|
An outline of the most important file formats for digital photographers and how best to use them.
When you save a digital image, you normally have to choose what format you want to save it in, regardless of whether you’re saving the shot in the camera’s memory card or to a computer. Most digital cameras provide between one and three file format options plus a range of ‘quality’ settings. And while all cameras allow you to save images in a compressed file format (which reduces the file size), top quality digital cameras also offer an uncompressed file format that delivers the best possible quality.
To get the best results from your digital photography, you should know the differences between the various file formats and when each is best used. You should also be aware that a digital camera may store your pictures in a different file format from the default format used for saving images after you’ve edited them with your preferred software package. This isn’t a problem as all software applications provide a selection of options whenever you save an image (although you may need to us the ‘save as’ or ‘save a copy’ menu to access them.
In this feature we’ll look at the most commonly-used file formats and explain when and how they are best used.
Capture Formats
1. JPEG
For image capture, the JPEG standard (denoted by the *.jpg extension) is the most widely used file format. Invented by the Joint Photographic Experts Group, it can be found in everything from the simplest point-and-shoot model to the most sophisticated SLR. Most compact-style digital cameras have JPEG as their default capture format; some allow you to select between JPEG and other file formats.
JPEG is a ‘lossy’ compression format. When you save an image as a JPEG file, a certain amount of picture information is irretrievably discarded. So if you decide to compress an image file to reduce its size, make sure you do so with a copy of the image; not the original.
The best features of JPEG are:
* It stores images with 24-bit-per-pixel colour, which maintains comparatively high colour fidelity;
* You have a great deal of control over how much your image is compressed and, hence, how much picture information is sacrificed.
The worst feature is that JPEG compression is a one-way street. If you try to recreate a large file from a small JPEG file, the results will be virtually unusable. This is one of the reasons files compressed for emailing should only be printed at 10 x 15 cm size or, preferably, smaller.
Because JPEG is so commonly used in digital photography, it’s important to understand how it works. The compression algorithms concentrate on the brightness information to which the human eye is most sensitive and break the image data into blocks of 8 x 8 pixels for processing. Any redundant information is discarded during this process.
When an image is JPEG-compressed all the areas that have the same (or very similar) colour and intensity are saved as a single set of values covering the location, colour and intensity of that set of pixels. This can have interesting consequences: an image saved at one particular setting can differ in size and quality from another image saved at the same setting. For example, shots with large areas of blue sky will be saved as much smaller files than shots containing lots of detail in the form of small areas of differing hues and tones. If you try to reduce the detailed picture to the same file size as the JPEG-compressed simple image, quality will inevitably be sacrificed.
The more you compress JPEG files, the cruder its selection settings become. At the maximum quality setting of 12, very little compression is applied, image files remain quite large and there is little noticeable quality difference between the JPEG file and a file saved in an uncompressed format. In contrast, at the minimum ‘1’ setting, files are made substantially smaller in size (this setting is really only appropriate when you want to compress large files for Internet applications).
If your camera only saves images in JPEG format, it’s best to use the highest quality setting when capturing images. If you’re working with an image in a software application, copy the file and archive the original before you apply any JPEG compression to the copy.
Always check the effect of JPEG compression you apply before you save the file. Magnify a detailed section of the image to 200% and look for compression artefacts, which show up as a blockyness in the image. If they’re very noticeable, the image has probably been compressed too much to be useful for anything but in emails. Note: some software applications will provide ‘before’ and ‘after’ views of the effect of the compression as standard, when you elect to save an image using the ‘save as’ option or when you convert an image to JPEG format from another type of file.
At the beginning of 2001, a new – and significantly improved – version of JPEG was finalised and made available to manufacturers. However, so far very few of them have taken advantage of the benefits JPEG 2000 offers and very few digital cameras have yet been JPEG 2000 enabled. But an increasing number of software developers are making the latest versions of their applications JPEG 2000 compatible so, with luck, we should see more of the new version in the future.
2. TIFF
Another popular capture format used in digital photography is TIFF (which stands for Tagged Image File Format). TIFF files can be identified by the *.tif extension and are generally uncompressed (a lossless TIFF compression format exists, but as it’s been patented by Unisys, it’s not in common usage). This means they retain all the information that was captured when the shot was taken – including any processing that was applied by the camera.
Saving images in TIFF format can be a useful strategy when you want to capture images at the highest resolution. However, if you’ve adjusted any of the camera’s capture parameters (saturation, contrast, sharpness, etc), the affects of these adjustments will also be saved in the TIFF file. TIFF files also tend to be very large. For example a TIFF file from a 3-megapixel camera can be around 9 MB in size, which is generally too much for the 8 MB memory card that is normally supplied with such units.
TIFF files can be used by most software applications and it is usually easy to convert images captured in TIFF format into JPEG files. TIFF files are also directly interchangeable between Windows and Mac PCs.
3. RAW
All digital SLRs and an increasing number of high-resolution compact style models offer a RAW capture option. Essentially, RAW files contain only the data that comes directly from the camera’s image sensor; no post-processing is applied (although this is not always the case; the TIFF format is, in fact, a proprietary Kodak RAW image format).
RAW files can be compressed or uncompressed. In the case of the former, a ‘lossless’ compression system is used that allows all the information contained in the captured image to be restored. Camera manufacturers have invested a great deal of effort in researching algorithms that enable data to be compressed and reconstructed so compressed RAW files are generally as good as uncompressed RAW files from a practical viewpoint.
