At ScanDigital I am constantly working with our customer service representatives to understand the questions our customers are frequently asking. One of the more common topics of conversation relates to the confusing world of resolution and how this is measured. Hopefully this post will shed some light on the topic.  My apologies in advance for a somewhat technical post, but I promise the next one will be much lighter reading!

First, let’s start with the commonly used terms: DPI, PPI and Megapixels. These terms are not as confusing as they may sound. In the common vernacular (and even on our website) we refer to resolution in terms of DPI or “Dots Per Inch”. What we really mean when we refer to DPI when speaking about digital images is PPI or “Pixels Per Inch”. DPI’s origin lies with traditional print methods which use patterns of dots to render photographic images on a printed page. While pixels on a monitor are square and in contact with the adjacent pixels, printed dots have space between them to make white, or no space between them to make black.

A digital image is simply a certain dimension: “x” pixels wide by “y” pixels tall. If you divide each dimension by 300, you will have the size of the image at 300ppi. When a digital image is prepared for print the pixels are converted to dots. Dots have spaces between them. 300 pixels become 150 dots and spaces, so 300ppi becomes roughly 150dpi and 150dpi is the accepted standard for printing photographic quality images.

However, it is also important to remember that everything is relative to viewing distance. 150dpi (or 300ppi) is accepted as photo quality because the average person cannot see the “dots” at a few inches away. When you move the viewer further away from the printed material, lower dpi is acceptable. A huge billboard might be printed at only 40dpi but no one notices because everyone is 50 yards away from it.

When we talk about viewing images on a monitor 72 dpi is often discussed. Technically speaking, referring to 72dpi should really be 72ppi because it has nothing to do with printing. On a computer display, a 72 pixel by 72 pixel image should take up about one inch of space on the screen. (This of course varies based on the size of the monitor and what resolution it is set to.) As a result, whenever we are asked about resolution on a customer call it is important for us to understand the intended use of the digital files. If a customer is going to only use them for digital viewing on a monitor or television, then our higher resolution option is completely unnecessary and will not be at all noticeable on a monitor. However, if the customer wishes to order large reprints than the higher resolution scan may make a huge difference in the print quality.

So, what about megapixels?

Converting dpi or ppi into megapixels simply requires a little math.  A megapixel is equal to 1 million pixels and is a measurement of area.   So lets assume a customer has a bunch of slides or negatives that they want to convert to digital format.  How many megapixels will the resulting image be?  A 2000 ppi scan of a negative or slide results in an image with an approximate size of 1,900 x 2,840 pixels or 4.7 megapixels (a very high quality image).

I hope this helps explain some of the confusion surrounding these terms!

This entry was posted on Friday, September 21st, 2007 at 2:31 pm and is filed under digital photography. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.