Image Quality


Most computer graphics terms have been imported from the printing industry and have a close correlation to their traditional meanings. Those which are not, are close enough for most people to use without any significant distortion or misunderstanding.

Resolution is the term used to describe the quality of an image, as displayed on a monitor, or a printed image. The higher the resolution the better the quality, and better looking it is.

Pixel is the term used to represent one spot of colour on a computer monitor or digital image. It is roughly equivalent to a dot in printing. Computer images use pixels per inch (ppi) or centimetre, whilst printed images use dots per inch (dpi). The more dots or pixels per inch the higher the resolution. Each pixel contains information on its size, location, bit depth, tone and colour values.

Bit Depth is the term to describe the amount of stored colour information. This affects the quality and colour of the image. Black and White line drawings are 1 bit, Black and White continuous tone photos are 8 bit (Greyscale), whilst full colour images are 24 bit or 32 bit.

RGB is the term for 24 bit colour images. These are the initials of the three colour tone mixing groups Red / Green / Blue.

CMYK is the term for 32 bit colour images. These are the initials of the four colour tone mixing groups Cyan / Red, Magenta / Green, Yellow / Blue, K / .

Gamut is the term used for the range of colours within a system.


Image Use

When scanning or saving an image you need to ensure that it is done in a manner that is suitable to your intended final use. This affects both the image type (.bmp or .jpg), colour type, and the resolution setting. Many adjustments can be made when scanning an image.

Bitmaps (.bmp) are the best for initial scans and manipulation as they give a higher quality image for working with. Because they use a lot of space they are not recommended for use on web pages or in e-mails. Jpegs (.jpg) are best for this use as they use 1/10th the space.

Resolution settings of 70 ppi to 100 ppi are good for monitor display but provide poor printing quality. Newsprint uses 97.5 ppi whilst most glossy magazines (like Vogue use 300 to 600 ppi) for their printed images.

Similarly an image that is 600 x 800 pixels at 150 ppi will look good, and large on a monitor; it will provide a good printed image only the size of a postcard.

You should consider all these aspects prior to scanning or manipulating an image to ensure that your finished product is of a suitable quality and meets common standards.



Printed materials are continuous tone images with a smooth and seamless transition between colours. Scanners and digital cameras create an electronic image by making a grid of pixel to represent the target. A high resolution image is created by dividing it into a larger number of pixels. Thus a high resolution image will give the appearance of a continuous colour when displayed on the monitor.

Computer monitors use pixels to indicate the display resolution level. Typically it is indicated as a set number of pixels wide and high, eg 800 x 600, 1024 x 768. These figure show the number of pixels across the monitor screen followed by the number up the screen, regardless of the size of the monitor, thus a bigger monitor would show the icons as being larger. Everything shown on the monitor is shown as either a set pixel size or a percentage of the screen size, absolute pixel size is the most common. As you increase you monitor resolution (more pixels on the screen) the icons appear to grow smaller as they are in an absolute pixel size and would thus represent a smaller portion of the screen when the number of pixels is increased. Most monitors are set at 72 or 96 pixels per inch.

The monitor appearance of an image is not always the way it will appear when printed. The image does not normally display on the monitor as the same size as it will print. The monitor will display the image as so many pixels not so many inches. Thus an image that is 4 inches at 150 ppi (total of 600pixels) would be displayed as 600 pixels regardless of the monitor setting and would display as 8 inches wide on a monitor set to display at 75 ppi.

Also you may have the monitor colour set differently for personal preference, or the printer may not be of high enough quality to replicate all the colours, or the monitor may be old and the colours being improperly displayed. The type of printer ink and printer paper can also have an affect on the quality of the printed image.

The best results are obtained by using a photo quality printer with photo quality ink and paper. However, most good printers will provide a highly acceptable finished image using plain copy paper and standard colour ink. The quality levels vary from printer to printer.