Most photographers view RAW files as the equivalent of ‘digital negatives’; in other words they’re seen as the ‘purest’ form of digital image capture. The RAW format is also popular for image archiving as it allows photographers to go back to an unmanipulated version of the image whenever they wish.
One problem with using RAW files is that each manufacturer has its own system for saving files in RAW format, which means different manufacturers’ RAW files tend to be incompatible. You also need special software to translate the manufacturer’s RAW files into something you can use – which will generally be JPEG or TIFF format or the native format in your image editing software (see below).
However, capturing images as RAW files can be a good strategy for a number of reasons. For starters, because no post-capture processing is applied, RAW files usually download quickly (faster than JPEG or TIFF files of a similar size). This means you can use the maximum burst speed of your camera to capture high-resolution shots in quick succession.
RAW files are also ‘cleaner’ to work with than files captured in other formats because they haven’t been sharpened or had their contrast or saturation adjusted. This is an attractive feature for those wanting to edit their images in one of the more powerful software applications (such as Adobe Photoshop, Corel Picture Publisher, JASC Paintshop Pro or Ulead PhotoImpact).
For Editing
Digital photographers who open images in a software application will often find that when they click on ‘save’ the image will be saved in a different file format from the one it was captured in. (You can do this deliberately by selecting the ‘save as’ option.) Sometimes the software will let you know this is happening; at other times it happens without your knowledge – which can be downright annoying.
In many cases, the default file format chosen will have been developed by the supplier of the image editing application you’re using and unique to that manufacturer – or even to that particular application. And occasionally, when you’ve saved the file, it can be difficult to get the image back into one of the more commonly used file formats.
The best known of the manufacturers’ file formats is the Photoshop (PSD) format, denoted by the *.psd extension. This has been developed by Adobe and is common to all of that company’s applications. Because of the wide usage of Adobe software, PSD files can also be opened in many different software packages covering a wide range of applications.
Other well-known manufacturers’ native file formats include Ulead File for Objects (*.ufo), the *.psp format used in Jasc Paint Shop Pro and *.ppf, which is used by Microsoft’s Picture Publisher.
Keen Internet users often come across two other file formats, GIF (*.gif) and Bitmap (*.bmp), both of which are mainly used for saving graphic images. The GIF (for Graphics Interchange Format) format was developed by CompuServe to make it easy to send graphical images such as logos and diagrams via the Internet.
GIF, like JPEG, compresses image data. It’s been designed primarily for producing small files that can be transmitted quickly over phone lines. Unlike JPEG and other file formats for photographic use, which will support over 16 million colour levels, GIF can only handle 256 levels of colour. It is often used to create animated clips from a series of shots and some digital cameras (notably many Sony models) include a GIF animation option for saving such clips.
Bitmap (BMP) is a rasterised format that was developed by Microsoft. It stores images as an array of individually addressable pixels, with each pixel defined as a bit, which means file sizes tend to be large. All pictures captured by digital cameras or scanners and images viewed on computer screens are bitmaps and whenever other types of images must be converted into bitmap form if they are to be printed. Unlike vector files, which are designed for quick re-scaling, bitmap images are resolution-dependent, with resolution defined as pixels per inch (ppi) for cameras and scanners or dots per inch (dpi) for printers. Because the image data is stored inefficiently, it is difficult to re-scale bitmapped images without losing picture quality. When you reduce the size of a bitmap by downsampling in a software application you have to discard pixels. When you upsample a bitmapped image, the software has to create new pixels through a process known as interpolation. Bitmaps come in several ‘flavours’, some of which compress the image data to reduce the size of the files. Common bitmap formats include JPEG, TIFF, GIF, PICT, and PCX as well as the standard *.bmp. Bitmap is exclusive to Windows PCs and files can only be read by Mac computers if special software is used.
Another file format that may be offered as a ‘save’ option is Portable Network Graphic, which is identified by the extension *.png. PNG is a patent-free replacement for GIF but, because it is newer and better, it can also substitute for some common uses of TIFF. It supports the latest indexed-colour, grayscale and truecolor images, has a more effective compression strategy than GIF and offers an optional alpha channel for transparency (which preserves overlays). It does not support multiple images and, therefore, cannot be used for animations.
Special Purpose Formats
Although never used for image capture, the Portable Document File format developed by Adobe is also frequently used by digital photographers.
Identified by the *.pdf extension, it allows both documents and images (plus documents containing images) to be transmitted electronically with high levels of security protection.
Professional photographers sometimes create image portfolios and save them as PDF files. These can be sent to potential clients, who can look at the images but can’t use them without the photographer’s sanction – even if they own a copy of Acrobat. PDF files are also commonly used by graphic designers to send page layouts and designs for advertisements and can be used by publishers for proofing publications.
One big advantage of PDF files is that everybody can open them for examination. The necessary Acrobat Reader software can be downloaded free of charge from the Adobe website (www.adobe.com). However they can only be created and edited in Adobe’s Acrobat program (which is not free) and the creator of Acrobat files can control the degree to which recipients of the files can edit them – even if those recipients have the latest version of Acrobat.
Finally, for Web applications, the HTML format reigns supreme and is the default setting whenever you use the ‘save for Web’ function in your software application.
|
