This is called compressing the file. It keeps the size of the file down so that more images can be stored on the camera's memory card. The level of compression is set by the photographer and will affect image quality. Less compression = less data thrown away = larger file size. The danger is that if a file is compressed too much (i.e. lots of data is thrown away), the quality of the image suffers. The photos look blocky and pixelated. On digital cameras the highest quality setting (usually called Best or Fine) results in less data being lost, and a good quality image is produced. The lowest settings (usually called Low or Economy) results in more data being lost, and a poor quality image is produced.

Lenses
The job of the lens is to throw a circle of focused light onto the image sensor. In the days of film cameras that meant the light fell onto the film. With digital photography the light hits the image sensor. Image sensors are usually smaller than a 35mm frame of film. This means the circle of focused light can be smaller too. Specific "digital only" lenses will throw a circle of focused light that will cover a digital camera's sensor, but is too small to cover a 35mm frame. They are therefore only suitable for use on digital cameras.

Lossy/Lossless files
When a digital camera takes a photo, the image data is stored on a memory card as a computer file. If the data is stored fully, the file is called a lossless file. These files are quite large in size. The most common type of lossless file in use are TIFFs. To cut down on large file sizes, the camera can throw away parts of the data. These files are called lossy. The most common type of lossy file are JPEGs. Caution should be taken when using JPEGs if image quality is important.

Low resolution
An image file that is generally small in size and is for screen or web use. Stored at either 72dpi or 96 dpi.

Macro
"Macro" means the ability for a camera (or lens) to focus so near to the subject that it is captured life size on the image sensor. The term macro is used more loosely these days, and usually refers to the ability to get close up shots (not necessarily life size). Typical macro subjects are flowers and insects.

Memory Card
Traditional cameras record photos onto film, digital cameras save photos onto a memory card. Memory cards come in different shapes and sizes. You need to use the correct memory card for your digital camera. The most popular formats are CF, Memory Stick, SD and xD. CF and SD are the most widely used.

Memory Stick memory card
A memory card designed by Sony, and used almost exclusively in Sony digital cameras. There are different types of Memory Sticks - Standard, Pro, Duo, Duo-Pro, and Magic Gate.

Metadata
When a digital camera captures an image it records two things in the file that is saved - image data itself, and data about that data: the "metadata". The information it contains varies from camera to camera, but typically includes, amongst other things, information such as date, time, shutter speed, aperture, ISO and file type. This information can be useful say when trying to find photos later (because you can search for your images by date), or adding a copyright notice.

Noise
Noise is the digital equivalent of film grain. It shows up on digital photographs as small coloured blotches, usually in the darker areas of an image.

Photography
The word photography is based on the Greek φῶς (photos) and γραφή
(graphé ) "representation by means of lines" or "drawing", together meaning "drawing with light

Pixel
A small dot in a digital photography image.Millions of pixels make up a digital photograph.

Post Processing
In the days of film photography this referred to work done in the darkroom. Not simply developing the photos, but relating more to correcting colour casts, lifting dark areas of a scene, adjusting the contrast. In digital photography terminology however, post processing means adjusting your digital photos using image editing software - Photoshop, for example. The same adjustments that were made in film can be reproduced with software.

Prime Lens
Prime lenses have one fixed focal length. They represent the best quality of lens available, but have the big drawback in that they cannot 'zoom in' to a subject.

Purple Fringing
Purple fringing is an undesirable purple "rim" that surrounds areas of high contrast in a digital image.

RAW
RAW files are the actual data taken directly from a digital camera's image sensor. They have not been processed by the camera at all. This means they are the purest image file possible. Often, quite correctly, the terminology digital negative is used to describe them. Different camera manufacturers have different names for their own RAW files. For example, Canon uses the digital photography terminology RAW, Nikon uses different digital photography terminology - NEF. They are incompatible with each other. Whereas every computer can show JPEG images, RAW files need specific software. This is always supplied by the manufacturer when you purchase a digital camera.

Resolution
Resolution is a measure of the number of pixels there are on a sensor. The resolution of digital cameras is measured in megapixels (millions of pixels). Multiply the number of pixels along the bottom of the sensor by those up the side. For example, a sensor with 3000 pixels along the bottom, and 2000 up the side equals 6,000,000 pixels in total. That would make it a 6 megapixel camera.

Secure Digital (SD) memory card
A popular type of memory card. Used in digital photography to store images. The amount of memory per card varies. Popular memory capacities range from 128mb to 32GB. Some SD cards have a clever feature where part of the card can flip up; this allows the card to be inserted into a USB port on a computer: images can be transferred to the computer without the need for a card reader or any special cables.

Sensor
The electronic chip that records the image in a digital camera. They come in two main types. CCD (Charge-Coupled Device) and CMOS (Complementary Metal Oxide Semiconductor) are the most common. CCDs are used almost exclusively in compact cameras, CMOS sensors are used in larger, and more expensive DSLRs. There are other types, but not well used. JFET (Junction Field Effect Transistor): developed by Nikon and used in a very small number of their cameras. X3: a new type of sensor developed by Foveon. Used only in Sigma DSLRs and a couple of Polaroid compact cameras.

Shutter Delay
Shutter delay refers to the time between pressing the shutter button to the camera actually taking the picture. It is not a term used in traditional photography because the delay was negligible.To overcome the problem of shutter delay most digital cameras allow you to pre-focus by pressing the shutter button halfway, and once the focus has been locked the shutter can be pressed fully. The problem is less apparent with digital SLRs.

Shutter Speed
Shutter speed is slightly different to traditional photography. Traditionally, the shutter opened when you pressed the camera's button, exposed the film behind the shutter, and then closed again; covering the film back over in the process. With electric sensors the "shutter" is controlled by the camera's computer. A short shutter speed means the sensor is charged for a fraction of a second. For longer shutter speeds, the sensor is kept charged for longer. The shutter noise in most compact digital cameras is created by a small speaker in the camera and mimics a real shutter. It has its use - it lets you know when the photo has been taken. Digital SLRs have no need to mimic a shutter as they work with a combination of shutter and electrically controlled sensor.

SLR
Stands for Single Lens Reflex. A design of camera where the photographer looks straight through the lens prior to taking a photograph. The view through the viewfinder of a SLR camera is therefore the same image that will be recorded by the camera.

Stereoscopic photography
A way of producing 3D images from 2D photography. Two photos are taken at the same time, by two separate cameras that are set a short distance apart. The resulting images are then swapped (i.e. the photo taken by the left-hand camera is placed to the right-hand side of the other image) before viewing. Viewing takes practice and involves crossing the eyes and merging the two photos.

TIFF
In digital photography terminology a TIFF is a type of lossless file format. Stands for Tagged Image File Format. Because they are lossless files the image data is stored whole (nothing is thrown away). This results in large file sizes, but also good quality photographs. TIFFs can be compressed to save space, but not as much as JPEGs. When they are un-compressed the software reverses the original compression process; the original large file is the result.

Vignette
A vignette is when the edges of an image are either darkened, or lightened, in relation to the rest of the photo. It has the effect of drawing the viewer "into" the photo. The vignette can be caused accidentally by obstructions (for example, the lens hood) creeping into the frame at the edges. Or, as is more common, a vignette is applied to an image after the photo has been taken using software.

xD Extreme Digital memory card
A small memory card (smaller than SD cards) introduced by Fuji and Olympus. They are designed specifically for digital photography (other memory cards were designed simply as data storage devices). They are not as widely used as either CF or SD cards.

Dick Makin Imaging

A glossary of useful photo terms