Most printers will have two or three print quality levels, they usually have a draft quality (often grainy) and a final quality. Some also have a photo quality level for images. Such printers usually have an option to use photo quality paper, and can also have an option to use photo quality ink. Final quality with normal ink is usually an acceptable print quality.


Image Expansion

When an image is displayed in an image manipulation program it can be expanded to show much finer detail. The higher the resolution the more it can be magnified. However, when an image is magnified too far, little squares start to appear; in an electronically displayed image this is called pixelation, in a printed image it is usually called grain.

Pixelation is due to the magnification enlarging the image to the point that you can identify each individual pixel. On the next page are two images, one has no pixelation whilst the other is heavily pixelated; notice the difference in magnification levels. Colour has been removed to make the images easier to save, store and print.

 pixels visible

As you can see the original appears to have two colours that make up a pattern, whilst the magnified image shows that the original colours are made up of individual colour squares that merge at the lower magnification level.


Colour Quality

An image can be stored or displayed with a number of colour qualities. They are:

32 bit or true colour This allows the use of up to 2 31 different colours.
24 bit or millions of colour This allows the use of up to 2 23 different colours.
16 bit colour This exists but is very rarely used today.
256 colours This utilises 256 colours, not usually used today.
16 colours Utilises the basic 16 colours, not usually used today.
Greyscale Various shades of grey used to show colour changes.
Black and White Uses only black and white for line drawings.

As you travel down the colour choices the size of the saved image file reduces, as does the colour choices and colour quality. Also, the lower quality colour resolutions can cause loss of image definition. Converting a high colour image into greyscale can often save image definition and also save space, whilst dropping to a lower colour level can sometimes lose the image definition.

File Types

There are a number of image file types, the type of file you use will affect the quality of the saved image, its ability to replace lost or damaged data, and its storage size. The smaller image files are usually referred to as ‘compressed’ files as various systems are used to compress, or squash, the size of the file. The most commonly used files types are:

Bitmap (.bmp), stores the file information by recording each individual pixel location and colour, starting in the top left hand corner, going across the line column by column, and then each line in turn.

Graphics Image File (.gif), stores the file in a compressed format by noting areas of similar colour as a group, eg position 1/1 to 5/50 are colour blue (denotes 5 lines of the same colour).

Tagged Image File (.tif, or .tiff), stores the file in a similar process to a .gif file.

Joint Photographic Experts Group (.jpg or .jpeg), stores the file in a compressed format similar to a .gif, but saves more space by noting the colour when it first appears and then referring to it. This runs the risk of losing more data if any is lost.

Photoshop File Format (PSD), this is the native image format used by Photoshop and is a proprietary Adobe design. It is quick and easy to save, and transport across platforms and Adobe programs. When creating an image within Adobe software it is best to use this format until the finished product is ready for distribution.

There are many other image storage formats, but those above are the most common used today. Jpeg files are what is usually used on the Internet or sent by e-mail as they use the lowest storage space, and thus less transmission time. Bitmaps provide the highest quality but can use from 10 to 50 times more space than a Jpeg image.


Storage Size

The storage size of an image can be affected by the colour quality, the type of file you save as and the resolution level chosen. A bitmap will need much more space than a jpeg file, whilst a colour image will need more space than a greyscale image. Also an image that is saved at 300 pixels per inch will use more space than one saved at 100 pixels per inch; a commonly used size is 72 pixels per inch. Increasing the number of pixel per inch will increase the size of the image in a square ratio.

That is a 4 inch x 5 inch image saved at 200 pixel per inch will need FOUR times the amount of storage space to a 4 inch x 5 inch image saved at 100 pixel per inch. This is because you are changing the image pixel size in two directions, sideways and down. The drawing below will demonstrate this.

4 inches x 5 inches x 100 ppi = 20 sq inches x (100 x 100 pixels) = 200,000 pixels

4 inches x 5 inches x 200 ppi = 20 sq inches x (200 x 200 pixels) = 800,000 pixels


Copyright by Ernest E Bywater