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 mega-pixels) 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 mega-pixels 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 from scenery) and much more. Another factor is the pixel quality/sharpness – you can make great and big prints from Sigma Foveon based camera files due to their special construction.
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 PPI (pixels per inch). PPI is actually the number of pixels along one inch. To get a good print you would need a certain PPI (on both X and Y axis).
Experience shows that the following qualities are usually associated with a specific PPI number:
- PPI 100 – fair to bad
- PPI 200 – good
- PPI 300 – very good
Of course we only would like to do very good prints, so all we need to do now is to figure out for each paper size how many mega-pixels translate to those PPI 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 PPI 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:
|9 x 13||0.7||1.6|
|10 x 15||0.9||2.1|
|13 x 18||1.5||3.3|
|20 x 30||3.7||8.4|
|30 x 45||8.4||18.8|
|50 x 75||23.3||52.3|
|60 x 90||33.5||75.3|
Again we would like to emphasise 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 fewer pixels.
Our best advice is if you are not sure just try one or two photos before printing a large batch.
Title image credit: rawpixel.com