John M. Setzler, Jr. Life in Black and White

Understanding Resolution

I run into a lot of discussion of resolution in terms of digital photography.  It seems that there is a great misunderstanding of what it means and when it matters.  Most of the discussion starts when a photographer looks at image size information in editing software, such as Adobe PhotoShop, and finds that a photograph is being represented there as 72 dpi, or some other number other than what they think it should be.  Comments like "My camera is only taking 72dpi photos!" start to fly.

A digital image file is a collection of pixels produced by a digital camera.  The number of pixels produced is fixed.  It does not vary.  A six-megapixel digital camera produces an image file that contains roughly 6-million pixels.  For the purpose of discussion, we will use the six-megapixel image file that is rectangular, containing 3000 pixels horizontally and 2000 pixels vertically.  Every time we press the shutter release on this camera, the image file that is created will be this size. 

What is the resolution of this image file?

The answer is very simple.  It has no resolution.  Resolution does not exist... yet. 

Let's define a few terms:

• DPI - Dots per inch (same as PPI - Pixels per inch)
• Pixels - digital dots that make up a digital photograph
• Resolution - the relationship between the number of pixels and the amount of space between them

The amount of space between pixels on a computer screen doesn't change, so the resolution of your digital image file simply doesn't matter when you are viewing it on the computer screen.  All that matters is the total number of pixels in the image.  When you print a digital photograph, resolution plays a huge role.  If your pixels are too far apart (lower resolution), the photograph may not look very nice.  If the pixels are very close together (higher resolution), the photo will look better.

We have to consider image resolution when we are ready to print.  We have a digital image file that is 3000 x 2000 pixels in size.  When we make a print, the first point of consideration will be the size.  We have 6-million pixels to put on paper.  We can spread them out and make a large print, or we can pack them tightly together to make a smaller print.  The following chart shows the resolution of our 3000 x 2000 pixel image at various print sizes:

The number of pixels in our image does not change.  The only change is the size of the printed image.  Changing the image resolution only changes the spacing of the pixels when the image is printed.  It has no impact on the image being viewed on the computer screen.  Higher resolution (more dots per inch) is generally preferred when printing. 

WHAT RESOLUTION SHOULD MY IMAGE BE FOR PRINTING?

This is a more difficult question than we might imagine, and there is no easy answer.  The generally accepted rule of thumb is 300 dpi.  With today's printing technology, you can't really go wrong with this figure.  If you want to really investigate this question, you need to be familiar with the printer that will be used to print your photograph.  Some printing systems like more or less resolution.  For photographs, a common printing system in use today is the lightjet printers like those found in most of the 1-hour digital labs.  Those systems don't usually require 300 dpi to get an excellent print.  In fact, a lot of them will automatically convert your image file to 150 dpi before printing.  Inkjet printers are capable of higher resolution printing, but it's really difficult to see any visible difference between a 300 dpi print and a 600 dpi print, unless the print is relatively large.