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(This entry was taken from Wikipedia and edited.)
Image file formats are a standardised means of organising and storing images. This entry is about digital image formats used to store photographic and other images.
There are two types of files – lossless and lossy
• Lossless & Lossless compression – are files that do not loose information when they are saved. Lossless compression algorithms reduce file size without losing image quality. When image quality is valued above file size, use this one. This is usually a TIFF format.
• Lossy compression – are files that loose information when they are saved. Lossy compression algorithms take advantage of the inherent limitations of the human eye and discard invisible information. There are varying quality levels that can be used, higher quality uses lower compression and therefore larger files size, or conversely lower quality uses higher compression and produces a smaller file size. At the highest compression levels, image deterioration becomes noticeable. A common format that uses lossy compression JPEG
Image file formats uses – These are all explained below
• Photographic & Print Media – Jpeg and Tiff are used for storing large photographic images.
• Web Based Media – PNG, JPEG, and GIF formats are most often used to display images on the Internet.
• Graphics file formats – EPS and PDF. These files formats are used by design professionals when producing brochures and other printed material.
File Format Explaination
JPEG (Joint Photographic Experts Group) files are (in most cases) a lossy format. Nearly every digital camera can save images in the JPEG format. When not too great, the compression does not noticeably detract from the image’s quality, but JPEG files suffer generational degradation when repeatedly edited and saved. Photographic images may be better stored in a lossless non-JPEG format if they will be re-edited repeatedly. The JPEG format also is used as the image compression algorithm in many Adobe PDF files.
The TIFF (Tagged Image File Format) is a flexible format. TIFFs are lossy and lossless, offering relatively good lossless compression, using the LZW compression algorithm. Later versions of the TIFF format use lossy compression in the form of JPEG & Zip compression algorithms. The TIFF image format is not widely supported by web browsers, but remains widely accepted as a photograph file standard in the printing business. The TIFF can handle device-specific colour spaces, such as the CMYK defined by a particular set of printing press inks.
The PNG (Portable Network Graphics) file format was created as the free, open-source successor to the GIF. The PNG file format supports 16 million colours while the GIF supports only 256 colours. The PNG file excels when the image has large, uniformly coloured areas. The lossless PNG format is best suited for editing pictures, and the lossy formats, like JPG, are best for the final distribution of photographic images, because JPG files are smaller than PNG files. All contemporary web browsers now support all common uses of the PNG format.
GIF (Graphics Interchange Format) is limited to 256 colors. This makes the GIF format suitable for storing graphics with relatively few colors such as simple diagrams, shapes, logos and cartoon style images. The GIF format supports animation and is still widely used to provide image animation effects. It also uses a lossless compression that is more effective when large areas have a single color, and ineffective for detailed images or dithered images.
The BMP file format (Windows bitmap) handles graphics files within the Microsoft Windows OS. Typically, BMP files are uncompressed, hence they are large; the advantage is their simplicity, wide acceptance, and use in Windows programs.
PDF (Portable Document Format)
The PDF is a file format created by Adobe Systems in 1993 for document exchange. PDF is used for representing two-dimensional documents in a manner independent of the application software, hardware, and operating system. Each PDF file encapsulates a complete description of a fixed-layout 2-D document (and, with Acrobat 3-D, embedded 3-D documents) that includes the text, fonts, images, and 2-D vector graphics which compose the documents.
EPS (Encapsulated PostScript)
EPS is a Vector file format that can contain images. files PostScript document with additional restrictions intended to make EPS files usable as a graphics file format. In other words, EPS files are more-or-less self-contained, reasonably predictable PostScript documents that describe an image or drawing, that can be placed within another PostScript document.
The idea is that an image colour balanced on a calibrated screen (and embedded with a ICC profile), will provide the same result on other calibrated screens and/or devices (eg. printers etc).This makes life easy, as the output result is known.
But use a screen that isn’t calibrated for colour balancing images and the result is unknown. For example, correcting images with a screen that is green to start with, will produce reddish (magenta) results. Use a screen that is too light, the images will be too dark. As there is no correct starting point, there cannot be a correct finishing point.
Most computer screens are not balanced for photographic or print output. The screens that come as standard with computers, are really only good for office style work.
There are several steps to producing colour correct imagery.
1. A quality computer screen – one that has a high contrast range and controls to make adjustments in the colour, density and contrast.
Eizo Screens are one of the better screens www.eizo.com.au. Good quality screens start at AUD $1,000.
2. A screen calibrator – This is a way of colour balancing a screen to an industry standard. We calibrate regularly to keep our screens in check. Xrite make a number of tools for calibrating screens. www.xritephoto.com.
3. A professional imaging program that supports colour profiling. While rough colour and density balancing can be done with the programs that come standard with Windows and OSX, they are really only good for the family photos.
4. Colour profiling workflow setup – This is important, if an image has an embedded ICC profile eg. sRGB or Adobe RGB, then the output result can be managed, either directly or by a third party (eg. a printer or lab). No profile and the results are less predictable.
Once the above is setup, images can then be colour balanced and managed.
Images taken from any digital camera are not colour or density correct straight out of the camera. They need to be profiled to be used in photographic, print or web media. That’s right, images for websites benefit from colour management too.
