Understanding Print Size and Resolution for Your Photos

December 05, 2016

Understanding Print Size and Resolution for Your Photos

With the holiday season here, you may be interested in printing some of your images. You might want to include a family photo in your holiday greeting card or maybe even print a special image in a larger size so that you can frame it and give it as a gift.

Whatever the reason, it's helpful to understand how print size works when you are dealing with a digital image. In particular, the question that a lot of people ask is, "How big can I print my digital photo?"

The answer to this depends on the resolution that you want to print in, the amount of pixel data in your image file and how much you can increase that data without degrading the image.

Print Resolution

A lot of folks throw around 300 ppi as the required resolution for printing. But that's not always the case. The resolution needed depends on how you are going to use the print and, most importantly, the distance it will be viewed from.

If you are printing something that will be viewed up-close such as a photo to slip into a holiday card or to frame for the mantel, a resolution of between 240 and 300 ppi will give you good results. If, however, you are printing an image that will be hung relatively high on the wall where it won't be viewed as closely, you can get away with 175 - 200 ppi. If you are printing a banner that will be hung on the side of the school gym, 72 ppi will be sufficient.

But What is Resolution?

We've mentioned 'resolution' several times now. But what is resolution?

All digital images are made up of tiny little dots of color called pixels. The number of pixels in an image is determined by a camera's image sensor. If you zoom into an image close enough, you can actually see them.

understanding print size and resolution for your photos

When we talk about resolution, we are talking about how densely those pixels are packed together. So resolution is measured as the number of pixels in a given area. For that reason, you will frequently hear the terms pixels per inch or ppi to describe resolution.

Changing Image Size and Resolution

There are two ways to change image's resolution and print size. You can do it with or without resampling.

Resizing without Resampling

When you resize a digital photo without resampling, you are changing the size and resolution of the image using only the pixel information in the digital file. The amount of data in the file is unchanged. No additional data is added and nothing is removed. You are simply taking the pixel data that's already there and rearranging it to change the image resolution and, therefore, the print size.

When you increase an image's resolution without resampling, the pixels in that image are rearranged so that they are packed more closely together. The opposite happens when you decrease resolution (again, without resampling). Decreasing resolution rearranges the image pixels so that they are more spread out.

This means that when you increase the resolution of an image without resampling, you will see:

  • The print size of the image gets smaller since the number of pixels per inch is getting larger;
  • The total number of pixels in the image, the pixel dimensions and the image file size stay the same;
  • The pixels in the image are getting packed together tight and tighter;
  • The image becomes crisper, with more well-defined lines.

You’ll see opposite effects when you reduce the resolution of an image.

These same kinds of relationships hold for print size too. When you reduce the print size of a photo (without resampling), you will see:

  • The resolution of the image will increase because the number of pixels per inch is getting larger;
  • The total number of pixels in the image, the pixel dimensions and the image file size stay the same;
  • The pixels in the image are getting packed together tight and tighter;
  • The image becomes crisper, with more well-defined lines.

Increasing a digital photo's print size has opposite effects.

Let's look at a quick example. Say that we have a 600 x 600 pixel image with a resolution of 100 pixels per inch. Given these numbers, the image would have a print size of 6 x 6 inches.

Now say that we want to increase the resolution to 200 pixels per inch. Without resampling, the pixel data and pixel dimensions don't change. So even with an increase in resolution, the pixel dimensions of the image are 600 x 600 pixels. Therefore increasing the resolution to 200 pixels per inch (ppi) produces a resized image with a print size of 3 x 3 inches.

The same reasoning applies if you want to increase the print size to 12 x 12 inches. Without resampling, the pixel dimensions will still be 600 x 600 pixels. That means that the resized photo will have a resolution of 50 ppi.

To see an illustration of this, click on the image below.

 

As you can see, as the resolution increases, the image itself gets smaller while the lines of the image get smoother and more well-defined.

Resizing with Resampling

When you resize a digital image using resampling, you use software to add or subtract image pixels. The software's internal algorithm increases or decreases the pixel dimensions of the image, changing the amount of data in the file.

Using resampling, you can increase or decrease the image resolution, the print size or the pixel dimension. These changes generally work this way:

  • Changing the resolution changes the pixel dimensions but not the print size.
  • Changing the print size changes the pixel dimensions but not the resolution.
  • Changing the pixel dimensions changes the print size but not the resolution.

In summary, resampling always affects the pixel dimensions, but only affects the resolution when it is changed manually.

Revisiting the example from above, say that we have a 600 x 600 pixel image with a resolution of 100 pixels per inch and a print size of 6 x 6 inches.

Now we'll increase the resolution to 200 pixels per inch. Since we are using resampling, the pixel data and pixel dimensions change to reflect the increased resolution. The pixel dimensions change but not the print size. So the resized image would be 6 x 6 inches at 200 pixels per inch with pixel dimensions of 1200 x 1200.

If instead we increase the print size to 12 x 12 inches, the resized image would have a resolution of 100 pixels per inch and pixel dimensions of 1200 x 1200.

So you can see that resampling can be a great option when you want to resize an image, but you don't have enough pixel data in the original file. Resampling gives you flexibly in terms of print size that you just don't have when you resize without sampling.

But there is a price to pay for that flexibility. Simply put, resampling degrades images.Image quality is reduced whenever data in added or removed from an image file. That's especially true when adding data.

Photoshop uses very complex algorithms for upsampling (adding pixel data) and it generally does a really good job. But, realistically, the software is creating pixels for you. And there's only so far that can go. So when you are resizing, keep in mind that resampling can only get you so far before your image starts to look blurry, jagged or blocky.

Resizing in Photoshop and Photoshop Elements

Resizing in Photoshop and Photoshop Elements is very easy.

Photoshop

In Photoshop, select Image>Image Size from the top menu bar.

understanding print size and resolution for your photos

The Image Size dialog box will open.

understanding print size and resolution for your photos

If you want to resize without resampling, uncheck the box next to Resample, change the image dimensions or resolution and then click OK.

understanding print size and resolution for your photos

If you want to use resampling, check the box and change the image dimensions or resolution. Click on the drop down box next to Resample and choose a Resampling method.

understanding print size and resolution for your photos

I usually leave the method on Automatic. But if you are on an earlier version of Photoshop, the Automatic option won't be there. In that case, I recommend Bicubic Smoother when enlarging and Bicubic Sharper when reducing.

BTW, when resampling an image, only resize it once.

Photoshop Elements

In Elements, select Image>Resize>Image Size from the top menu bar.

understanding print size and resolution for your photos

The Image Size dialog box will open.

understanding print size and resolution for your photos

If you want to resize without resampling, uncheck the box next to Resample, change the image dimensions or resolution and then click OK.

understanding print size and resolution for your photos

If you want to use resampling, check the box and change the image dimensions or resolution. Click on the drop down box next to Resample and choose a Resampling method.

understanding print size and resolution for your photos

I usually leave the method on Automatic. But if you are on an earlier version of Photoshop, the Automatic option won't be there. In that case, I recommend Bicubic Smoother when enlarging and Bicubic Sharper when reducing.

BTW, when resampling an image, only resize it once.

For Further Reading…

How to calculate print size for your photos

How to Properly Resize Images in Photoshop




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