1. Computing

Discuss in my forum

A Real-World Problem of Image Resolution

How to Calculate Resolution for Publishing Photos

By

Here's a question and answer from a reader's real-world problem of dealing with image resolution. This is pretty typical of what most people have to deal with when they are asked for an image to use in publication...
"Someone wants to buy a photo from me. They need it to be 300 DPI, 5x8 inches. The photo I have is a 702K, 1538 x 2048 jpeg. I figure it has to be big enough! But how do I tell? The only photo program I have is Paint.NET, and I'm not sure it's telling me what I want to know. If I don't mess with it, it tells me that my resolution is 180 pixels/inch, at a size of approximately 8 x 11. If I make it 300 pixels/inch (is that the same as DPI?) I can get a print size that works, about 5 x 8, and it changes the pixel width to 1686 x 2248. Is that what I'm supposed to be doing??? It doesn't seem like much of a change to the human eye."
A lot of this confusion is because most people don't use the right terminology. They say DPI when they should be saying PPI (pixels per inch). Your photo is 1538 x 2048 and you need a print size of 5x8 inches… the math you need is:
pixels/inch=PPI
1538/5=307
2048/8=256
That means that 256 is the maximum PPI you can get from this image to print the longest side at 8 inches without letting your software add new pixels. When your software has to add or take away pixels, it is called resampling, and it does result in a loss of quality. The more drastic the change, the more obvious the loss in quality will be. In your example it is not very much, so the loss won't very noticeable… as you noted. In a case of this small of a change, I generally prefer to print the lower PPI image. It usually prints fine. But since you are sending this out to someone, you'll just have to accept the resampling to make it 300 PPI.
More on Resampling

What you did in Paint.NET is fine as long as you know and understand that the software is going to resample the image. Anytime the pixel dimensions are changed, this is resampling. There are many different algorithms for resampling, and different software uses different methods. Some software even offers you a choice of different algorithms. Some methods work better for reducing image size (downsampling) and some work better for increasing image size (upsampling) like you want to do. "Best Quality" in Paint.NET should be fine for what you need to do.
More on Upsampling Methods

My resizing practice exercise might help to make all this clearer to you. It was written as part of my Photoshop CS2 course, but the resize dialog box in other software may be similar enough that you can still follow along.
Resizing Practice Exercise

Also see: How do I change the print size of a digital photo?

Another problem you have is that your dimensions are a different aspect ratio from the print size that has been requested. That means you'll have to crop the image yourself if you want control over what is shown in the final print.
Aspect Ratio and Cropping to the Proper Print Dimensions

Here's some additional follow up clarification:

"When I tried to make the photo a higher PPI, I expected the pixels numbers to decrease rather than increase. I guess I thought that if there aren't enough pixels to get the size I want at the resolution I want, it would 'spread them out' somehow, not give me more. Now that I've read your resampling definition, I understand why there are more pixels, not less."
What you said about spreading out the pixels is basically what happens when you send a lower resolution file to the printer. At lower resolutions, the pixels get more spread out and you lose detail; at higher resolution pixels are squished closer together, creating more detail. Upsampling causes your software to create new pixels, but it can only make guesses as to what is accurate — it can't create any more detail than what was there originally.
  1. About.com
  2. Computing
  3. Graphics Software
  4. Digital Photography Tips
  5. Image Size and Resolution
  6. A Real-World Problem of Image Resolution - How to Calculate Resolution for Publishing Photos

©2014 About.com. All rights reserved.