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There is often confusion over the way the resolution of digital images is expressed. One of the simplest and most useful ways of expressing the resolution of a digital image is in pixels. A pixel is one of the many coloured dots that make up a digital image. As an example, you may have an image that is 1500 x 1000 pixels. Our example would have 1,500 dots in the horizontal direction and 1,000 dots in the vertical direction.

An image that has a large amount of pixels also has a higher resolution. High-resolution images will generally use up more disk space, occupy more memory and take up more space on your computer's monitor when displayed in full size. Having high-resolution images becomes necessary, however, if you want to print your image and have it look like a photo, rather than a large collection of little coloured squares!

Another very common way of referring to the resolution of digital images is by dpi, which stands for dots per inch. You may also see ppi, which stands for pixels per inch and has a nearly identical meaning. You can estimate the number of dots per inch in an image by dividing the images dimensions in pixels, by the dimensions of the image in inches. The important thing to keep in mind about dpi is that it is always relative to something.

Resolution(pixels) = dpi x image size (inches)


If someone tells you they have an image that is 300 dpi, the actual image resolution could be just about any size depending on the height and width of the original scanned image. As an example, an 8x10 print scanned into a flatbed scanner at 300 dpi, would be 2400 x 3000 pixels in size. (8 x 300=2400; 10 x 300=3000). But a 35mm slide scanned at 300 dpi would only be 405 x 270 pixels, which would not be suitable for print use unless the printed image was very small.

Converting DPI Measurements to Pixels and Vice Versa
Resolution (pixels) / Image Size (inches) = DPI
DPI x Image Size (Inches) = Resolution (pixels)


Beyond resolution, an important expression of the quality of an image is its color depth. Color depth is expressed in 'bits.' For example, most high quality image files are saved in 24-bit color. Bits are an expression of binary data and the number of bits in an image is an expression of the number of colors that are possible in each pixel of the image. In a 24-bit image, each pixel in the image has the potential of 16,777,216 different colors. (Each bit represents a doubling in quantity, so an 8-bit image would have 256 colors, a 9-bit image would have 512 colors, etc.)

From looking at the examples below, you will notice that images with lower color depths may appear to be "grainy" and the color transitions are not as smooth. Needless to say, you will want to have your display settings configured to show the maximum amount of colors available from your video card, otherwise you may not be viewing the true potential of images you see on your monitor.


So why use images with lower color depth if they don't look as good? Simple. They take up a lot less space. You will often see 8-bit images on the web, as they can be downloaded a lot faster than larger-bit images. A process called 'dithering' is often used when converting an image to a lower color depth. Dithering involves inserting a pattern of different colored pixels into an image to create the illusion of a single color. This process is designed to make color transitions appear smoother in images with low color depths. Dithering can also increase file sizes, but not as much as an image with a higher color depth will, so it's often worth the tradeoff. Dithering is also used when printing images, as most digital images contain a lot more colors than printers have colors of ink.

Image Resolution vs. Print Quality

To get an idea of the potential print quality of a digital image before you print it, you will want to estimate the number of dots per inch relative to the size at which the image will be printed. To do this, divide the number of pixels in each direction of the image by the number of inches in the print dimensions. For example, if you have a 1500x1000 pixel image and print it at 6 inches by 4 inches, it will work out to be 250 dpi when you print the image.

For most purposes, a file of this resolution will work out great. At 250 dpi, you should get 'photo quality' results. It may be worth noting that for professional publishing purposes, the standard is to have images that are a minimum of 300 dpi. You'll find you can get by with less than that and still have pleasing results for average everyday use, however.

On the low end, you probably never want to print an image at much less than 100 dpi. Below that you will start to see degradation. If you shoot for 200 dpi or higher, that should be suitable for most general applications.

It may be helpful to work backwards sometimes. This way, you can estimate how big you can print your digital image based on its pixel size. As an example, if you have a 900x600 pixel image, you could divide that by 300 dpi and figure that youíll need to print it at 3 inches by 2 inches or less to get professional quality publishing results. Similarly, if you divide it by 100 dpi, you can figure that you definitely donít want to print it any larger than 9 inches by 6 inches, otherwise the result will not be satisfactory.

Other Printing Factors to Consider

Now comes the confusing part. You may be thinking, "I just bought a new inkjet printer that prints at 1200 dpi. What good does that do me if I only need a 300 dpi image to get professional results?" This is where the color depth of your images comes into play. (See Resolution 101 for more information on color depth.) If you have a 24-bit color image, there are over 16 million shades of colors that can be used in each pixel of the image as you see it on your monitor. Unfortunately, your printer only has a handful of different ink colors.

In order for your printer to get your image onto paper, your image needs to be converted from "RGB" color to "CMYK." RGB stands for Red Green Blue. Colors you see on your monitor are projected optically, and are made up of different combinations of Red Green and Blue. But printed images are made up of four different ink colors: Cyan, Magenta, Yellow and Black (CMYK).



The conversion from RGB to CMYK is a rather complicated process. In order to simulate the millions of different colors in your original image and smooth color transitions, your printer puts many dots very close to one another in order to simulate different color shades. This process is known as 'dithering' or 'half toning.'

Thankfully, the software that comes with your printer will probably take care of everything for you. It's helpful to understand, however, that some colors possible in one format cannot be duplicated in another and therefore substitutions must be used. This is one reason you will often find that an image on your monitor almost always looks different once you print it out on paper. The software that came with your printer may provide provisions for color synchronization. This may help you to get your desired result, but for the average userís applications this is not usually necessary.

Sending/Receiving Pictures

Have you ever received or tried to  send an email that had a picture attached? Has it taken  you 2 hours to receive the picture BadCompANI.jpg (42367 bytes)  and when it  finally finished downloading,
 and you tried to view it, the computer protested?

  There are many different ways of saving a picture (called  'formats') onto your hard drive. Some of the more popular  formats are jpg, gif, pcx, tif, bmp. If you are planning on  sending a picture via email to somebody, you will need to  take special care to save the image in either a gif or jpg  format.

Why use only gif or jpg formats?

 There are two reasons.  The first and probably most important is that almost anybody  on the Internet can view these two formats. That is not true  of the others. It would be very easy to send a file in a format  that the person on the other end could not view...and it  happens frequently. By sticking with a gif or jpg, you'll be  assured that the person on the other end can see your  picture.

The second reason to use one of these two formats is size.  Generally, these two formats take less space on your hard  drive, which means it will take less time for you to send the  picture and less time for the person on the other end to  receive the picture.  So, the next time you scan some pictures to send, remember to save them as either a gif or jpg. Generally you  will use the jpg format for photographs and the gif format for  pictures that have fewer colors, like drawings or comic strips.  

You may need to consult the manual for your graphic/image  program to learn how to save images in the different formats. 

Sometimes, to convert a picture from any format to a gif or jpeg, you select that picture in its own programme, Copy it, and then open another imaging programme, like Microsoft Image Composer, Adobe, Photoshop, and others....then Paste your pic there...SAVE AS...name.jpeg...in most modern imaging software, the gif/jpeg extensions are part of the drop down menu options...Then it is simply a matter of identifying your file, when you are inserting it in your email

There are two ways of inserting pictures in your Email letter:-

  • As part of your letter...this means that wherever your cursor is, you Insert a picture...how you do this exactly depends on the browser you are using..

In Internet Explorer, for example, click Insert/Picture/Browse to find your pic.. If your Picture command is not active, make sure Format/Rich Text (HTML) is activated. If the recipient can still not see your picture, click Tools/Options/Send/HTML/Send pics with messages is selected.

2010