A lot of confusion surrounds the use of the term resolution in relation to digital images. The confusion lies in the various definitions attributed to resolution. All definitions are valid but are incomplete unless taken into consideration with each of the other definitions. This particularly holds true when we consider resolution for the purpose of some form of output.
Resolution as Pixel Quantity
At its most basic definition, resolution refers to the number of Pixels comprising a digital image. The term mega-pixel refers to a million pixels. Thus an 8 mega-pixel image has 8 million pixels. Conventional wisdom tells us that the more pixels an image has, the more detail it can hold.
Resolution as Pixel Dimensions
Images are two dimensional. They have width and length. Thus, images are also measured by their pixel dimensions. In fact, computing for Pixel Quantity is made easier once we know the Pixel Dimensions. We just multiply the width by the length. So if an image has 3000 pixels as its width and 2000 pixels as its height, when they are multiplied we have a figure of 6,000,000 pixels or 6 mega-pixels. Knowing the Pixel Dimensions is essential if we need to determine an output size for our images.
Resolution as Pixel Density
Resolution is also often used to describe how densely arranged are the pixels in an image. The standard unit of measurement for Resolution as Pixel Density is PPI or Pixels per Inch. (The term DPI, which means Dots per Inch, is more often used interchangeably with PPI.) An image with a resolution of 72ppi (or dpi) means that the pixels in that image are arranged such that 72 pixels fit in one linear inch. A 300ppi (dpi) image would have its pixels more densely arranged such that 300 pixels fit into one linear inch.
Take note that PPI refers to pixels in a linear inch of an image. What does that mean? It means that in a 300ppi image, there are 300 pixels per inch vertically and also 300 pixels per inch horizontally. Thus the total number of pixels in one square inch would be 90,000 pixels.
Resolution as Pixel Density is essential when preparing an image for output to a particular media. Computer monitors display images at a default of 72ppi (dpi). A sharp and detailed image can be rendered at that resolution. Images for printing usually require a resolution of 300ppi (dpi).
One major error a lot of people make is that they consider any image set at 300ppi (dpi) as a high resolution image. Thus, they think they can print very large prints from their image simply because it has a high resolution. The error here lies in their failure to take into consideration the Pixel Quantity together with the Pixel Density. In other words, I can have a 300ppi (dpi) image but it might only be one inch in size. That means my image has a pixel quantity resolution of 90,000 pixels, not even one tenth of a mega pixel. Thus, ‘high resolution’ is relative to the intended output. If I wanted to print a 1x1 inch ID photo then my 90,000 pixel 300ppi file would be of high resolution for that particular output. But if I wanted to print that image at 8x10 inches, that same file would definitely be of very low resolution.
Calculating for Output
So how does one correctly gauge the size and resolution of an image for a particular output? Basically, one takes into consideration all three definitions of resolution – pixel quantity, pixel dimensions, and pixel density.
Once we know the pixel quantity we can have an immediate estimation of the print size capabilities of an image. 3 mega-pixel images can easily make sharp 5x7 inch prints, 6 mega-pixel images can make 8x10 inch prints, 8 mega-pixel images can make 8x12 inch prints, etc.
To be more precise, we need to make use of the Pixel Dimensions. ( You can get the pixel dimensions in Adobe Photoshop by clicking IMAGE / IMAGE SIZE and looking under Pixel Dimensions. In Windows, you can see the pixel dimensions of a JPEG image by clicking once on an image in My Computer and looking under ‘details’ on the left side of the screen. ) Once you have the pixel dimensions, you just have to divide the width by the recommended output resolution, and do the same for the length.
For example, if your output is to a 300dpi printer, then you can determine how large you can print a sharp image from your file by multiplying each dimension by 300. So if you have a 6 mega-pixel image with dimensions of 3000 pixels x 2000 pixels :
3000 pixels divided by 300 pixels/inch = 10 inches
2000 pixels divided by 300 pixels/inch = 6.67 inches
Ideal output = 10 x 6.67 inches
*Some printers can print sharp and detailed images at resolutions lower than 300dpi.
Interpolation and Pixel Size
How big is a pixel? The size of a pixel is not fixed. It is determined by how densely arranged they are in an image. Images with pixels set at 300ppi (dpi) are more densely packed than an image set at 72ppi. 300 pixels have to be squeezed into one linear inch, to do so, they become smaller. At 72ppi, pixels are stretched to fit into one linear inch, thus they become bigger. Technically, a pixel can even be as large as an inch, if you set an image at 1ppi although you wouldn’t be able to display it at that size on your computer.
When pixels in an image are stretched, they can reach a point wherein they are big enough to be seen as an individual pixel. When this happens, we call this pixelation. The result of this is a lessening of visible detail and sharpness.
If we have an image a 6 meg-pixel image that prints at 300dpi with a size of 10 x 6.67 inch, can we print it at a much large size? Can we print it at 30x20? Yes we can! But the quality of the image will not be as sharp as the original. Still, we can come up with a relatively sharp print at that size if we interpolate.
Interpolation is a process wherein pixels in an image are duplicated to create a larger file. The advantage of this is that more pixels are created to keep the pixel density of an image constant even while the pixel quantity increases. By doing this, the pixels remain small and are not stretched.
How does this differ from not interpolating? If we don’t interpolate, our option would be to stretch an image causing the pixels to be larger. For example, the 10x6.67 image at 300ppi would become only 100ppi if we stretch it to print at 30x20 inches. The pixels would be bigger and less detail and sharpness would be seen. Through interpolation, the same image would be resized to 30x20 inches while still retaining a pixel density resolution of 300ppi. Compared to stretching, this would produce a sharper image.
Take note though that interpolation does not create new detail, it just duplicates detail that is already present in the original image. So, an interpolated image cannot be more detailed than the original. In fact, the larger we interpolate an image, the less detail and sharpness begins to become obvious. Still, it will produce better quality than stretching your image.
Interpolation can be accomplished in some photo editing software. In Adobe Photoshop, you can interpolate an image by clicking IMAGE / IMAGE SIZE, and by inputting your intended width, length, and pixels/inch resolution. Make sure though that Constrain Proportions and Resample Image are checked. Under Resample Image, the Bicubic method would do an excellent job for most purposes.
By the way, if you uncheck Resample Image, you can reset the pixel density of the image without affecting the pixel quantity. This is the tool to use if you want to reset your 72ppi image from your digital camera to a 300ppi file and also for seeing what size it could be print at.