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Image Size & Resolution for printing and screen viewing:

This is an issue that perplexed me for the longest time when I would try to create a picture for my website or print.  Every time I would make a picture that looked good for printing it would look huge if I added it to my website, and pictures I downloaded from the web would print out really small if I didn't want them to look pixely and jagged when I printed them.  After a lot of tinkering and reading on the web, this is what I learned:

The whole problem is the difference between what resolution your monitor displays and what your eyes are capable of distinguishing, as well as how the monitor handles picture information compared to how the printer handles picture information.  Ultimately it comes down to the number of dots (pixels) that your picture has, and how your eye blurs dots that are really close together.  Your computer monitor can only display between about 72-100 dots of information in every inch of screen (72-100dpi) depending on the screen resolution you have chosen in the control panel.  Once you select it, that is literally the number of dots your screen has in every inch, so it can do no better.  This does not mean that you cannot look at pictures that are higher resolution, but the monitor distorts the image in order to do so.  I have a 17" monitor set to 1152x864 (1152dotsx864dots) which equals a display resolution of about 90dpi, so I will use this in my explanations.  On the other hand print quality all depends on your eyes' ability to distinguish between denser and denser groupings of dots on a page.  In order to get non jagged edges on pictures you print, you have to print out at a dense enough grouping of dots that your eyes blur together the difference between them.  It turns out that your eyes blur together anything greater than a certain number of dots per inch (dpi), depending on your distance from the image, and the quality of your vision.  I wrote a page on the limits of your vision and how that relates to digital images as well.  You should probably read that section  before continuing on with this section.  Print quality images should be created at at least 175dpi at the absolute minimum and preferably at least 300dpi.  If you want really really clean quality printing that ensures absolutely smooth edges, 600 dots of information in every inch will absolutely guarantee a great print out.  However most printers do not print out at this quality, especially ink jets, and some laser printers do not either.  I happen to have an HP Laserjet 1100A that allows me to select between 300dpi and 600dpi printing, the tradeoff for selecting 600dpi is that the printing is slower than with 300dpi.

So, now that you know how your eyes and the monitor works, here is why good quality print images appear huge on a monitor and pictures that appear good on a monitor typically print off with jagged edges.  For example, I will use an image I created for printing that I defined as 600dpi 6" wide by 3.42" tall.  At 600dpi, 6"=3600 dots (6x600) and 3.42"=2050 dots.  My computer monitor only displays 90 dots in every inch but interprets picture information based on total dots and opens the picture as 3600dotsx2050dots not 6"x3.42" that the picture is defined as. Therefore, on my screen the picture would appear as if it were (3600/90) 40"x22.77".  This is why the picture appears so huge on my screen and will on yours if you click the link.  So, in order to compensate so that I can look at the image all at once, I set the zoom on the picture to 29% when I am working on it.  Then the picture editing software only displays about 1out of every 3 dots and I can see the entire image on my screen, although all of the information is still there.

Now for the printing side.   My printer is set to print up to 600dpi, but the printer does not use the number of dots to determine the image size like the screen does.  The printer uses the dimensions that I input when I created the picture 6"x3.42".  So when I print out the image, the printer uses this size and it prints out how I expected it to.  The printer uses the resolution in a different way.  It forces the picture to be the dimensions I set, and uses the resolution (dpi) to fill in the dots within those dimensions.  So, the more dots I defined the picture as, the clearer the image.  I defined 600dpi, so my printer was able to completely fill in the 600dpi that it is capable of printing with information, and the picture is crystal clear. It would probably actually be crystal clear at 300dpi as well.  Most images that people have on their web pages are only defined as 72-100dpi because they are optimized for screen viewing, and whatever they draw looks exactly how it shows up on their web page, but when you print this out the printer uses the defined dimensions of the picture and only fills in 72-100dpi out of its capable 600dpi, which is way less dense than the point at which your eye blurs the dots and therefore the picture looks jagged.  To make it appear clean you have to resize the picture's defined size.  If you want to print out 300dpi of info for a clean image, you have to change the resolution to 300dpi, and at the same time resize the dimensions to (90/300) 30% of their original size. Then the picture will print out clear, but the image is small (30% of its original size). This is why this picture business is so confusing to most people, because no one ever told them how the different devices interpret picture info.

Just to ensure you that this is the way it works, I am attaching a few pictures for tests.

Test 1:  (Test 1 600dpi 6x6.jpgTest 1 90dpi 6x6.jpg)
Download the pictures by right clicking on the link and selecting "save link as" or "save target as".  Then using your picture editing software, look at them on the screen then print them out.  I will define both of them as 6"x6", but I will define the image resolution as 600dpi for one of them and 90 dpi for the other. The 600dpi image will appear huge on your screen and the 90 dpi image will appear approximately 6"x6". Then print them out, they will both print out the same size, but the 300dpi image will be clear but the 90dpi image will probably be jagged.


Test 2:  (Test 2 600dpi 6x6.jpgTest 2 600dpi 3x3.jpg)
Download the pictures by right clicking on the link and selecting "save link as" or "save target as".  Then using your picture editing software, look at them on the screen then print them out.  I will define both of them as 600dpi, but I will define the image size as 6"x6" for one of them and 3"x3" for the other.  Both images will appear huge on your screen the 6"x6" larger than the 3"x3", but both huge.  Then print them out, they will both print out clear, but the the 6"x6" image will print out 6"x6" and the 3"x3" image will print out 3"x3".

Summary:

When making a picture for print remember that the dimensions of the picture determine the size it will print and the resolution determines the clarity.

When making a picture for monitor viewing remember that the resolution is constant depending on the settings on your screen, so the picture will display at the total number of dots that it is defined as.  The dimensions of the picture only multiply by the resolution to determine the total size on the screen. (a 90dpi 6"x6" image will display the same size as a 180dpi 3"x3", image 540dotsx540dots).

If you have any questions please feel free to email me at: johnatblahadotnet.

Now that takes us to another question, when I buy a digital camera, how many megapixels do I have to have to print off good pictures?

If you didn't read the section on the limits of your vision (Visual Acuity) and how that relates to digital images, you can read that now.

 

Check ebay's "Business Graphics" section for low cost auctions of imaging software

Check ebay's "Desktop Publishing" section for low cost auctions of imaging software

Check ebay's "Digital Camera & Accessories" section for low cost auctions of Digital Cameras & accessories

 

 

Links to other people's description of this issue:

Dummies guide to digital imaging

EPIcenter digital imaging basics