I decided to update the article somewhat and offer it here on my blog. Hopefully it will be helpful to more people this way. If you notice any inaccuracies, please bring them to my attention so that I can have this article as complete and accurate as possible.
If you have any further questions, please feel free to leave a comment. I will update this post with additional information as needed.
Many people find themselves with a certain level of confusion when it comes to the terms surrounding digital imaging. The biggest confusion seems to relate to resolution and pixels.
A Pixel, by definition, is a picture element, the smallest possible part of the picture.. Resolution is normally expressed in terms of pixels, and refers to the fineness of detail. The more pixels you have, the higher your resolution is.
There are four main types of resolution, which leads to some of the confusion. They are: image, screen, output and printer resolution. It is important to understand how they relate to each other to achieve the best results with your digital images.
Image resolution is most commonly referred to in megapixels (million pixels). You can purchase cameras with 10, 12, 14, and even higher megapixel counts. This pixel count, when paired with your output resolution, lets you know how large of a print you can make. Your true pixel count can be found by multiplying the number of vertical pixels by the number of horizontal pixels.
Screen resolution is the number of pixels that particular screen is able to display. Even the newest 3 inch digital camera screens usually have no more than 150K-230K pixels, making them poor displays for truly judging the quality of the photo you just took. Some of the higher end cameras now have screens with 900K to 1 million pixels, which does indeed result in better on camera viewing. Computer monitors generally have the capability of using a number of different resolution, generally referenced by the number of horizontal pixels by the number of vertical pixels. My 17” monitor is set at a resolution of 1024x768, or a total of 786,432 pixels. The multi-megapixel images taken with the digital cameras of today have to be scaled down, or shrunk, to fit completely on a screen.
Output resolution is normally expressed in DPI (dots per inch), or, more accurately, in PPI (pixels per inch). This allows you to determine how high your image resolution must be for a particular print size. Optimal output resolution varies somewhat by personal preference, but generally is in the 200 to 300 ppi range. Anything higher than 300 ppi is unnecessary. It is possible to get acceptable prints at slightly lower settings(150 or 180 ppi), but these should be reserved for a last resort.
Not everyone will agree with me in regards to what output resolution is optimal for print quality. It's also important to note that as your print size goes up, your viewing distance should also increase. This increased viewing distance gives you more “play” with the output resolution since you will not be as likely to discern the individual pixels at that distance. When I am printing posters for customers I generally don't recommend making the print if the output resolution goes below 100 ppi.
Printer resolution, when referring to ink jet printers, is correctly referred to in dpi. This is the number of individual ink drops that are laid down on 1 inch of paper. A printer with 5760 x 1440 dpi is capable of laying down 8,294,400 drops of ink in one square inch. These numbers do not correlate with output resolution. Continuous tone printers, such as dye sublimation or, better yet, the fancy schmancy digital printer at The Shutterbug (where I work), do use output resolution (usually 240-300 dpi) as the printer resolution.
For the purpose of printing we need to be concerned with the image and output resolutions. Simply multiply the dimensions of the print you want by the output resolution that you want to use. A 5x7 at 300 ppi comes out to 1500 x 2100 pixels. Multiplying this to get the total pixel count gives us 3,150,000 pixels (3.15 megapixels). By using a slightly lower output resolution, 200 ppi for example, we can still get an acceptable 5x7 print from a 1.4 megapixel image.
In Photoshop, the Image Size command makes it easy to see what your image and output resolutions are set at. Keep the “resample” box unchecked and you can play with various output settings to see what size prints you can make with the image you have.
Use the highest output setting possible (up to 300 ppi) to get the print that you want. Beware though of trying to get a larger print by using a lower output resolution. Try to stay at or above 200 ppi at all times.
The digital printer at The Shutterbug uses the 300 ppi standard. With that in mind though, the Kodak digital printing kiosk, which provides a resolution warning for images that are too low of quality for a good print, will allow images set at 150 ppi, or higher, with no warnings. Keep in mind that if you need to print with that low of an output resolution, you may not be happy with the results.
Resolution can be confusing, but if you understand the difference between image, screen, output, and printer resolution, you will be on your way to doing more with your photos.