Explaining Digital Image Resolution, Effective Resolution and How They Affect the Appearance and Quality of Digital Images when Printed

Explaining Digital Image Resolution, Effective Resolution and
How They Affect the Appearance and Quality of Digital Images

Image resolution is one subject that confuses a lot of people but it is not really so difficult to understand. Pictures should always be supplied at an effective resolution of 300 pixels per inch (PPI) for best results when printing on professional printing presses. Just what does that really mean? Below we will explain just what resolution is, what it looks like when printed (high and low resolution) and how it is affected when pictures are placed within a document and scaled up or down. When we are through you will understand effective resolution and will be able to figure scale and place pictures just like a pro.

The Basics

Let’s start with the basics. All images from digital cameras or scanners are made up of PIXELS. The word PIXEL is a contraction of the term PIcture ELement. A pixel’s appearance can be defined as a tiny square of color. You may find it helpful to think of it as a very small tile, such as a floor tile. If you magnified a high resolution digital image to 1,600 percent you would see the pixels (or tiles) that the image is made up of.

When preparing files for printing, the problem that most people encounter with their digital pictures is resolution. What is resolution? Simply put, resolution is the amount of information contained in an image. A good way to think of resolution is to think of a digital photo as a design mosaic made up of those very small tiles we mentioned earlier. The smaller the tiles are, the greater the detail and the more intricate the design can be. The larger the tiles, the less detail and thus the simpler the design must be. The pixels we talked about earlier can be thought of as those tiles.

PPI or DPI?

We will discuss pictures in terms of pixel density or the number of pixels per inch (measured horizontally). This is referred to as PPI  (or pixels per inch) but is also often called DPI. While this term is commonly accepted, it is not technically correct. DPI (or dots per inch) refers to the resolution of output devices such as laser or ink jet printers and platesetters or film imagesetters. For the sake of accuracy we will use the term PPI (or Pixels Per Inch).

Resolution: High or Low?

In printing, we require a 300 PPI image at 100% to obtain good quality printing. Here is an example of a high quality 300 PPI file at 100%. Note the crisp detail. This picture will look very good when it is printed.

High Resolution 300 PPI image

Below is an example of a low resolution 72 PPI image placed at 100%. This file will also not look any better when printed.

Low Resolution 72 PPI Image

Effective Resolution: What is it and How do I Figure it out?

Effective resolution is simply the final resolution of any picture at the actual scale that it is placed within the document. This is really quite simple. If you place a 72 PPI image at 100% its effective resolution is 72 PPI. If you place it at 50% (half size) the effective resolution doubles and the result is 144 PPI. If you place it at 25% the effective resolution is quadrupled and is then 288%. You can determine the effective resolution of an image by dividing the actual image resolution by the scale.

Here are a few examples:

300 PPI divided by 200% (or 2) = 150 PPI (unacceptable)

72 PPI divided by 150% (or 1.5) = 48 PPI (unacceptable)

300 PPI divided by 60% (or .6) = 500 PPI (good)

72 PPI divided by 25% (or .25) = 288 PPI (acceptable)

Another problem we often encounter is the overly enlarged image. This occurs when someone tries to enlarge an image that is too small to fit their design. While it is true that you can enlarge an image safely, going beyond a certain size will result in a degraded or even a “pixelated” (or blocky) image. We recommend that you do not enlarge pictures more than 125% of the original size (at 300 DPI). Pictures enlarged beyond 125% will show noticeable degradation, and enlarging an image beyond 200% can result in the actual pixels that make up the photo becoming visible.

Image Resolution, Printed Size, and CMYK File Sizes 

Below are some examples showing the pixel dimensions, appropriate printed size and the approximate CMYK Tiff file size for images to be used at the industry standard effective resolution of 300 pixels per inch. Dimensions shown below are landscape, simply reverse the width and height for portrait format images. 

