If you change the image document size (the size the image will be printed at) the image size in pixels will also change as long as the Resample Image checkbox at the bottom of the dialog box has a check mark. If you clear the Resample Image checkbox, the pixel dimensions will remain unchanged when you enter new values in the document size fields. When the image is not being re-sampled (no pixels added), the setting for pixels per inch must change if the document size is changed.

 

As mentioned before, one way of quantifying the idea of resolution is pixels per inch. For the most part this approach to image resolution applies to printing. If you have more pixels per inch being printed, the quality of the results will generally be improved. Notice though, that if you increase the pixels per inch value in the Image Size dialog box, the actual pixel dimensions of the image can change. Remember, if you change the actual size of the image in pixels, it must be re-sampled, with a possible loss in quality. Consider that if you try to improve the print quality by increasing the pixels per inch, you may actually change (reduce) the quality of the image because of the reconstruction of the pixel array.

 

Say for example, that you wish to print a 10 by 10 inch picture from an image file that is 400 by 400 pixels. For a print that is 10 inches wide, 400 pixels can only support a pixel per inch value of 40. This would translate into poor print quality. If you specified 100 pixels per inch, the original image would have to be resized to 1000 by 1000 pixels. The re-sampling required would almost certainly degrade the picture quality. It is next to impossible to print a large high quality image from a small low resolution file.

 

The best solution for printing in high quality is to begin with an image that was captured with plenty of resolution, so there are a lot of pixels to work with. If you must work with low resolution images (images with fewer pixels), reduce the size of your prints to produce a high quality result. In the example above, changing the document (print) size to 4 inches by 4 inches would increase the pixels per inch to a value of 100.

 

For high quality prints, a setting of 180 pixels per inch is recommended as a minimum.

 

In this example, the resample check box has been cleared. Because re-sampling is not allowed, changes to the pixels per inch resolution will not change the pixel dimensions of the original image. If the number of pixels per inch is increased, the document size will decrease. This is because the amount of pixels in the image cannot be changed (no re-sampling). To achieve the desired number of pixels per inch, the print size of the document must decrease. Similarly, if the resolution (pixels per inch) is reduced, the document size will increase.