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How Many Pixels Do I Need?or How Large Can I Print? Version 1.1, © 2005 by Dale Cotton, all rights reserved This is one of the most common questions on photography forums. If you are thinking of buying, for example, a 5 megapixel digital camera, you want to know how large a print you can expect to be able to produce from its image files. Unfortunately, we can't simply say 8x10" or 16x20" or any such single answer. Assuming an excellent lens and flawless technique, maximum print size still varies enormously depending on your subject matter and how close you are to your subject. It also depends on how acute the viewer's close vision is and how close she gets to the print. Billboards are routinely printed at 15 pixels per inch - which would make a 5 mp image weigh in at 10x14 feet (3x4 meters)! - but assumes the viewer is many yards away. For the following discussion, however, we'll assume excellent vision and close inspection (12" or 30cm from eye to print). Pixels per inch is an essential concept for this discussion. For example, my Pentax DS is a typical 6 megapixel dSLR, which means it produces image files that are 2000 pixels by 3000 pixels in size. If I tell the printer I want a print that is 10 inches wide, the printer simply divides 2000 by 10, which tells it to use 200 pixels per inch. The length of the print will then be 3000 divided by 200 or 15 inches. Consider the following close-up portrait:
A 5 mp image like this can be enlarged almost indefinitely. Certainly, a 16x20" (40x50cm) print should be possible. Now consider the opposite extreme:
In this picture the fine detail of the tree branches and houses needs to have absolutely crisp edges. I went through several sheets of paper printing the 6 megapixel original of this image to narrow this down. A 9x13" (22x33cm) print is crisp; a 10x15" (25x40cm) print begins to show marginal enlargement blur; anything larger will not pass inspection by anyone able to read a typical paperback book. In fact, you may have difficulty even getting to 9x13" - here's what I did:
Everyone will come up with her own upsampling and sharpening recipe; but I doubt it is possible to eek out more than a few percentage points further enlargement out of such an image. So, if you want a simple answer to how large you can print while still maintaining reasonable quality at close inspection: the best that I can give you is: For photo-realistic prints you will need a minimum of 220 pixels per inch for your most highly detail-dependent images to a minimum of 120 pixels per inch for your least detail-dependent images. (These ppi numbers are before any upsampling.) This translates to:
Please remember that these numbers are meant as guidelines, not decrees. Also keep in mind that detail rendering will improve by printing smaller than maximum size. 300 pixels per inch (uninterpolated) is usually considered to be the finest print resolution that the human eye can fully resolve, so I used this figure to calculate the ideal print size column. Old timers might think of this as contact print quality. And of course printing at even higher pixels per inch for even smaller print sizes will retain the impression of extremely fine detail and may give the impression of even greater fineness of detail. |
