Digital technology is becoming increasingly important in all areas of photography - from taking pictures, through image retouching and processing, to printing. However, although the equipment and software are rapidly gaining ground, understanding of the fundamentals is sometimes sadly lacking; this article attempts to fill in some of the gaps.
Image Size - Display Resolution - The Magic 72 dpi ? - Print Resolution - Extrapolation - The Right Resolution - File sizes - File and Compression Formats
The size of an image is determined by the number of pixels it consists of vertically and horizontally, pixels being the smallest components of an image - those little squares you see when you enlarge a picture on the screen. We might say, for example, that an image measures 450 x 300 pixels as a way of describing its size in absolute terms, as opposed to the amount of space it will occupy on the screen.
The amount of space an image will occupy on the screen depends on the resolution of your monitor (see below).
For example, an image measuring 750 x 550 pixels displayed at 100% will almost fill a screen with a resolution of 800 x 600 pixels, but will take up much less space on a 1024 x 768 monitor. On the other hand, it would not even fit on a 640 x 480 screen.
There is a limit to the number of pixels graphics cards and computer monitors can display across their width and height, depending on the number of colours (or shades of grey) you opt for. The most common resolutions are : 800x600 – 1024x768 – 1280x1024 pixels. You should bear in mind, however, that most of today's internet users browse the web at a resolution of 1024 x 768 so, taking account of the fixtures (menu bars etc) that take up space in all browsers, images destined for the web should be around 900 x 650 if you want your audience to be able to view complete pictures without scrolling.
The Magic 72 dpi ?
It's (all too) often said that images intended for the web should have a resolution of 72 dpi. This is quite simply wrong. The figure comes from the first Macintosh screens, which did indeed have a fixed resolution of that order, but it makes absolutely no sense today. The dpi unit of measurement (see below) is relevant only to printing.
Don't believe me? Well, a picture is worth a thousand words, and here are three identical ones measuring 250 x 390 pixels. The first is 10 dpi, the second 72 dpi and the third 300 dpi. Can you see a difference on the screen?
Note in passing that the file size is exactly the same in each case: 288 k (39 k after JPEG compression - see section on compression), which goes to show that dots per inch have no part to play. Under each image you will see a note of what size it will be when it's printed - and it's only in printing that the difference will be noticeable.
If you have a high-speed connection (ADSL or cable) and you're still sceptical take a look at another example I've created specially for you. But beware, this page is huge because it contains extracts from the images above at much larger sizes and much lower levels of compression.
If you think you've got what it takes, click here. The page will open in a new window, which you just have to close to get back to where you were.
Another measure, the number of dots per unit of surface area, is used to describe the resolution of the printed image. This could be expressed in millimetres but normally it's in inches (25.4 mm). We therefore talk about dpi, or dots per inch. Dot density is very important at the printing stage because it's this that determines the quality of the final image.
Take an image measuring 2240 x 1680 pixels (what you would get from a 4MP digital camera); if you print it at 300 dpi, you will get a picture 7.5 inches (19 cm) wide by 5.6 inches (14.2 cm) high; if you print it at 100 dpi, it will come out at 22.4 inches (57 cm) by 16.8 inches (42.7 cm). As the number of individual dots, or pixels, will not have increased, it is clear that the quality of the resulting image will be considerably lower because the size of each pixel will have to be artificially increased to fill the space separating it from its neighbours.
'Rule of thumb'
People often ask what size prints they can expect to get from such and such a digital camera or such and such a file. As a rough guide, if you take dimensions of the native file in pixels and divide by 100 the result will give you the size of the prints you can expect to get without loss of quality. Example: 2000 x 3000 = 20 x 30 cm [See 'The Right Resolution' below].
Some applications will let you 'resample' an image so that you can produce quite large prints without the unwanted side-effects of enlargement. All this means is that the software analyses the composition of the existing pixels (colour, contrast) and tries to find matching pixels to fill the empty spaces left by increasing the surface area. This can be quite successful if there aren't too many pixels to extrapolate, particularly if the image consists of large areas of uniform tone. However, it nevertheless creates pixels artificially and I don't think anyone has yet come up with a perfect way of generating pixels that are missing from the original image.
