Resolution (PPI or DPI) and Image Size (W x H)

*** the most misunderstood concepts in Graphics ***

Resolution - a numerical value that indicates the sharpness and clarity of an image. Difficult to understand because there are several different types of resolution, as follows:

Resolution in terms of Density (PPI and DPI)

Resolution in terms of size (the total number of pixels or dots - W x H)

 For graphics monitors, the screen resolution signifies the number of dots (pixels) on the entire screen.  This translates into different dpi measurements depending on the size of the screen. For example, a 15-inch VGA monitor (640x480) displays about 50 dots per inch.

Calculating your Monitor Resolution

Example - my own monitor is a 22" (diagonal).  The viewable area is 13 3/4" x 11 1/2" (Width x Height).  I keep the display resolution size set at 1024x768 pixels.  At that setting, it displays a 500x500 pixel image at 8" x 7 1/4".  So as with most monitors the pixels are rectangular, and are wider than they are tall.  Therefore:

But we will use width as the predominant factor and round down:

my monitor resolution = 62 ppi


How the External devices deal with Resolution

Printers, monitors, scanners, and other I/O devices are often classified as high resolution, medium resolution, or low resolution. The actual resolution ranges for each of these grades is constantly shifting as the technology improves.

Scanner resolution differs from printer resolution in regards to how it relates to the PC's pixels per inch (ppi).  

Forget about DPI unless Printing - in reality, the industry should have never referred to scanned images using DPI.  DPI has always been a term used solely with printing.  Scanners use ppi, since they bring in an image as a bitmapped image file, and ALL bitmapped images have a resolution that is ALWAYS measured as PPI. 

The best way to think of your scanned images is to use PPI and just forget about DPI unless you are dealing with printing.

If printing - make sure Scanner DPI = Printer DPI:  if you are thinking of printing, and plan to print the scanned image - then it is best to scan the image at the same resolution as you will be printing it at (typically 300 dpi).  However this does create huge files, so if you will be saving the image, then just go ahead and scan it a a lower resolution such as 150 or 96.

So with scanners, dots do equal pixels !!  Remember, with scanners, dpi=ppi !!  If you scan a 4x4" image at 72 dpi, and your screen resolution is set to 72 ppi, then the image will take up 4x4" on your screen.  However, often, it is better to scan at higher resolutions than that - especially for high detail nature images, or people.  Therefore - the resultant image on your screen will be huge, and you may be only able to see a small portion of it at a time.

I believe, in general, you may as well go ahead and scan in high resolution - I use 150 dpi usually, and use 300 dpi for the large, nature images that I scan for Windows backgrounds.  It does result in a huge file size - 10 MB up to 30 MB.  However, you will be reducing the image size after the scan, within your software - through "resampling".

The problem of scanning at lower resolutions - even if that is the final resolution you want - is in the resizing.  Resizing a huge image to a smaller image results in clear, fine detail.  Resizing small-to-medium images always degrades the quality substantially.  For example, a 100x100 image, resized to 90x90, has to use a very small number of pixels, and interpolate them to the new size.  Conversely, a 1000x1000 image, resized to 90x90, has thousands of dots to work with in it's interpolation algorithm, and can much more accurately depict the final image.

You lose nothing by scanning at high resolutions - just adds a short amount of extra time to the scan.

Should I change the dpi (which with scanners is really ppi) after scanning ??  depends on what you will be doing with the image.  If you just want to save family pics on your PC for viewing on your PC, then there is no need to mess with the dpi.

Remember, the ppi of the image is only a value used when the image is sent to the printer.  "Some" applications, such as Word, will display images as WYSIWYG (What You See Is What You Get), which means it displays exactly what will come out on the printer.  So Word will show a 300 ppi image that is 200x200, as being smaller than a 72 ppi image that is 200x200.  

Microsoft Word image Sizes and Printouts

All image viewing applications, and for that matter, most computer application, ignore ppi - it is a printing concept only !!  The only parameters that your PC cares about, when displaying an image, is the dimensions of the image in pixels (Width x Height).  

