Print sharpness depends upon viewing distance & resolution

How Image Quality affects Maximum Print Size.

Clearly, a higher-quality image can be printed bigger than a fuzzier one. But maximum print size is subjective, depending upon your viewing distance and acuity standards. Surprisingly, a giant billboard along a highway can be printed from a little 3-megapixel camera!

Subjectively speaking, an excellent 8 to 10 megapixel camera made back in 2007 can create images for prints perceived as sharp when viewed at a distance equal to the print’s longer dimension or further away. For example, for most people a 20- by 30-inch print from an 8-10 megapixel DSLR camera looks sharp when viewed at about 30 inches.

Further, for a good 3+ megapixel camera, maximum print size is unlimited when viewed from at least twice the print’s longer dimension away. But when viewed closer, a subjective formula for maximum print size is suggested further below. Consider this surprising example:

  • A 20-foot-wide billboard produced from a good 3 megapixel camera can look great viewed from 40 feet away, but can look fuzzier when viewed closer. Different observers will judge the “fuzziness” differently, subjectively. A typical 3-megapixel image might be 2000 pixels wide by 1500 high. When you print that image 8 inches wide (at 250 pixels per inch, or PPI) and view it 16 inches away, it will look just as sharp as the more distantly viewed billboard. Stand 40 feet from the billboard and hold the print 16 inches from your eye, and they will line up perfectly. Both have the same viewing angle relative to your eye and both have 3 megapixels of information, so they will look equally sharp, theoretically.

Image quality directly affects maximum print size, depending upon many factors:

  • shooting with sharp focus
  • sufficiently fast shutter speed to freeze subject motion and prevent shaky hands moving the camera during exposure
  • proper exposure, brightness
  • sufficiently low ISO settings to reduce noise
  • better raw file optimization: in-camera JPEG conversion versus superior raw conversion on a computer
  • better sensor & processor: larger sensor, latest technology (such as backside illumination, BSI), better processor efficiency, higher signal-to-noise ratio
  • good lens quality with large diameter

Formula for minimum print-viewing distance:

The following subjective viewing distance formula for the acuity of the human eye is for well-printed images, from any high quality camera of any megapixel specification. As most prints are viewed at a distance of at least 15 inches, printing a camera’s native pixels at 240 ppi seems sufficiently sharp for most people (or somewhere between 180 and 300 ppi):

Optimal viewing distance in inches = 3500 divided by optimal ppi.

  • For example: a 240 ppi print can look sharp when viewed at 15 inches or greater, which is about the closest that most people look at prints (14.6 inches = 3500 / 240 ppi). In this example, the pixels designated within the file at 240 ppi are assumed to be native (not digitally enlarged), from a well-shot image.
  • Another example: a 10-megapixel Nikon D40X camera captures 3872 x 2592 pixels, native size. Take the longer dimension of 3872 pixels and divide by 240 ppi, which equals a 16 inch print, which should look sharp when viewed at 15 inches or further from your eye. Enlarging this image, by doubling its long side to 32 inches, will look sharp when viewed at least twice as far away (30 inches from your eye). The same native optimal image size enlarged by tripling the long side should be viewed at least 3 times as far away (45 inches).
  • This formula is subjective, meaning your judgment of image sharpness may differ from mine. Adjust your optimal ppi as needed to customize this formula.

Printing tips:

For best prints of up to 48 inches on an ink jet printer, resize the file’s resolution in the range of 240 to 300 ppi (pixels per inch), at your desired length and width. This optimizes the print for the acuity of typical human eyes at common viewing distances, such as for small handheld prints, and also for large prints on a wall viewed a step or two away. Adobe Photoshop’s “bicubic” resizing works great; or use Adobe Lightroom.

If more than doubling a dimension, resize in steps, to avoid imperfections. For example, to increase an image from 16 inches to 48 inches high, resize height to 32 inches first, then resize again to 48. Maintain image proportion of height to width. After resizing, sharpen (or Unsharp mask) the image as the last step before printing.

  • As you smoothly enlarge the image in steps in an editor, keep the file’s recommended 240-300 ppi constant, in order to avoid the appearance of pixelation in the print upon close examination. Enlarging will increase the number of pixels beyond native camera size by the square of the increase in the long side. For example, doubling the long side quadruples the number of pixels beyond native size. Tripling the long side increases by 9 times the number of pixels beyond native size.
  • File ppi versus printer DPI: Setting a file’s resolution to 240 ppi is unrelated to setting the printer’s dpi (dots per inch) for laying down ink, which is a different topic — for final prints, simply choose the printer’s “Highest Quality” setting, which will automatically optimize dpi for the printer’s ink.

Compare cameras:

Seeing is believing. Compare images from two cameras side by side using 100% pixel view at

www.imaging-resource.com/IMCOMP/COMPS01.HTM

  • Compare compact camera images (with sensor size 1″-Type or less) at ISO 400 and 800, where blotchy noise becomes obvious.
  • For DSLR and mirrorless cameras, compare at ISO 1600 and 3200. 

For better quality images, buy the latest-model camera (read my recommendations). I prefer a camera with a larger diameter lens and physically larger sensor which is better optimized than its competitors, as judged when 100% pixel views are compared at ISO 800 or higher. Back in 2007, sensor designs exceeding about 6 to 8 megapixels in small cameras (subcompacts less than 10 ounces) usually didn’t help increase effective print size, due to hardware limitations of tiny lenses and sensors at that time. But by 2016, technological leaps allowed a 16mp sensor in a little Samsung S6/S7 smartphone to make good 18-inch prints! In this example, upgrading to the latest model pays off handsomely.

Historical camera comparison from year 2012: Based on review sites dpreview.com and imaging-resource.com, I rate the following travel cameras on ISO 400 quality, starting with best first, and worst last, with roughly equivalent cameras connected by slash / marks: 

Canon EOS Rebel T3i/T2i/Nikon D5100/D3100 > Nikon D5000 > Nikon D60 > Nikon D40X or Canon EOS 400D Rebel XTi SLR > Fujifilm FinePix S6000fd > Panasonic DMC-FZ50 > Canon PowerShot G7 > Panasonic DMC-FZ8 > Canon PowerShot A710 IS > Panasonic DMC-LX-2 > Canon PowerShot SD700 IS > Canon PowerShot Pro1 (which requires ISO 50 to best the G7).

Look up the LPH for cameras, sometimes reported on dpreview.com: resolvable lines per picture height (sometimes abbreviated LPPH). A camera with higher LPH can make sharper large prints. LPH is a good empirical measure of real resolution of a camera’s sensor for a given lens (independent of pixel pitch or megapixel count). dpreview.com reports the absolute vertical LPH judged by photographing a PIMA/ISO 12233 camera resolution test chart under standardized lighting conditions. Note which lens and camera body was used in each test, and compare with others within the same web site.

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