Get it right, it’s your product that’s on display.
Resolution relates to the number of pixels that go to make up the file and is measured in dots per inch (dpi). The higher the dpi the higher the image quality.
To make it even more confusing, different resolutions are required for different purposes.
Web 72 dpi RGB
Brochure 300dpi CMYK
Newspaper advertising 240dpi CMYK
Poster (inkjet or offset) 300dpi CMYK
Wall prints (photographic or inkjet) 250-300dpi RGB / CMYK
On a computer screen, all that is needed to produce a large and clear image, is a resolution of 72 dpi (screen resolution).
Say your original file is 21 cm x 30 cm at 72dpi, it looks great on the screen.
But when it goes to print in a brochure, which requires a dpi of 300 dpi, the file will only print at ¼ of the original’s size.
This is why you sometimes run into the problem of the file looking good when viewed on the computer screen (which is essentially ‘low resolution’) and looking dreadful when it is run in print at the same dimensions, where it needs to be high resolution (300 dpi).
While everyone has a camera and a computer, not everyone knows what is required to get images into print. Below are the main components that you need to know.
1. Resolution relates to the number of pixels that go to make up the image file and is measured in dots per inch (dpi). The higher the dpi, the higher the image quality.
2. ‘High Res’ generally means a resolution of 300dpi or greater, suitable for offset printing, brochures, photos and large wall prints. But this does not tell you the dimensions of the image.
3. ‘Low Res’ generally means a resolution of 72dpi, only suitable for the web and previewing. But again this does not tell you the dimensions of the image.
4. Image size refers to the actual dimensions of the image at a set resolution. eg A4 hi Res is 29.7 x 21cm @ 300dpi A4 Low res is 29.7 x 21cm @ 72dpi
5. File size is measured in kilobytes (kb) or megabytes (mb). But the file size does not tell you the dimensions or resolution of an image. (see image size)
6. File compression vs Open image file. As an example a 25mb Image file (photo) opened in your favorite image editor can be saved as the following file sized jpegs 5.8mb, 2.5mb, 1.4mb and 736kb. Size depends on the Jpeg compression used, the more compression the lesser the quality.
7. When requesting a digital image, you should know what the file is to be used for!
a) What size do you want the final image to be? eg A4, A3, wall size.
b) How is the image to be used? Will it be a photographic print, in a brochure or on a website? (This tells us the resolution and colourspace required)
c) What file format required – Jpeg, Tiff, EPS, PDF. Ask for Jpeg unless otherwise instructed.
Feel free to contact us for more information.
“Send it as a Hi Res Jpeg” …..a common request, sounds good but doesn’t mean much.
‘Hi Res’ is commonly translated into meaning a file with a resolution of at least 300dpi.
‘Jpeg’ is one of the various file formats used for images.
The statement mentions nothing about the physical dimensions of the image, nor how the file is to be used.
What could have been said is – send me an A4 300dpi RGB file saved as a Jpeg.
This implies that the image file needs to be:
1. 21 x 29.7cm in dimension,
2. has to have a resolution of 300dpi,
3. has to be supplied as RGB, so it could be used in a photograph, downsized to web or converted to CMYK for use in a brochure or newspaper,
4. lastly the file needs to be saved as a jpeg for emailing or transportation convenience.
When requesting a digital image file, know what the file is to be used for.
Then supply the following information relevant to your situation:
1. What size is the image going to be used?
2. How is the image going to be displayed – photograph, brochure or website?
(This tells us the resolution and colourspace required)
Web, wall print, newspaper, brochure?
3. The file format required – Jpeg, Tiff, EPS, PDF
Ask for Jpeg unless otherwise instructed.
If you don’t know, just ask us for advice.
‘Image size’ , File Size and ‘Resolution’ are three terms that are often and easily confused.
Image size is measured in kilobytes (kb) or megabytes (mb) and is relevant to the open uncompressed image.
• Compressed file size – using compression algorithms various file formats reduce file size of the image file. Some keep the quality of the image (Lossless) others, loose information as a compromise for file size (Lossy). See the ‘Image file formats’ article in this blog for more information.
• Uncompressed or open file size – This is the actual size that the image comprises of when open in a program like photoshop.
File size is measured in kilobytes (kb) or megabytes (mb) and is relevant to the the saved file size of an image. This size depends on what if any compression is used. See the below table showing the varying file sizes and compressions used.
Based on a A4 image open in photoshop which is 24.9mb
File format Compression File Size mb Quality Jpeg 12 5.8 mb excellent Jpeg 10 2.5 mb excellent Jpeg 8 1.4 mb good – excellent Jpeg 6 1.1 mb average Jpeg 4 736 kb poor Jpeg 2 564 kb rubbish TIFF nil 25 mb excellent TIFF LZW 13.1 mb excellent TIFF Zip 11.5 mb good – excellent
See the ‘Image file formats’ article in this blog for more information.
Resolution is measured in dots per inch (dpi).
An A4 300dpi RGB file is 24.9mb and can be reproduced in any number of dimensions relevant to the different resolutions used.
Resolution Width Height 300dpi 21cm x 30cm 150dpi 42cm x 60cm 75dpi 84cm x 120cm