Changing the Display Size and/or Print Size of an Image

to make the image appear on your screen in the size you want, you must "resample" it, using Photoshop or some other image editor - to the lower dimensions that you prefer.  

to make the image appear on your printer ins the size you want - do not resample - instead, using Photoshop, go to Image/Image Size, uncheck the "Resample" box, and change the ppi - you will see that this effects the "Document Size" (the print size):


Computers ignore PPI !!

Here is proof that the PC ignores the ppi of an image - these images also appear in the Monitor 2 section, since it is a very important concept - which few understand.  Feel free to download these by right-clicking on them, and open them in an imaging application.  You will see that they are at 72 dpi and 300 dpi respectively - if you insert them into a WYSIWYG application such as Word you will then see their size differs greatly, because Word shows you the size that they will print out at.  Nevertheless, the dimensions of both are 200x200 pixels, so they are identical on the screen, and the files sizes are also identical (each is 15 kB) :


  Image1 - 200x200  72 ppi                                 Image2 - 200x200  300 ppi 

*** these look the same here because Web browsers are not WYSIWIG for images - they display the image at it's dimensions - and ignore the ppi.  

So how will these print?? (my printer default to 300 dpi)

Image 1 should print out larger than Image 2, because it is less "dense".

Image1 is 200x200 pixels, at 72 pixels per inch, and therefore it will print out at a size of 200/72 = 2.77x2.77 inches.  Since the printer lays down 300 dots of color per inch, this image prints at 2.7 x 300 = 833x833 dots.

Image 2 is 200x200 pixels, at 300 pixels per inch, and therefore it will print out at a size of 200/300 = .666x.666 inches.  Since the printer lays down 300 dots of color per inch, this image prints at .666 x 300 = 200x200 dots.  Does 200x200 sound familiar?  Well, the ppi of the image = the dpi of the printer.  Therefore the printer prints out the image with the same number of dots as there are pixels.

OK - then prove it !!  Print them !!

No problem.  I printed both, tore off the excess paper, and took a snapshot so you can see.  Even though the printer is "set" to 300 dpi, it does take note of the image ppi setting, and the image dimensions.  It printed both of these these at 300 dpi, which is the preset physical number of sots it will lay down per inch.  But it sized the images according the their ppi (72 and 300 respectively).  


When you send a job to the printer, it will send it at whatever dpi you have configured in the printer "properties" box.  That is a "physical" concept, and the 300 dpi is just the density of the print job.  The printer will also look at the ppi and the dimensions of the image, and size the job accordingly.


NOTE:  since all printed images are sent to the printer at a high resolution (most printers default to 300 dpi and have options for 600 or even 1200 dpi), it would seem that it is best to use images that have a ppi equal to the dpi so that no conversion will take place (conversions generally always result in at least some loss in quality).  But in "general", so long as the image is crystal clear on the screen - it should be crystal clear on the printout.  For small, detailed jobs, however - such as business cards, you should use a ppi equal to the dpi of your printer.  300 dpi is more than enough for any image IMO !!  Print shops make their $$$ based on quality - so they would never think of using images at 72 ppi.


Sample Images at Varying resolutions and Sizes

*** all measurements done on my monitor, which was set at 1024x768, and is a 13 1/2" wide.  It will display a 500x500 pixel image at 8" x 7 1/4".  So as with most monitors the pixels are rectangular, and are wider than they are tall.  Keeping with this study, where I only look at the width of the images, we will ignore the height for the purpose of simplicity.  Therefore, my monitor "width resolution" = 500 pixels/8 inches = 62 ppi


Base File to be used for Comparison
The Pixel Dimensions Width and Height are the dimensions of the file - also called it's "Display Size"
The Document Size WxH is the actual printed size
The resolution is in PPI and is stored in the image file - no effect on print size


Increased the ppi, but left the display size (pixel dimensions) the same
Note that the image on screen size stays the same, but it prints out smaller


Increased the ppi, but left the document size (print size) the same - which forced the display size WxH to increase
Note that the on screen size is bigger, but it prints at the same size


Increased the document size (print size), but left the ppi the same, which forced the display size WxH to increase also
Note that the image on screen display size is bigger, and it prints out bigger