How Can You Have "Dots Per Inch" if You Don't Have Inches?
When you open image files from your digital camera, are they 240dpi, or are they 300dpi? Or perhaps only 72dpi? If you open them in a different program, does the resolution change? This can happen, but that doesn't mean that anything is wrong. After all, resolution is just a number. And it can be changed.
Allow me to explain.
The sensor in my Nikon D2x measures approximately one inch by two-thirds of an inch (23.7 x 15.7 mm). Full sized D2x images are 4288 x 2848 pixels. If I were to print an image at the size it was captured, I would end up with a mighty fine, inch by two-thirds of an inch print with an extremely high resolution, limited only by what my printer is actually capable of printing. If I were trying to make my own postage stamps, prints this size might be of interest, but larger ones are likely to be more in demand otherwise. Those same 4288 x 2848 pixels could also be printed more realistically at 300dpi to yield a print approximately 14 by 9.5 inches. Or at 240dpi for an 18 by 12 inche print. Or a gigantic but undoubtedly less sharp 60 by 40 inch print at 72dpi. In each, the pixels never change, only the way I would be interpreting them would.
So if the same image can be printed at such wildly differing resolutions with no change to the pixels, what is the resolution of the image itself? The truth is, while a digital image has pixel dimensions, it doesn't really have a resolution. It's really just a bunch of data in your computer and doesn't have a physical size in inches. The entire concept of "dots per inch" or "pixels per inch" breaks down when you don't really have inches. You can choose to interpret the pixels you have as being any resolution you need.
Image formats do have a resolution field as part of the file header. Some applications use this as defaults when you open the file, but you are not bound by what is recorded there. You can change resolution as needed without altering the image pixels at all. To reiterate, all you are doing is changing how the existing data gets interpreted.
When I open a raw file from my D2x in Nikon Capture, it uses the default resolution of 300dpi that is recorded in the image by the camera. Resolution can be changed as needed though using the Size/Resolution dialog. To do so, select "Keep the output size" and enter the desired new resolution in the "at" box. The Width and Height will be adjusted accordingly. If I open the same image in Adobe Camera Raw, it defaults to 240dpi, but I can easily change it by using the Resolution edit box in the lower left-hand corner of the ACR window.
Resolution can also be changed In image editing applications such as Photoshop and Photoshop Elements. To do so in Photoshop, go to Image >> Image Size (Image >> Resize >> Image Size in Photoshop Elements) and uncheck the "Resample Image" checkbox. You can then enter your desired resolution and the Document Size Width and Height will be calculated for you. The pixel dimensions will not be able to be changed while resampling is turned off.
All of the ways to change resolution we have looked at this week don't actually change the image pixels at all. They merely let you decide how you want to interpret the data you have. Resolution and size are inversely linked together. Lowering resolution will correspondingly increase the height and width in inches, and vice versa. But until those inches become real inches by printing the image out, both resolution and inches remain just descriptive fields in the file header. The image pixel data remains unchanged. Next week we'll take a look at what to do if you want to change either resolution or inches independently of the other.