# A Real-World Problem of Image Resolution

## How to Calculate Resolution for Publishing Photos

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.

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

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