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How many pixels does a photograph really need?

The early HP LasetJet printers produced text and graphic output at a resolution of 300 dots per inch (dpi).  Output from these printers looked sharp and a resolution of 300 dpi was standard for about the next ten years before technology and reduced memory prices made 600 dpi possible.   That original value of 300 dpi was not selected at random.  The conditions necessary for a good viewing experience is a combination of several factors including, print resolution, viewing distance and visual and emotional content. 

Someone with perfect eyesight of can distinguish about 720 ppi at a viewing distance of around one foot.  We can see a maximum of 300 dpi when the viewing distance increases to around two feet.  As viewing distance increases, we perceive lower resolution images to be sharp and a poster or billboard that might be viewed from around sixty feet will be considered as sharp even if printed around 10 dpi.  The choice of 300 dpi was made for printed text where sharp lines and edge contrast are required and expected.  That is not usually the norm for photographs and printing at lower dpi print resolution will give results that appear sharp.  Moreover, modern digital cameras including those in phones have pixels to spare allowing images to be cropped to size without loss of detail.  This lets us increase the recommended print resolution as indicated in the table below.

Print Size
(inch)
Viewing Distance
1.5 x Diagonal
Calculated
DPI
Typical
DPI
Vertical DimensionHorizontal DimensionMegapixels
4 x 67.21313300120018002.16
8 x 1012.81181240192024004.61
8 x 1214.42156220176026404.65
11 x 1417.80156180198025204.99
16 x 2025.6189120192024004.61
16 x 2428.8480100160024003.84
20 x 3030.0664100200030006.00
40 x 6072.1132803200480015.36

Note that if a 4×6 (or 5×7 or smaller) image is included in a page of text, the likely viewing distance will not be the 12-15 inches typical for a snapshot.  Instead, viewing distance will be that comfortable for a page of letter-size paper.  The acceptable resolution for the image will decrease from 300 dpi to around 220 – 240 dpi depending on the subject.

Screen resolution if typically defined as 72 dpi.  However, as anyone who has looked at a new cellphone screen knows, actual pixel density can be much higher. Thankfully, screen dpi is irrelevant and is a just a confusing red-herring!  Image pixels are mapped onto the screen pixels.  If screen resolution for a laptop is 1920×1080 pixels, then that is the required image size to fill the screen without requiring software to enlarge or reduce the picture to fit.  Similarly, if an image is to fill a 4K screen, it will require 3840 x 2160 pixel (four times the size of a 1080p resolution).  Thankfully, most web articles and blogs do not require the full resolution and content management systems like WordPress are geared around images with long edges less than 1600 pixels.  This standard should be maintained to keep image file sizes reasonable and ensure that web pages load quickly.

In summary, most digital cameras, including those in cell phones, produce more pixels than necessary to produce a typical printed image or screen display.  For images that will be included on webpages or included in a PDF read on screen, nothing larger than about 1600 x 1200 is necessary.  Time taken to crop or downsize the original image will be rewarded with faster email upload/download, smaller PDF files and snappy webpage loads.   For stand-a-alone prints destined for display, a little more thought is required but even here, not all those pixels may be required as larger prints are normally viewed from a longer distance.

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