DPI or PPI ???

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DPI / PPI  What does it all mean?


There are a fair number of numbers in this article, but stick with it.  It is not nearly as difficult as it first appears.


Dots or Pixels per inch are often used interchangeably but they mean different things.
PPI are the number of pixels available and, with camera sensors a good rule of thumb is more = better, however more also = bigger file size = slower editing and more storage.   Also, for web images = longer loading times = “a bad thing”.


A camera sensor may be 8,256 pixels wide by 5,504 pixels tall.  Multiplying these gets 45.5Mp (Do not get confused between Mp (Megapixels) and Mb (Megabytes).  Mp are the number of pixels across your image (or sensor) times the number of vertical pixels.  Mb are how much space that image takes up in your memory, and is different for every image type (TIFF, JPG, RAW etc.).


DPI are the number of dots you ask the printer to print per inch - usually 300 is considered the industry minimum requirement for photographs.


300 also depends on the paper used and, although used as a standard, on glossy paper (where the ink sits on the paper) 300 is good but for matt paper (where the ink soaks into the paper slightly) 200 is probably indiscernible.


So, thinking about screens, 1600 @72ppi and 1600 @ 300ppi are the same thing. The @???ppi number is meaningless for screens, screens will display 1600 pixels at whatever the SCREEN resolution is, (and how you zoom in and out) not the picture resolution.


Coming on to printing 1600 @72dpi and 1600 @ 300dpi are VERY different.  72dpi is fairly blocky and you will probably see artefacts the image. It will be 1600/72 = 22.25 inches long. 300dpi is fine printing and you will get an image 1600/300 = 5.3 inches long.


Looking at size, the default (uncropped from the camera, see below) is a hefty 38Mb whereas scaling that to 1600 wide (by 1067 (calculated by the image program) at either 72 OR 300 dpi) is a much more manageable 2Mb.  Why then would we have a camera that has so many ppi?


8256x5504 (divided by 300dpi) gives a mahoooosive default size of 27.5 x 18.4 inches - A3 paper is 16.5x11.7 to put that in context. Alternatively you could ask the printer to use 500dpi and get an excellent quality A3 print on aluminium (totally non-absorbent) for example.


Knowledge Image 1

Also having spare pixels allows you to crop your image and still have enough for quality printing.  This is the crop I chose (your milage may vary) coming down to an image 5072x3504 which, dividing by our now standard 300dpi gives a great print up to 16.9x11.69 inches - OH! Look at the requirements above for A3 printing (16.5x11.7 inches) :-) What are the chances?


You can see the face is far more visible and is the better image of the two, but still retaining enough pixels to print at a high resolution.


Knowledge Image 2

To keep file sizes manageable, keep us all to one standard (and help webmasters displaying LOADS of images) the UK wide camera club standard is 1600 pixels wide (for landscapes) OR 1200 pixels tall for portrait shots.  This also means if anyone steals your image they have a rubbish resolution for printing! 


Please DO NOT save your original image at this resolution because if you want a different crop, or you want to print it, you will need the higher resolution.  Always keep the original as read-only and save copies of changes you make.


To save you firing up your calculator here are some standard photographic paper sizes IN PIXELS assuming 300dpi;


Knowledge Image 3

Finally don’t get too hung up on 300dpi.  


If you divide the pixels you have by the print size you want (in inches) and come up with say 283dpi don’t sweat it - you will hardly notice the missing 17dpi.  


Think of 300 as an “ish” number not an absolute.