One of the areas of photography that we find people can get confused about is understanding the difference between the picture they take and what size they can print that picture at. The goal of this article is to explain the concepts of digital camera resolution, DPI and PPI and what that means for getting the most out of your photos..
On a occasion we talk with customers who feel that our printing quality is not what it should be when in fact the issue is that a low resolution image that has been blown up to a size where the photo becomes blurry (our Photobooks editor does warn you when you are pushing things a little too far). So what does this mean? What is resolution and how can an image on my monitor not print out with the same quality?
The answer is a little complicated and it has to do with how cameras capture an image and then how that image is displayed on a monitor compared with printed on a page.
All digital cameras are constructed with a specific resolution that they capture an image at. When the first digital cameras appeared on the scene they were a lowly 2 Megapixel whereas these days even entry level cameras are now at 8 Megapixel. What this means is the number of points of information that are captured by the camera. So in an 8 Megapixel camera there are 8 million individual pieces (i.e. pixels) of light information that are captured. This information in most 8MP cameras is in 3,264 columns and 2,448 rows.
So in order to produce your photo book this is the input information that is then translated to the page.
Straightaway you can see the issue - there is a limited amount of information that is available in order to print the image. If you try and produce the image at too large a size the amount of information per inch (yes, the printing world still primarily works in terms of the number of dots per inch) falls to a level where the quality of the image begins to fall away.
So, for example if you were to produce an 8 megapixel photo full page on one of our A4 landscape album pages (which measures 11.7 inches wide) there will be 3,264 pixels divided by 11.7 inches or 279 pixels per inch that is available to print the image to the page. Quality at this size will be excellent.
However, if you decide to stretch the photo over 2 pages, which means the image will need to be stretched to 23.4 inches wide, your photo will contain 134 pixels per inch (PPI) of information to be used to produce the image. The albumworks Photo Book Editor has an inbuilt warning that warns you if an image falls below 125 pixels per inch of information. This means that if you decide to crop your image (which means you only use a portion of the photo) you may fall below our minimum recommended resolution level and it is up to you as to whether you decide to go ahead with using this photo at this resolution.
You will often see people refer to PPI (Pixels Per Inch) as DPI (Dots Per Inch) and this is where a lot of the confusion springs from. We find that its best to keep these two concepts completely separate.
The reason is that how an image is captured on a camera, produced on a screen and reproduced on a printed page are all completely different technologies but many people, even in the photo and printing industry, use DPI to describe the same thing.
The pixel captured by your camera is a piece of information which can have 256 different intensities in 3 different colours (you may have heard of RBG which refers to Red, Green, Blue). Of course a dot of ink on a printed page can only have one of four different colours (which is called CMYK which refers to Cyan, Magenta, Yellow, Key [Black]). Printing produces an almost infinite rainbow of colours that you see on a printed page by assembling lots of dots of colour 'very' close together. The sum of those dots together work together (are dithered) to create what we perceive as colour. (This is a very simplified description as the science of how to produce colour on a page is a massive topic with many different methods as to how that colour is assembled so that we perceive beautiful colour reproduction, but that is outside the scope of this article).
The printing presses that we use to produce photo books, calendars, posters and so on actually print at 812 Dots Per Inch (DPI), massively greater than the 8 Megapixels that our camera captured but they bear no relationship as they are completely different technologies. For this reason its best to refer to how much information your camera is giving to the printing press to work with (PPI) and then refer to how high a resolution your image is then printed (the DPI) which is a measure of the quality of the printing press.
So, going back to your original 8 Megapixel Photo which you want to print stretched across 2 A4 pages in a photobook the translation is:
* 8 Megapixel (MP) image (measuring 3,264 x 2,448 pixels)
* Stretched across a total distance of 23.4 inches wide x 8.5 inches high
* Producing 134 pixels per inch (PPI) of input information
* Output on a Hewlett Packard Indigo Digital Press at 9,600 Dots Per Inch (DPI)
For further reading on the subject wikipedia has a good entry at http://en.wikipedia.org/wiki/Dots_per_inch with further links to other related topics.
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