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Image size is composed of three related parts:
1. Pixel Dimensions
2. Print Size
3. Resolution
1. Pixel Dimensions
Pixel Dimensions refers to the total number of pixels that make up a digital image. ("Pixels" stands for "picture elements", i.e. the elements that make up a picture.) Pixel dimensions are referenced in terms of width x height, e.g. 600x400. The greater the number of pixels, the more information (or detail) an image contains.
2. Print Size
Print Size is perhaps the easiest component to understand because it is simply the dimensions (in inches or centimeters) that an image is printed at. When someone refers to a 4"x6" image, they are referring to the print size.
3. Resolution
Resolution refers to how much information is contained in a given amount of space. Resolution is referenced in terms of dots per inch (or dpi). The important thing to remember about resolution is that the higher the resolution, the more detail you will see in your printed images.
To better understand resolution, it may be easier to think of the dots as squares - or more specifically as tiles in a mosaic. Imagine that you are remodeling your kitchen and you've decided to put a tiled mosaic on the wall behind your stovetop in a space of about 3'x3' (about the size of the stovetop itself.) You've drawn the outline of a sunflower on the wall to guide the placement of the tile:
Now it's time to actually choose the tile for your mosaic. Your goal is to create the mosaic with as much detail as possible while minimizing the amount of tile cutting you will need to do. When you get to the store, you discover that tile comes in many different sizes, e.g. 4", 2", 1" and 1/2" (to name just a few.)
Imagine trying to create a mosaic of the sunflower using only the 4" tiles. Would you recognize the design as a sunflower or just a bunch of different colored squares? Now imagine trying to create the mosaic design using the 2" tiles. You may be able to recognize the sunflower in the design, but it will probably take some imagination.Now imagine creating the entire mosaic using 1" tiles and then 1/2" tiles. Think of how many tiles that would be!! The following examples provide a visual example of the differences using each of the different tile sizes. (Move your mouse over each of the images, to see the sunflower sketch superimposed on the mosaic.)
Sunflower mosaic using 4" tiles
Sunflower mosaic using 2" tiles
Sunflower mosaic using 1" tiles
Sunflower mosaic using 1/2" tiles
Why have we spent all this time explaining mosaic tiles? Because pixels are like mosaic tiles! The more pixels you fit into a given space (i.e., the higher the resolution), the more detail your printed image contains.
You may have read or heard advice that images destined for the web or e-mail should have a resolution of 72 dpi; likewise images destined to be printed should have a resolution of 300 dpi. While these are not hard and fast rules, there is a reason these numbers exist. A computer monitor cannot display more than 72 dots per inch. (Actually, advances in technology has changed this, but that is another discussion.) So creating an image with a higher resolution does not actually provide more detail for the viewer because the computer monitor cannot display more detail than 72 dpi. Printers on the other hand (including inkjet printers) can print at least 300 dots per inch (and often more.) That is four times more information (i.e. detail) that can be included in an image to be printed than one destined for the web:
Why does all of this matter?
Having a good understanding of the relationship between pixel dimensions, print size and resolution will give you greater control over the quality of your images: Pixel Dimension = Print Size * Resolution. A resolution that is too low may result in your printed images looking pixelated, i.e. you can see the individual pixels. A resolution that is too high (thus resulting in a larger number of total pixels) will increase the processing time on an image because more pixels have to be processed than are really necessary.
Examples
Let's take a look at some examples to visually see how this all fits together. In each of the examples below, the size of the image is shown relative to a "printed" 6"x4" grid.
Example 1 - We start with an image of 300x225 pixels at 72dpi. If we increase the resolution while keeping the pixel dimensions the same, the print size will decrease. It is important to note that because the pixel dimensions remained the same and the resolution increased, the perceived quality of the image stays the same (or may look better), though the printed image size will be smaller.
At 72dpi, the print size is 4.17"x3.13":
Print size = Pixels / resolution
Print size = 300 px/ 72 dpi= 4.17"
Print size = 225 px / 72 dpi= 3.13"
Increasing the resolution without changing the pixel dimensions results in a decrease in the print size.
Print size = Pixels / resolution
Print size = 300 px / 150 dpi = 2"
Print size = 225 px / 150 dpi = 1.5"
Example 2 - We start with an image of 900x675 pixels that prints at 3"x2.25". If we increase the print size while keeping the pixel dimensions the same, the resolution will decrease. It is important to note that when the pixel dimensions stay the same and the resolution decreases, the quality of the image may degrade because the same number of pixels are spread out over a larger area which has the effect of increasing the size of the pixels. If the change is large enough, you may see some pixelation of your image when printed.
At 3"x2.25", the resolution is 300 dpi:
Resolution = Pixels / print size
Resolution = 900 px / 3" = 300 dpi
Resolution = 675 px / 2.25" = 300 dpi
Increasing the print size while keeping the pixel dimensions the same results in a decrease in resolution.
Resolution = Pixels / print size
Resolution = 900 px / 6" = 150 dpi
Resolution = 675 px / 4.5" = 150 dpi
Example 3 - We start with an image that is 3"x2.25" and 300dpi. If we decrease the resolution while keeping the print size the same, the pixel dimensions will decrease. It should be noted that any time the pixel dimensions are changed, there is a decrease in the quality of the image because the graphics application must average the correct color values for the remaining pixels. In this case, the averaging of the color values in addition to the decreased resolution further degrades the image quality. Look closely at the stamens of the flower to see the effect of this.
At 3"x2.25" and 300dpi, the pixel dimensions of the image are 900x675.
Pixels = Print size * resolution
Pixels = 3" * 300 dpi = 900
Pixels = 2.25" * 300 dpi =675
Decreasing the resolution while keeping the print size the same results in a decrease in pixel dimensions.
Pixels = Print size * resolution
Pixels = 3" * 150 dpi = 450
Pixels = 2.25" * 150 dpi =338
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