Glossary of Terms
Digital photography is full of jargon and strange “three- and four-letter words”. Here is an explanation for some of the technical terms used in articles on this part of the site
JPEG is a standard file format invented by the Joint Photographic Experts group. It is a file format that was designed to save computer disk space by throwing away ‘redundant’ image data and approximating colour and tone information in small blocks. Because they may be compressed, JPEG files are useful for images that are going to be loaded up on the web where small file size gives a speed advantage and where they will be viewed only on computer screens which have relatively low resolution. On a computer JPEG files are followed by .jpg, for example: _MG_5678.jpg
Tagged Image File Format is a standard format for image files that allows for lossless compression. Maximum quality TIFF files may be edited and saved without losing image quality. Because they are lossless, TIFF files are good to use when you want to maintain maximum quality for printing. On a computer TIFF files are followed by .tif, for example: _MG_5678.tif
The data actually generated by a digital camera’s sensor. Camera makers all use different sensors and different electronics so no two cameras raw files are in the same format. So that images can be manipulated by standard software and printed raw files need to be translated by software into a standard file format before anything can be done with them. Most digital cameras have an in-built computer that converts their raw image files into JPEG files in an RGB colour space: usually either sRGB or Adobe RGB.
Some digital cameras allow the user to save the raw files and convert them to a standard format using a software program (raw converter or raw processor) on their desktop or laptop computer. A good raw processor can translate raw files directly into the file format required by a printer without needing to translate the file into JPEG or TIFF. On a computer RAW files will be followed by a suffix that depends on the make and model of camera that made it: some examples are : _MG_5678.NEF (Nikon), _MG_5678.CR2 (Canon), _MG_5678.ORF (Olympus), _MG_5678.PEF (Pentax), _MG_5678.SR2 (Sony), _MG_5678.RW2 (Panasonic/Lumix)
Megapixels, Megabytes, and print size.
Most fine art prints that are made to be viewed close up, are printed using at least 300 inkjet dots per inch (300 dpi). You can’t see the individual dots with the naked eye. (By comparison, computer screens typically display images at between 72-120 dots per inch). If you want to make a 15” X 10” print at 300 dpi your image will be printed with 13,500,000 individual ink dots, 4500 dots wide X 3000 dots high. The best possible quality print will match each dot in your print exactly, with a corresponding pixel in your image file. In the case of the 15” X 10” above, your image file would have to have been 4500 pixels wide X 3000 pixels high. It would have had a total of 4500 x 3000 = 13,500,000 individual pixels which translates to 13.5 MB of data. Any image data file that is smaller than 13.5 MB will need to be scaled up (upsized) to make a 15 X 10 print. Most people’s images will be in this situation so image quality is really important.
The amount of colour (or the number of different colours) that a device can record, or retain, or display. A digital camera has a very large colour gamut: it can capture most of the colours that the human eye can distinguish. An ordinary computer monitor has a very limited colour gamut and is incapable of displaying deeply saturated colours that the human eye can see. An issue that makes managing colour in digital photography interesting, is that an ordinary monitor is incapable of displaying highly saturated blues, cyans, and greens, that a good colour printer can print.
Photographers know that the 'colour temperature of light' changes during the day affecting white balance in photographs. In the early morning and late afternoon when the sun is low, the light has a warm feeling and has a slightly orange-yellow feel. It has a colour temperature of around 5500oK At midday, when the sun is high the light is more blue and has a colour temperature around 6500oK. Monitors are usually adjsted so that their white point mimics midday daylight at a setting called D65.
The gamma of a monitor refers to the shape of its tonal response curve. The output brightness of an uncorrected monitor is not linear — if you double the value of the input signal (voltage) you don't get double the brightness out. However the way we try to represent double the brightness in an image file is to double the colour values. The 'gamma correction' is applied to the input voltage so that when the colour values double the percieved brightness on the monitor doubles. The best setting for photography is gamma= 2.2