Image Size and Resolution

[Sunite]

Image Size and Resolution
 

I have noticed a lot of confusion about size and resolution of digital images. I'll try to provide some clarification here. Note that this discussion applies only to bitmapped digital images, which are the kind you are dealing with when you use a scanner or digital camera. Vector images are another matter but they are not relevant in this context.

 
First of all you need to realize that a digital image really does not have a physical size. To say that a digital image is, say, 4x6 inches is meaningless. After all, the image is just a collection of pixels stored in memory or on your hard disk. A digital image does, however, have a pixel size - the number of pixels horizontally and vertically. This is the only "size" that is inherent in any bitmapped digital image.

It can be useful, however, to assign a physical size to a digital image. Suppose that you scan a 4x6 inch postcard. If the resulting image is assigned the size "4x6 inches" then people viewing the image will know the actual size of the postcard. It also means that when you print the image at "normal" size it will print at this size (at least with all the software I have seen). All digital image files include this size information with an image. Scanned images are assigned a physical size that corresponds to the actual size of the area that is scanned - which makes perfect sense.

Then there's the matter of resolution, which is usually expressed in terms of pixels per inch (ppi, sometimes referred to as dots per inch or DPI). Perhaps you have already realized that once a digital image is assigned a size it automatically has a resolution. Here's the formula for horizontal resolution:

Horizontal pixels per inch = (number of horizontal pixels) / (horizontal size in inches)

Note that the vertical resolution can be different from the horizontal resolution, but usually they are the same.

When you are scanning, here's how it works. There are two things under your control. One is the size of the area being scanned - a single stamp, a postcard, whatever. The other is the resolution that will be used for the scan. With some scanner software you set the resolution directly - 96 dpi or 150 dpi, for example. With other software you select the type of document being scanned (color document, color photograph, etc.) and the use you will make of it (printing, web page) and the software selects the best resolution for you. In any case, when you make the scan the resulting file has a pixel size that is determined by these two settings:

number of horizontal pixels = (horizontal pixels per inch) x (horizontal size in inches)

Thus, if you scan a 4x6 inch area at 150 ppi the resulting image will be 600x900 pixels in size (4 x 150 is 600, 6 x 150 is 900).

What about digital photographs, which unlike scanned images do not have an inherent physical size? These images are assigned an arbitrary resolution either by the camera or your software. For example, when I open an image from my digital camera in Photoshop it is assigned a resolution of 72dpi. Since the image is 2560x1920 pixels, this results in the image having a "physical size" of about 36 x 26 inches. This "size" is essentially meaningless, of course.

In your graphics program you can change any of these three image size parameters: physical size, pixel size, or resolution. Because they are all linked to one another, changing any one of them means that one or both of the other parameters must change as well. Let's look at an example. Start with the following image:

Size: 4x6 inches
Pixel size: 400x600
Resolution: 100 ppi (pixels per inch)

Suppose you want to change the size to 8x12 inches. In order to do so you must also do one of the following:

Change the pixel resolution to 800x1200 (resolution remains at 100 ppi).
Change the resolution to 50 ppi (pixel size remains 400x600).
Your graphics program will do this automatically, but you must tell it which one to do - change the pixel size or change the resolution. Note that the process of changing an image's pixel size is called resampling. The specific instructions for this differ from one graphics program to another, but the concept is the same.

Resampling of the image is controlled by the Resample Image option. If this option is on:

Changing the width or height changes the pixel size while leaving the resolution unchanged.
Changing the resolution changes the pixel size while leaving the width and height unchanged.
Changing the pixel size changes the width and height while leaving the resolution unchanged.