Tutor Tanith

Wrapping Your Brain Around DPI

A lot of people seem to have trouble wrappng their brains around the concept of dpi. OK, it can be confusing. Let's see if this can help.

Part of the confusion is that creating on a computer is very different from use tools such a pencil, brush or pen.When we create using tools that put down pigrment we never think in terms of dpi. The image is the size we create it. We know that our tools (pen, palette knife, pencil, brush, etc) and our medium (ink, pastel, watercolor, oil, etc) influence the image, but we still never think in terms of dpi, we still think in terms of what size our image is.

Your image isn't any size in inches when it is in your computer. It is electronic. It consists of a file that with instructions as to the placement of dots of color. That's it. Those instructions can go to a printer, or to your computer screen.

A printer creates an image by laying down dots of color. We define the quality of the image by how small those dots are. The smaller the dots, the better the image quality. We describe the size of the dots by how many fit within a defined area. dpi=dots per inch. An image that is 300 dpi is made up of dots sized so that 300 dots fit inside an inch. An image that is 200 dpi is made up of dots sized so that 200 dots fit inside an inch. The dots at 300 dpi are smaller than the dots at 200 dpi.

Not quite sure you have the concept?

Get yourself a handful of M&Ms (candies). Put them on a piece of paper in nice rows each touching one another. Maybe use 10 M&Ms wide and 5 M&Ms high.

Now get yourself a bag of Oreos (cookies) - or maybe two bags. Do the same thing. You can see that it takes fewer Oreos to cover the same space. Arranging the Oreos 10 wide by 5 high results in a MUCH larger area being covered. There are also bigger gaps between the cookies than there are between the candies.

All that is important right now is that you understand that:

  1. smaller dots require more dots to cover an inch, larger dots require fewer dots per inch.
  2. your image on the computer screen is made up of dots
  3. your image when printed is made up of dots
  4. the dots when printed are smaller (M&Ms) than the dots on the computer screen (Oreos)
  5. therefore when you look at the image on the computer screen it will look larger than when it is printed.

When you create an image on the computer you are creating a set of instructions that says to lay down dots of color in the pattern you create. You are not yet saying how large to make those dots.

The size of the dots can (and will) change between the computer and the printer. Your computer screen is not capable of showing you dots at the rate of 200 per inch. Typically it will be 72 dots per inch (or pixels per inch), sometimes 96 ppi (pixels per inch) or dpi (dots per inch). So even if your image program says it is showing 300 dpi, it is not. All it is doing is showing you an approximation of what the image will look like when printed at that rate.

Lets try another exercise. Draw a square 6 inches by 6 inches. Fill that square with your Oreo cookies. Count them. Now count out the same number of M&M candies. You can't fill that square because you don't have enough M&Ms. It takes more because they are smaller. So when you create an image the question you need to answer is how many dots of color do I need to cover the area I want? The answer to that is provided when you are told the number of dots that it will take to cover an inch. In the CafePress environment that could be between 100 dots per inch and 300 dots per inch.

When you are planning to create an image for a specific product you want to make sure it is the right size. For example, if you want to use the entire image area on a CafePress® T-shirt you want to cover an area of 10 inches by 10 inches. You know that when your image is printed it will be printed as a series of dots. Smaller dots make a better quality image. It takes more small dots to cover an inch. So when you create your image make sure you have enough of those dots to cover the area you want. 10 inches times 200 dots per inch equals 2000 dots. So you want your image to be 2000 dots wide by 2000 dots high. That is the same as 2000 pixels by 2000 pixels.

Your computer application might help you by doing the math for you. You set the dots per inch, you set the size in inches, and then it will show you the size of the image in pixels. Once you know the number of dots (the size in pixels) the inches and dpi your application shows you isn't important anymore. As long as you keep the number of pixels (dots) the same, the way it is actually printed depends on what size you choose for the image.

You can choose to display 2000 dots by 2000 dots like this:

2000 pixels = 10 inches at 200 dpi (2000 divided by 200)
2000 pixels = 6.66 inches at 300 dpi (2000 divided by 300)
2000 pixels = 27.7 inches at 72 dpi (2000 divided by 72)

Notice that when you make the image smaller the dpi goes up. Why? Because you have a fixed number of dots. You WANT a fixed number of dots. That is what keeps the image the way you designed it. Dots are what makes up your image. If the computer changes change that number of dots, that can have an effect on your image quality. Sometimes it is very noticeable. sometimes it is hardly noticeable.

After you have the right number of dots, it doesn't matter if your application then shows a different number of inches or dpi, because all CafePress® knows when you submit the file is the number of dots. It has no idea whether you created your image using M&Ms or Oreos, it doesn't know the size of the dots you are sending up because all you are sending is instructions to create the dots. Just the same way as I didn't send you Oreos or M&Ms when I made the examples. All I gave you was the instructions. That is all you are sending to the CafePress computers, is the instructions on how to create the image.

Now you say "I want these image dots to fit inside an area 8 inches wide by 4 inches tall." With the information of how big to make the image NOW the program decides how big to make the dots so that all of them fit inside the dimensions you stated.

Again, keep in mind that when your application says something like 10 inches at 200 dpi all it is doing is doing the math for you. It does not and cannot actually show you 200 dots per inch on the screen. It will show you an approximation of what the image will look like when the dots are printed at that rate.

As long as you have an adequate number of dots you can make the dots larger or smaller as necessary to cover your image size when you apply it to the printing process. Yes, smaller dots do have better image quality. In printing on fabric for the most part you won't notice any difference between 200 dpi and 300 dpi. For printing on paper you might notice some slight difference between 200 and 300 but usually 200 is quite acceptable. Most people, however, will notice the difference at 100 dpi. So your goal in creating images to apply to CafePress products is to create images with enough dots for the largest product you want to apply the image to, at the quality you want to use. CafePress uses 200 dpi as the default. You can let your application program do the math, or you can do it, or you can use the CafePress charts. http://www.cafepress.com/cp/info/help/help_image_sizes.aspx (For comparison see the Zazzle guidelines)

In the CP environment DPI information is not retained in the file. CP has no clue what DPI your image is. They only know its size in pixels.

dpi = dots per inch. mph=miles per hour

DPI means nothing until it is applied.
MPH means nothing when your car is parked at the curb.

So what happens if you make your image larger than the maximum dpi? The application at the CP end intelligently selects which dots to ignore so that it reduces the number of dots to what will fit. An image that is very large can be simply reduced in size to cover an area at the correct dpi.

A vehicle that is capable of 200 mph can be driven more slowly.

No provider is going to have problems with an image just because your application showed ... say ... 600 dpi. Because once you send it, it is like the car parked at the curb. It only has potential dpi.

Some providers might have a limit on file size (number of dots) just like some bridges have a limit on the weight of a vehicle. That limit has nothing to do with how fast that vehicle can go, it has to do with burden on the infrastructure. CafePress does not limit file size.

See also "Say Not to 72dpi", "DPI, PPI, SPI - What's in a name?", "Display, Printing, DPI and PPI".

Image Size


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