Photo Resolution - by Jae Le
Jae, our technical evangelist in the office, has gone from student to professor to ZenMaster. Some of you may know him from our video tutorials or met him at one of the recent trade shows. Jae has agreed to share some of his expertise with us.
People often ask me how big a digital image needs to be for printing. This is a very relative question, based on variables such a print quality and viewing distance. But I’ll try and answer it first on the basic level of resolution.
First off, I want to make it clear that we are not talking about file byte size. We cannot judge an image based off the file byte size, since most image files are often compressed. But we can judge an image based on the total number of pixels that make up an image, which can be determined by the height multiplied by the width of the image in pixels. These pixel dimensions are usually measured in megapixels.
Now that we have made this clear, it’s easy to understand why most people do not talk about digital images in terms of PPI. They usually only refer to pixel dimensions. This is because you can’t mathematically assume there will be a specific number of pixels in an inch. You simply can’t. They are relative points in space! The concept of “per inch” can really only apply to a physical entity, such as a dot. Thus DPI resolution.
The lab Mpix uses photo imaging equipment that will print with a minimum resolution of 100 DPI. It’s important to understand this is a minimum for the printing equipment, not necessarily a generally accepted minimum for a visually pleasing printed image. That standard is considered to be 300 DPI.
At 300 DPI the eye at an average small photo viewing distance stops distinguishing between dots and creates a resulting shape. Computer monitors were found to work well at a minimum of 72 DPI, Newspaper text at 125 DPI, and more recently 600 DPI has become more of a photography printing standard. But for over a decade we have thought of 300 DPI to be a visual minimum for printing a standard photo.
300 DPI multiplied by 4 inches + 300 DPI multiplied by 6 inches
(300x4)x(300x6)=1200x1800=2,160,000 pixels or 2.1 megapixels when rounded down.
So to answer the question of how big a digital image needs to be for printing, I recommend a minimum printing quality of 300 DPI for any 4”x6” print, which means the pixel dimensions of around 1200x1800 or 2.1 MP. Some may want to take this information and extend it out to larger sized images:
For 4”x6”: 2.1 MP (1200x1800=2,160,000 pixels)
For 10”x12”: 10.8 MP (3,000x3,600=10,800,000 pixels)
For 16”x24”: 34.5MP (4,800x7,200=34,560,000 pixels)
For 24”x36”: 77.7 MP (7,200x10,800=77,760,000 pixels)
So what about a large 24”x36” wall mounted photo printed with heavy color inks on canvas? I would be seen from quite a distance away and the dots making up the photo would be larger with less space between. I think the answer is relative when it comes to best perceived quality, but remember Mpix states they have a minimum printer requirement of 100 DPI.
Now forged Magic The Gathering cards, which are small and held closely to your eye, were often printed at 3000 DPI. Not that I ever accidently bought a forgery, or have ever played Magic the Gathering, or have even been to the Game Stop in San Francisco. I’m too busy counting pixels.
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