What is Resolution?

Resolution is a term that relates to the density of pixels or, number of pixels per inch of a digital image (ppi).

The resolution of an image is dependent on 2 other values. Firstly, the total number of pixels within an image. Secondly, the image size.
Although the number of pixels within an image is fixed the image size and resolution can and do vary. Both are dependent on each other.

PPI (pixels per inch) is one term used for image resolution though most products or people may refer to the dpi (dots per inch). DPI is a term that has more relevance to inkjet printers or computer monitors and refers to the number of ink/light dots created per inch by the printer/monitor. To all intents and purposes although ppi and dpi mean different things, the value itself can be regarded as the same.

Resolution can vary depending on how the image is being viewed. Images on some computer monitors are always viewed at 96dpi, while others produce an image at 72dpi while many inkjet printers produce an image at 300dpi.

What's a pixel?

Definition: the smallest discrete element of a digital image or picture.

A digital image is made up of a grid of coloured squares. Each of these squares is referred to as a pixel.

A pixel is a piece of information that is assigned a location and colour within a digital image. Pixels are not fixed in height or width but vary according to the resolution and total image size.
eg: if you have a square image of 96 pixels by 96 pixels and you are viewing it on a monitor set to 96dpi (dots per inch) the image size will look 1inch by 1 inch. If you then print it on an inkjet printer set to 300dpi it will print out at 0.32inches by 0.32inches - the pixels have decreased in size.
Alternatively, if you have the same original image and change the image size to 2inches by 2inches you will now have an image of 48ppi (pixels per inch) - the pixels have increased in size.

Above are four versions of the same image. The image size is the same but the number of pixels per inch (ppi) has been altered, and therefore the total number of pixels.
Looking from left to right the total number of pixels in the images decreases, resulting in an image that has little definition, in fact it looks like nothing more than a bunch of coloured squares.

Suffice to say, more pixels results in greater image quality.

What affects image quality?

The quality of a digital image is affected by 5 variables.

Total number of pixels
The output media chosen and its optimum resolution
The size of the output media
File format used to save image
The type and amount of image compression used

If you are planning to print your images on a small scale you firstly need to decide what print size you would like, for example 4inch, 5inch, 6inch, 8inch, 12inch width. Next take a look at the total number of pixels of your image. This figure will consist of the 2 values of a grid, the total number of columns (larger number) and total number of rows (smaller number), for example 800 x 600. If you take the number of rows and divide this by the width of your chosen print size you will find the resolution of your output, for example 600 divided by 4inch gives 150ppi (pixels per inch). Because the human eye is more critical of detail on the small scale you should aim to keep your output resolution reasonable high (200 - 400 ppi).

Once you have your image prepared and your chosen print size decided you must select the appropriate file format. Jpeg make small images for easy storage and transportation, though if you are planning on opening and saving the image several times you will compress the image further each time. BMP and TIFF format offer the ability to save without compression but can leave you with a massive file, certainly no good to sending via e-mail or the internet.

What size can I print?

Judging whether an image is adequate is subjective. The size of the print, the viewing distance, and the type of digital reproduction all play a part. Image size and viewing distance are often related. We will move closer to a small print and stand back from a larger print. The human eye can be highly critical of image detail on the small scale and yet forgiving on the larger scale.

For example, digital images projected onto a large screen may be readily accepted by an audience several metres away, though when viewed on a computer monitor it may appear pixilated. Similarly an image that appears convincing on a computer monitor seems pixilated when printed. Being aware of the optimum resolution of your output makes a big difference to your final result.

When printing digital images on a small scale take the number of pixels in one dimension (width) and divide it by the print size you desire (in inches). This will give you the final print resolution (ppi). The resolution can act as a guide as to whether the image will appear pixilated. Lets say you have a digital image that is 1200 by 1600 pixels. If you divide this first value by the print width dimension you will end up with the final print resolution. So, for a 4 inch print you will have 1200 pixels divided by 4 inches which equals 300ppi. Alternatively for an 8 inch print you will have 1200 pixels divided by 8 inches which equals 150ppi.
The pixels in the second option (150ppi) will be larger than the first (300ppi). Therefore they will be easier for the eye to detect, making the print look pixilated.

As a general rule ensure the final print ppi is somewhere between 200ppi and 400ppi. Any lower than this and the images will appear pixilated. A higher resolution will go beyond the abilities of most printers and will not affect any noticeable difference in image quality.

What file format?

Images can come in 2 forms. Bitmap and Vector formats. Bitmap images are made up a rectangular grid of pixels. Vector images objects and shapes defined mathematically.

Vector images are resolution independent, making them very efficient in terms of file size and can be scaled up and down without loss of quality. Their mathematical nature limits these images to simple shapes, no good for a photograph. The big advantage of a bitmap format is that each pixels value can be arbitrarily different to the next. This is perfect for continuous tone photographic work. A handful of the more widely used bitmap formats are listed below:

JPEG (Joint Photograph Experts Group) is a format that compresses images by deleting some pixels and can render 16 million colours. Ideal as this format is for photographs there is some loss of image quality depending of the amount of compression chosen. Repeatedly saving the same image inJPEG format gradually removes more and more information. Though the file size can make storage or transfer of images far easier.

BMP (Bitmap) is the standard windows format, generally used without compression. It can represent over 16 million colours. This format is ideal for photographs but produces large file sizes.

TIFF (Tagged Image File Format) is traditional used for traditional print graphics. This format can fulfil all your needs, over 16 million colour rendition, compression (lossy and non-lossy) or non-compression saving, but beware not every program will support the full compliment of options. If in doubt save without compression.