Image Dimensions in Pixels

Printed Size (W x H)

Approximate File Size (CMYK Tiff)

600 x 400 pixels

2” x 1.33”

938K

640 x 480 pixels

2.13″ x 1.6″

1.17 Mb

600 x 600 pixels

2” x  2”

1.37 Mb

800 x 600 pixels

2.67″ x 2″

1.83 Mb

1024 x 768 pixels

3.41″ x 2.56″

3 Mb

1280 x 960 pixels

4.27″ x 3.20

4.7 Mb

1200 x 1200 pixels

4” x 4”

5.5 Mb

1600 x 1200 pixels

5.33″ x 4″

7.32 Mb

1800 x 1200 pixels

6″ x 4″

8.24 Mb

2100 x 1500 pixels

7″ x 5″

12 Mb

2400 x 1600 pixels

8” x 5.33”

14.6 Mb

3000 x 2400 pixels

10″ x 8″

27.5 Mb

3300 x 2550 pixels

11” x 8.5”

32.1 Mb

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Resolution: Let’s Play the Numbers Game

Let’s take an example using Photoshop (other applications menu selections may vary slightly but will be similar). Make a new 4″ square image that is 72 PPI and change its resolution to 288 PPI by doing the following:

1. Go to Menu > Image > Image Size

2. Uncheck the Resample Image option

3. Change the Resolution to 288 PPI

Notice that the width and height changed from 4″ to 1″ (or 25% of their original size). What you have actually done is to reduce the pixel size which reduces the overall size of the image (from 4″ to 1″) which in turn increases the pixel density (from 72 PPI to 288 PPI). This is because the total number of pixels has not changed.

Resolution is really just a numbers game in the end: one number (pixel density) goes up while the other (image size) goes down in direct proportion. The picture isn’t really any different except that now the computer thinks that it is a smaller and higher resolution image than it was before you made the change. Now it will print well at 100% but it will only be 1″ square. It would not have printed well at 4″ before you made the change. No picture information has actually changed and nothing is lost or gained in this resizing process. Only the printed size has changed.

Important Note: This would not be the case if we had not unchecked the Resample Image option. We do not recommend that you resample images when resizing. You cannot improve the appearance of a low resolution image by resampling. Don’t even bother to try. You’ll only end up with a crummy high resolution image that still looks like the low resolution version, only now its file size is larger.

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Image Quality

Images vary considerably in quality. Below is an example of a high quality 300 PPI image.

High Quality 300 PPI Image

We recommend that all files be saved using the TIFF. We do not recommend using JPEG because it is a “lossy” format. That means that to reduce the file size image data is actually thrown away. If you re-save a JPEG file repeatedly you are continuing to throw away more and more image data and the image will degrade further and further as a result. The JPEG format is really intended as a one time use only format. If you must use the JPEG format please set the quality setting to Maximum to prevent uneccessary image degradation.

Next is an example of a low quality 300 PPI JPEG image.

Low Quality 300 PPI Image

This image has been over compressed (JPEG) or has been enlarged (resampled) from a smaller original file. Note the general blurriness and distortion. It looks a bit like looking as though you are viewing the image through water. This is generally due to lower quality JPEG compression. This file will not look any better when printed. At best, pictures copied that have been from the web will look something like this. At worst they will look like the example below.

Low Quality 72 PPI Image

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Using Digital Photos

Most digital photos cannot to be utilized properly at 100% scale. The reason for this is the difference in resolution between your computer monitor screen and the resolution required for effective print reproduction. Because they are intended to be viewed on a computer first, most digital photos are saved by default as 72 PPI images. This is because a computer monitor only needs to display an image at 72 PPI (most Macs) or 96 PPI (most PCs). What looks great on your screen at 100% will not print well at all when scaled at 100%, but it may print very well if used at 25% because the resulting effective resolution is the 288 PPI.

If you want to use photographs from your digital camera in your print job then you need to set your camera on the highest pixel count possible. A higher pixel count (or resolution) will mean better photos, but it will also mean that your camera can store fewer of them, because the individual size of each photo will be larger.

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Special Note About Resolution And Text

When using raster based graphics software such as Photoshop to set type, it is recommended that you set your document resolution to 400 PPI. Even though text will remain vector in native Photoshop documents (.psd) and in .eps files, the file will become rasterized during the production process just as it would when flattening as a .tif or .jpg file. Setting the file up at 400 PPI will insure the text stays crisp.

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