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A real verdict, not a score
Many so-called quality checkers return a percentage or a star rating with no explanation of where the number came from. This checker does something different: it computes the effective resolution of your image at your print size, works out the resolution your eye can actually see at your viewing distance, and compares the two. The verdict has three levels. Sharp means the print will look crisp at that distance. Acceptable means it will look fine at a glance but fine detail may soften on close inspection. Too low means it will look soft or pixelated, and you should use a larger image or print smaller.
The viewing distance is the key input most checkers ignore. A billboard looks sharp at 30 DPI because nobody stands 30 centimetres from a billboard. A photo book page needs 300 DPI because readers hold it close. The same image can be sharp on a gallery wall and too low in a book, and only the distance decides which verdict applies.
How to use the checker
- Enter the image size in pixels, width and height.
- Enter the intended print size in inches, width and height.
- Choose the viewing distance that matches how people will actually see the print.
- Read the effective DPI, the DPI your eye needs, the verdict, and the pixel size that would reach the sharp level.
How the verdict is calculated
First, the effective DPI is the pixel count divided by the print size in inches, computed separately for width and height, and the lower of the two is used, because the weaker direction limits the whole image. Next, the required DPI comes from the resolving power of the human eye, about one arcminute of detail. At a distance of d inches, the smallest detail the eye resolves is about d divided by 3438 inches, so the required DPI is 3438 divided by d. At 12 inches that gives about 287 DPI. At 36 inches it gives about 96 DPI. At 120 inches it gives about 29 DPI. If the effective DPI meets or beats the required DPI, the verdict is Sharp. If it reaches at least 60 percent of the required DPI, the verdict is Acceptable. Below that, the verdict is Too low.
A worked example
Take a 3000 by 2400 pixel image printed at 10 by 8 inches and viewed from 12 inches away. The effective resolution is 300 DPI in both directions, and the eye needs about 287 DPI at 12 inches, so the verdict is Sharp. Now print the same image at 20 by 16 inches and view it from the same distance. The effective resolution falls to 150 DPI, which is about 52 percent of the required 287 DPI, so the verdict is Too low. Move back to 36 inches, where the eye needs only about 96 DPI, and the same 150 DPI print becomes Sharp again. Nothing about the file changed; only the distance changed, and the verdict changed with it.
The checker also tells you the pixel size that would reach Sharp at your print size and distance. In the 20 by 16 inch example at 12 inches, you would need about 5730 by 4584 pixels. That number is honest and sometimes unwelcome: it may mean the photograph you have simply cannot print that large at close-viewing quality, and the right move is a smaller print or a different image.
What the verdict does not cover
- Sharpness of the original photograph: a blurry photo stays blurry at any DPI.
- Colour accuracy, which depends on the printer, the paper and the colour profile.
- Compression damage: a heavily compressed JPEG can show artefacts even at high DPI.
- Viewing light: prints viewed in dim light hide flaws that bright light reveals.
The verdict answers one question well: is this image big enough for this print at this distance? For that question, the arithmetic is exact and the answer is trustworthy. For everything else, look at a test print. Print a small section at full size on the real paper, hold it at the real viewing distance, and judge with your eyes. The checker tells you where to start, and the test print confirms it.