Print sharpness depends upon viewing distance & resolution

How Image Quality affects Maximum Print Size.

A higher-quality image can be printed bigger than a blurrier 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 3-megapixel camera!

I’ve found that a good 8 or 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, a 30- by 20-inch print from an 8 megapixel DSLR camera looks acceptably sharp when viewed at about 30 inches. As of 2017, midsize cameras having 20 to 24 megapixels can make 30-inch prints that look sharp at about 18 inches away.

In general, for a good shot on a 3+ megapixel camera, maximum print size is unlimited when viewed from about twice the print’s longer dimension away (or further). Further below are subjective sharpness formulas suggested for determining optimal viewing distance and corresponding maximum print size. Consider this surprising example:

  • A 20-foot-wide billboard produced from a good shot on a 3 megapixel camera (2000 x 1500 pixels) can look great viewed from 40 feet away, but may subjectively look fuzzier when viewed closer (according to formulas suggested further below). Now print that same image 8 inches wide (at 250 pixels per inch, or PPI) and hold it 16 inches away from your eyes, while standing 40 feet from the billboard. Both of these prints have the same viewing angle relative to your eye and will look equally sharp, despite the huge difference in print size (40 feet wide versus 8 inches).

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

  • emotional impact of the subject matter
  • sharp focus where needed on the subject
  • sufficiently fast shutter speed to freeze subject motion and prevent shaky hands moving the camera during exposure
  • appropriate exposure, brightness
  • sufficiently low ISO setting 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 optimal (minimum) print-viewing distance

Because most prints are viewed at a distance of at least 15 inches, printing a camera’s native pixels at 240 Pixels Per Inch (PPI) appears sufficiently sharp for most people (or somewhere between 180 and 300 PPI).

The following subjective viewing-distance formula for human eye acuity is for well-printed images, from any modern high-quality camera of any megapixel specification (preferably with 1-inch Type sensor or larger):

Optimal viewing distance in inches = 3500 divided by optimal PPI

or: 14.6 inches = 3500 / 240 PPI

  • Example A: 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. 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.

This formula is subjective, meaning your judgment of image sharpness may differ from mine. Adjust your preferred optimal PPI as needed to customize this formula.

Formula for maximum print size when viewed as close as 15 inches

Maximum print size = longest dimension in native pixels divided by optimal 240 PPI 

  • Example B: a 24-megapixel Sony A6300 camera captures 6000 x 4000 pixels, native size. Take the longer dimension of 6000 pixels and divide by the optimal 240 PPI, which equals a 25 inch print, which should look sharp when viewed at about 15 inches or further from your eyes. Maximum print size and minimum viewing distance have a linear relationship: for example, doubling the observers’ minimum viewing distance to 30 inches lets you print the longer dimension up to 50 inches for this 24 MP camera.
  • Example C: a 10-megapixel Nikon D40X camera captures 3872 x 2592 pixels, native size. Take the longer dimension of 3872 pixels and divide by the optimal 240 PPI, which equals a 16 inch print, which should look sharp when viewed at about 15 inches or further from your eyes. Enlarging this image by doubling its long side will look sharp when viewed at least twice as far away (30 inches from your eye). Tripling the long side should be viewed at least 3 times as far away (45 inches) to avoid subjective blurriness.

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 in Photoshop, resize the image in steps to avoid introducing imperfections. For example, to increase an image from 16 inches high to 48 inches, resize height to 32 inches first, then resize again to 48. Maintain image proportion of height to width. After resizing, sharpening (such as Unsharp mask) is the last step before printing, to optimize human perception of edges.

  • 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 of APS-C size and larger sensors, compare test images of the same subject shot at ISO 1600 and 3200. 

For better quality images, buy the latest-model camera (read Tom’s 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.

In an impressive 2016 feat of miniaturization, daylit-image quality from the pocketable 20-megapixel Panasonic LUMIX ZS100 camera (buy at Amazon) can rival all of my cameras used over 34 years through 2012 (beating my cameras up to 4 times heavier, up to 11x zoom range, up to 12 megapixels, at base ISO 100).

Historical camera comparison from year 2012Based 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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