The Right Resolution
In absolute terms, there is no such thing as the right resolution for an image. It all depends on what you intend to do with it. There is no point in having a large, print-quality image if all you're going to do is put in on the web, especially since the size of the file would be a huge drawback [see below under: 'File Sizes'].
As we saw in the section on display resolution, a 600x400 image will fill most screens without users having to scroll to see it.
The problem of the right resolution for printing is slightly more complex because other factors enter into the equation, in particular the type and quality of the printer, the printer drivers, the paper, the inks etc. Without going into detail, a print resolution of 300 dpi is considered best (even if your printer can manage 1200 dpi or more, because these aren't the same dots per inch), but it is usually possible to get excellent results from a personal inkjet printer with a resolution of 240 dpi. However, you will still need to do a series of comparative tests to work out what the optimum resolution is for your equipment and materials. Ideally, the printed image should be the same size as the original image, without any artificial enlargement that relies on extrapolation. A 2240 x 1493 photograph will print out at 19 cm x 12,6 cm at 300 dpi, 22 cm x 14.5 cm at 260 dpi and 22.7 cm x 15 cm at 250 dpi; in all three cases the file size will be 9.5 MB. To get close to the size obtained with film, the image would have to be 2700 x 1800 pixels, yielding prints of 22.8 cm x 15.2 cm at 300 dpi, 26.4 cm x 17.6 cm at 260 dpi and 27.4 cm x 18.3 cm at 250 dpi. The file size would then be 14 MB.
To make a print measuring 20 x 30 cm at 300 dpi, the dimensions of the image you start with would have to be 2362 x 3543 pixels and the resulting file size would be 24 MB. A 24 x 36 cm print would correspond to a file size of 36 MB, while a 30 x 45 cm print would weigh in at 53 MB. As we saw above, the size of the file can be reduced slightly if you opt for a print resolution of 250 dpi or a little less, but only a test run with the same image will allow you to determine the lowest resolution at which your printer will give good results. Clearly, files of this size cannot readily be transmitted over the internet or transferred from one computer to another, except by using high-capacity storage media (for example, CD-ROM, compact flashcards, or ZIP disks). They also take a long time to open, display and 'process' unless you have a computer with a good graphics card (128 MB RAM); a fast high capacity hard disk will also make things easier. On the other hand the performance and speed of the processor make little difference unless you're using complex effects filters such as those found in image-processing programs like PaintShop Pro and PhotoShop. Finally, the time taken to print an image will increase considerably with the size of the file.
File and Compression Formats
Image-processing software, scanners and a few cameras allow you to choose different file formats. The main ones are these :
To solve the problem of file size, image-processing software comes with compression features, some of which allow you to reduce file sizes dramatically. This is particularly true of the JPEG format, which can make an image file up to 50 times smaller than the original. However, you will have to choose the best (i.e. the least damaging) compromise between quality and file size because the image will be subject to varying degrees of degradation depending on the level of compression, and there is no way of going back.
If you have a high speed connection, click here to see the effects of different levels of JPEG compression.
Finally, it is important not to keep saving the same image in lossy format: the cumulative effects of repeated compression are particularly devastating; the page mentioned above will give you some idea of what I mean.
Although its compression ratios cannot compete with the those of JPEG, the TIFF format allows LZW compression, which does not degrade the image. This is the format of choice, with or without compression, for storing images and preserving their original quality.
* A word of advice: if you use a digital camera, there's a good chance that it will save images in JPEG format without giving you the opportunity to use TIFF or choose the level of compression. In cases like this, as soon as you have transferred the images to your computer, you should open them and save them as TIFF files (with or without compression) before doing anything else; this will enable you to manipulate, crop or otherwise change the image later without running the risk of the original picture being ruined as a result of successive compression operations. When you've finished working on the picture you can always save it in JPEG format again if you intend to publish it on the internet. But make sure you save the TIFF file. You never know when it might come in useful!
English translation : John Loydall
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