300 PPI and Output sizes

“The magazine that is going to publish my pictures says the images must be at least 300 ppi.  How do I change the resolution to 300 ppi?”

Editorial Note: While DPI 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).

I’m asked this question a lot and it is based on a misunderstanding that I hope we can clear up in this newsletter.

First of all, it is rare that you need to do anything.  They are requesting an image with a resolution of 300 ppi when printed to a specific size.  A request for a 300 ppi or any ppi without any associated width and height dimensions of the print it is nonsense.  The dimensions of the print need to be know.

The Formula:  

Width x 300 = a
Height x 300 = b

Multiply a times b to determine the minimum size mega pixel camera needed to make the photo.

Here’s an example for an 8 x 10 inch print.  Width 8 x 300 ppi = 2400.  Height 10 x 300 ppi = 3000. 

You need an image of at least 2400 x 3000 pixels.  Multiply 2400 x 3000 = 7,200,000 or 7.2 mega pixels.
Now we know the minimum size mega pixel camera that’s needed, but how do we change resolution?  Photoshop to the rescue…. again.
The dialogue to change the ppi-number in Photoshop is Image->Image Size.  When you choose it, you get a dialogue box like the one shown Example 1.  Be sure the Resample Image checkbox is left unchecked.  This prevents accidentally throwing away pixels that will diminish the quality of the image.

The Image Size box is a resolution calculator.  Your camera manufacturer has a default setting.  It may be set at 72 ppi or something else.  In this example the default setting is 240.  Remember you can change this to 300 ppi and it will then tell you the maximum size print you can make with this image.  As long as the Resample Image checkbox is unchecked you will not accidentally damage your image.

Take a look at Example 2.  Here you can see that the Resample Image box is unchecked and the Resolution was changed to 300 ppi.  Note: the Width and Height remain the same in Pixel Dimensions.  The actual image didn’t change.  You now have a calculator that tells you the size print you can make at any ppi.

There are many different output devices.  Let’s look at printers. Canon and HP desktop inkjets are usually optimized at 600 ppi, for Epson it is 720 ppi. The Fuji Frontier, used in many minilabs, is usually optimized for 300 ppi, some laser jet printers offer a choice between 200 ppi and 400 ppi.  You should refer to the manufacturer’s specifications or ask the lab to determine the optimum resolution for a specific printer.

You need to know the optimum ppi for your printer to determine the largest size print you can make.  Just be sure the size image is not larger than your printer’s quality capabilities.  You want it large enough, but there is a slight chance it can be too big.
An often forgotten aspect of quality prints is the distance from which the prints will normally be viewed.
The viewing distance changes everything.  You do not need a 300 ppi image to produce a billboard.  Actually you only need about 6000 total pixels for a Billboard. 
Use the chart below to help you in making display prints for trade shows or other places you need extremely large images.

Determining PPI by Viewing Distance

To determine an optimal PPI (resolution in Photoshop) for a photo the calculation is as follows:

7000 / viewing distance in inches = PPI

This is for optimal results.  Using half that number will still provide good quality results. Any lower and you will begin to sacrifice image quality.

A Quick Chart for Determining PPI by Viewing Distance

Under 24 inches = 300 PPI

24″ to 36″ = 200 PPI

36″ to 60″ = 120 PPI

5′ to 10′ = 60 PPI

10′ to 20′ = 30 PPI

Billboard = 10 to 20 PPI

I hope this bit of information about ppi, image size, viewing distance and printer capabilities has been of use to you.
Nothing is EVER simple, right?