Rescaling vs. Re-Sampling Photos and Why it Matters


The subject of resizing your photographs to make a high quality print or to share it online continues to be question that comes up in forums. Part of the confusion stems from the terminology itself. In this post/post, I will discuss the subject from the standpoint of scanning a negative or slide. First, Are a couple of comments about the terminology.

Resolution – Dots Per Inch

Resolution is generally referred to as either dots per inch (dpi) or pixels per inch (ppi). What can lead to confusion is that dpi is used to describe the resolution of a scanner. For example, a scanner that is capable of scanning 35 mm negatives may have a optical resolution of 3200 dpi or greater. The word optical here means there are no mathematics being used to artificially increase the resolution. For the best quality, you should scan at or below the optical resolution of the scanner.

Resolution – Pixels Per Inch

Referring to a scanned image, once you open the image in say, Photoshop Elements or Photoshop, the terminology changes. Now the same resolution is measured in and referred to pixels per inch (ppi). The resolution is the same as you scanned, the term just changes. For example, if a negative is scanned at 3200 dpi and then opened in PSE 15 using the Image > Resize > Image Size command, the dialog below is opened. The resolution of 3200 ppi is shown.

3200 ppi ex

Print Resolution – Dots per Inch

When describing the resolution of an inkjet printer, dots per inch or dpi is again used. However, at this point in the process, the term takes on a whole new and more literal meaning. Now it is used to describe how many drops of ink are placed on the paper per inch. It is one of the prime, but not only parameter that is used to describe how well the printer can reproduce the image. And, this number has nothing to do with the resolution of the image file (in pixels per inch) that was sent to the printer.

This tutorial is an exercise that will not only give you a better understanding of rescaling an image versus re-sampling it, but will also demonstrate the loss of quality that is a bi-product of up-sampling an image, or adding to the number of pixels that were not in the image when it was scanned.

To view or download the tutorial, click on the image below. I suggest you actually follow along with the tutorial using one of your own images.

Resampling Comparison

In this post and tutorial I have focused on resolution as it impacts the printing of your digital photographs. In a future post, I will cover the topic of how to properly size your photos for sharing them online.

If you found this tutorial to be helpful and would like to see other tutorials in the future, please give it a suitable star-rating and  share it with your friends.

Until next time..

 

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