How Many Pixels Make A Good Print?
One of the more common dilemmas for people is choosing the paper size for printing their photos. Everybody knows that if your digital camera does not produce enough pixels (or actually megapixels) printing its photos on a large paper size will yield poor quality and you will be able to see the actual pixels (also known as pixelation)
So how many megepixels do I really need in order to print on a specific paper size? there is no one right answer for that. The actual quality of the print depends on many factors other than the number of pixels. For example the paper quality itself the printing process that is used the lighting conditions when the photo was taken the photo itself (i.e. portraits are different than scenery) and much more.
However a rough estimation of how good a picture will be based on the number of pixels can be calculated and is actually pretty easy to do. When evaluating how good a print will be there is a measurement that is simple to use and provides a good estimation for the quality – it is called DPI (dots per inch). DPI is actually the number of pixels along one inch. To get a good print you would need a certain DPI (on both X and Y axis).
Experiments show that the following qualities are usually associated with a specific DPI number:
DPI 100 – fair to bad
DPI 200 – good
DPI 300 – very good
So all we need to do now is to figure out for each paper size how many megapixels translate to those DPI numbers. To calculate this we need to simply multiply the page length by its width in inches. The result is the number of square inches on the page. Now multiply this number by the square of the DPI number and the result is the number of pixels on the page which is the number of pixels we want our source photo to have. Here are the numbers calculated for some common sizes (for 100,200 and 300 DPI respectively):
page 4X6 0.24MP 1MP 2MP
page 5X7 0.35MP 1.5MP 3MP
page 8X10 0.8MP 3MP 7MP
page 11X14 1.5MP 6MP 14MP
page 16X20 3MP 12MP 28MP
page 20X30 6MP 24MP 54MP
Again we would like to emphasize that these are just ballpark numbers. Factors like the ones mentioned above and also like the compression ratio used (low or high compression) and the aspect ratio difference between the paper and the camera can result in a need for more or less pixels. Our best advice is if you are not sure just try one or two photos before printing a large batch.
About the author:
Ziv Haparnas is a technology veteran and writes about practical technology and science issues. You can find more information about photo printing on http://www.printrates.com