Pixels are simply the building blocks of an image. Any given digital image has an inherent number of pixels like bricks in a house. The number of pixels an image has is initially established when the image is digitized (either captured with a digital camera or scanned).
When you capture images with your digital camera or scanner you have several resolution settings available to choose from. The question many people wrestle with is which setting to choose. Low resolution allows many more images to be taken and stored on a memory card but the quality is not as good as high resolution images. The trouble with high resolution images is at the higher pixel settings, they can fill up a memory card very quickly.
The challenge is that images require a certain amount of pixels to be present in order to look good depending on the image purpose. The bigger the image you want to print on paper or publish on the web, the more pixels you will need to have. You can always delete pixels after the image is created, but trying to add them later isn't a good idea if you want to maintain a high quality image. This process is called interpolation. The image software makes a “best guess” at the color parameters based on the pixels present and just adds them throughout the image.
Image resolution can be described in two ways: In terms of pixel dimensions or physical dimensions. Web designers talk about pixel dimensions, while print professionals want to know physical dimensions. Essentially, it's two different ways of looking at and talking about the same thing.
An image's pixel dimensions refer to the number of pixels that comprise its height and width. It says nothing of physical size, just simply the number of pixels present in that specific image. Therefore a web designer might want a jpeg (or jpg) image that measures 300 by 400 pixels. Because inches don't exist in the web world, web designers are only concerned with pixel dimensions. An image that is 100 pixels wide will always take up 100 pixels of screen space in that dimension on your display monitor.
In print, the physical dimensions of an image are more complicated. Just a fixed number of pixels for the height and width won’t suffice. When talking about the physical size of a printed image, printers express it in terms of inches, followed by the number of pixels per inch. Therefore a printer might want a photo to be 3 x 4 inches @ 300dpi.
The tricky part of printing is that two different images can be made to have the same physical measurements, for example, 3 x 4 inches but their pixel dimensions can vary greatly. For example, a web image can measure 300 pixels wide (or 3 inches @ 100 pixels-per-inch for a printed image) and 400 pixels tall (or 4 inches @100 pixels-per-inch for a printed image). On your display monitor this would be enough pixels to produce a nice quality image. However, if you doubled the number of pixels to 600 and 800 respectively the web image would of course be twice as large as displayed on your monitor. However, if you also changed the pixels per inch to 200, the printed image would still be 3 x 4 inches. Why? 600/200 = 3 inches and 800/200 = 4 inches.
It may help to think of pixels like water in a glass. For example, imagine that you have a glass of water and the water volume represents an image with a fixed amount of pixels, e.g. 4 million. The glass then represents the physical dimensions of the image e.g. 2 x 6 inches which approximates the circumference and depth of the glass.
Now imagine you take the same 4 million pixels from the image and pour them into a “glass” that is 5 inches wide and 1 inch deep. What happens to the pixels? Like water they spread out to fill the size and shape of its container. Now, instead of a deep glass of pixels (higher resolution photo), the same number of pixels have a bigger area to fill. This “shallow” glass of pixels will therefore be a lower resolution photo.
In the end, it's all relative. Given an image with a fixed number of pixels, if you increase the physical size of the image the number of pixels per inch goes down. If you decrease the physical size of the image you've made the pixel pool deeper, thereby increasing the number of pixels per inch.
Conversely, given a fixed physical size for an image, if you increase the number of pixels in the image you’ve made the pixel pool deeper as well. This also means you have more zoom depth. You can experiment with this by using the ZoomInto Viewer on several different pixel sizes of the same image. These are commonly available on photo sharing websites such as Flickr®.
We hope you enjoy using our free All-in-one Image Zooming Add-on from ZoomInto.com