Knutsford Photographic Society
Knutsford Photographic Society This article is aimed at assisting anyone considering displaying images on a web site. This can be confusing and technical so here we have tried to simply the various advice available. It has been written by the KPS drawing on various reference material and plenty of personal experience from the club’s webmaster!
Executive Summary – for most web publishing purposes, jpeg is the recommended format.
| Introduction | Technical Stuff to Remember |
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The formats : |
There are a number of key points to bear in mind when considering posting images on a web site :
v The higher the resolution at which the image is saved, the better the quality/realism of the image
v The higher the resolution/colour depth of the image, the more file space it takes up ... and vice versa
v The higher the resolution of the image/colour depth, the longer it takes to download and thus the longer it takes to display on a surfers screen...and vice versa
v If the image is designed to display with dimensions that are too big the image will not fit onto the surfer’s screen without them having to scroll down/to the right
v Conversely, if the image is designed to display too small then its impact will be lost and it may be difficult to see properly
To summarise : the art is to compromise between image size/resolution and the desired result; choosing the correct file format for your image is an important contributor in this process.
The saving grace is that, for an image to be displayed on a computer screen, there is quite a good resolution tolerance, more so than for an image which is designed to be printed. For example, for an image to be printed with any degree of quality, it would generally have to be set at a resolution of at least 300 dpi (dots per inch). For display on the Internet, one can get away a resolution of 90 dpi, sometimes even 76 dpi. Return to top Return to Contents
v The visible area for displaying information/images (if they are to fill the available display area in a browser screen) should take up a maximum area of 800x450 pixels
v Many digital cameras operate at a resolution of 1280x1024 (data ref. 2001)
v “8 bit” colour depth = 256 colours
v “16 bit” colour depth = 65,536 colours
v “24 bit” colour depth = 16.7 million colours
v The 4 major image formats to consider are :
o .bmp .jpg (jpeg) .tiff .gif
o As you can see the different image formats are usually denoted by their file name extension e.g. myphoto.jpg
o Fundamentally the different formats for storing images compress (to varying levels) images in different ways, to try to bring optimum quality with optimum/minimum file size
v compressing files reduces quality as well as disk space required and also download times
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This is the original computer format for storing all pictures/images built into Windows. It caters for all sorts of pictures (including photos and clipart) and treats them all the same regardless of type. Thus could be considered a jack of all trades and master of none!
Although there will be no loss of quality with a bitmap, file sizes for photographs will be huge because there is no compression involved.
Summary - Not recommended for storing photographs for any purpose, especially for publication on the web!
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Given the existing option of using bitmaps (not suitable for internet use because of the big file sizes), CompuServe (one of the original internet service providers (ISP)) developed the gif format back in the mid 1980s. The gif format has powerful compression properties to reduce the size of images.
It lends itself well to the storing and display of clipart as it allows background colours (often white) to become transparent, for better blending/presentation on a website.....not a quality usually required for photographs!
Its powerful compression also allows clipart to be compressed without real loss of quality because clip art generally only uses comparatively few different colours (again unlike photographs, with their wide scale of colours/shades, even in monochrome (aka grey-scale in computing speak)). For clipart, gifs also offer the option to interlace (i.e. load the overall image bit by bit within the image) and the animation of clipart images (again not a requirement for photographs).
Summary – idea for clipart but too much loss of quality for most photographic work.
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The Tagged Image File Format is designed for use with photographs as it compresses without loosing detail by just disregarding unneeded colour information. It is therefore the default format used for professional photographic publication. Pick up any magazine these days and it will probably have been produced via “desktop publishing”. All the photographs will have been held and output as tiffs. Aldus (now Adobe) take the credit for the invention of the tiff format (given the short falls of bitmaps and gifs for professional photographic work). Its other advantage (unlike bitmaps) is that tiffs can be easily transferred between Windows PCs and Apple Macintosh PCs (the latter being predominant in the professional publishing world).
So, what does this mean for we amateur photographers who want to publish photographs on the web? Well, as a professional format tiff does tend to have ‘professional’ overheads. The main disadvantage is that tiffs are still very large files and thus require a lot of storage space (both locally on your PC and on the web server that you use) e.g. one tiff can easily require more than 50MB of disk space, which is as much as/more than most individuals receive as their total web space allowance from their Internet Service Provider. You also will need a very powerful PC to use Tiffs.
Summary – use for professional photographic work for printing; do not use for web sites.
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So, is there any hope for those of us who want to publish photos on the web? Well, the answer is ‘yes’ (fairly obvious really as so many people do put photos web sites!). Here’s the answer.....
The Joint Photographic Experts Group’s format, as the name suggested, is a format dedicated to photographic images. It does compress images, and can loose quality (but variably at a level of loss that you can control) but not to the extent that will be detrimental to the quality of images that you will be posting for the purposes of Internet distribution. Yet, it compresses to a level that means that you get quality images with an acceptable and relatively small file size.
You will still have to experiment in the art of choosing the level of quality versus the file size/download speed, but you are in control and can easily vary these. There will always have to be a trade-off. When saving the file as jpg, most software will ask you to declare the quality level that you want to retain (which it may refer to something called lossy level).
One important tip – always save your original photograph at the highest level of quality/least compression and then experiment with saving a copy of that original at various levels of quality until you reach the desired compression between quality and file size (and thus download time). Never save a photograph at a low level of quality/resolution and then try to resave it at a higher resolution – this will adversely affect the quality of the image. Always keep working from copies of your original, which you have saved at the highest quality level (after which the only way is down!). Generally for web display you can take the quality level of a photograph saved as a jpg down considerably!
Summary - jpg – for web images – use me!!
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The photographs that you want to put up on the internet may be derived from a number of sources including : a print which has been scanned using a desktop scanner, a slide which has been scanned in a slide scanner, an image which someone has e-mailed to you, a third party commercial image (e.g. sourced from the cover CD/DVD of a magazine), directly from a digital camera, from a disk supplied by a high street/mail order photo processor as part of the processing of a film, produced by A.Nother on their website which you have downloaded and wish to reuse for your own purposes (beware of copyright if you do this!) .... and so on
Whatever the source, you will end up with the photograph as a singe file, most probably with a filename extension that is one of the 4 formats mentioned above. You then need to use a photographic software package through which you can edit the image. These can range from the simple and inexpensive (often supplied with your computer or scanner/digital camera) through to the expensive/professional packages. At the time of writing the most high-end package probably used amongst the amateur is Adobe PhotoShop, but just to change a photograph from one format to another a low-end package will easily suffice – you don’t need all the photographic manipulation facilities that PhotoShop brings to convert the file format.
In most cases, changing the format is as easy as using the “Save As” option in the package you are using. To get an image into a gif (not recommended above) you may have to use an export option. It is really that simple. Remember, the difficult part is balancing your file size with the minimum quality level you can get away with on the web. Good luck!
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