How to Change an Image Size
Many digital photos will open into your photo editing software with a resolution of 72 pixels per inch, because your digital camera does not store resolution information when it saves the photo, or the software you are using can’t read the embedded resolution information. Even if your software does read the resolution information, the embedded resolution may not be what you really want. Look in your photo editing software for an Image Size, Resize, Print Size, or Resample command. When you use this command you will be presented with a dialog box where you can change pixel dimensions, print size, and resolution.
How to Change an Image Size Without Losing Quality
To change print size without loss in quality, you should look for a “resample” option in this dialog box and make sure it is disabled. Resampling effectively adds pixels based on nearest-pixel averaging, so the resulting product will tend to look a bit less sharp.
How to Constrain Image Proportions
To change the print size without stretching or distortion, look for a Constrain Proportions or Keep Aspect Ratio option and enabled it.
Raster vs. Vector
Images come in two broad flavors: raster (also called bitmap) and vector. Raster images consist of pixels; changing the size of a raster image affects the quality of the image. A vector image, which is common with logos, charts, and line art, consists of instructions for redrawing the image. Vector images perfectly resize to any dimension, but photographs cannot be effectively vectorized.
Choosing an Image Resolution
When the resample option is disabled and the constrain proportions option is enabled, changing resolution will alter the print size and print size will alter the resolution as expressed in pixels per inch. The ppi will get smaller as the print size increases. If you know what size you want to print, enter the dimensions for the print size.
If the ppi changes to 140 or less, you will get a low-quality print at that size.If the ppi changes to 141-200, you will get an acceptable quality print at that size.If the ppi changes to 201 or higher, you will get a high-quality print at that size.
Resampling an Image
If you do not have enough pixels to get an acceptable or high-quality print, you will need to add pixels through resampling. Adding pixels, however, does not add quality to your image and will usually result in a soft or blurry print. Resampling by a small amount is generally acceptable, but if you need to increase the size more than 30 percent or so, you should look into other methods of increasing image resolution.
