Easy Basic Photography
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Pixels, Image Resolution, and Print Sizes
The quality of a digital picture is largely dependent upon the number and size of the pixels contained in the image. Image resolution is
simply the information about the pixels and the pixel density of the image. This tutorial will describe how pixels and image resolution relate to
viewing images on a computer screen or as prints.
What are Pixels? A pixel is a shortened and combined version of the words Picture and Element. Digital cameras have image sensors with
millions of light capturing cells. The light and the quality of the light captured at each cell location, are called pixels. Digital Single Lens
Reflex cameras like the Nikon D5100 have larger image sensors than a digital compact camera like the Canon Powershot ELPH 300 HS.
The larger sensors produce larger pixels and can produce higher quality images. Check the Digital Camera Image Sensors tutorial for
more details about sensors and pixel size.
The colors and intensity of light for each of the millions individual pixels appear to be blended together when we view them on a print or on
a computer screen. The picture below to the left is just a regular picture taken with a compact digital camera. The picture to the right is a
portion of that same picture with the pixels greatly magnified and exaggerated so you can see how pixels form an image. However, there will
be thousands or millions more pixels in an area equivalent to picture #1B when you take your pictures.
Image File Size: The file size of an image is expressed by the total number of pixels in the width and height of an image. For instance a file
size might be written as 3,456 X 2,304. That will mean there are 3,456 pixels in each row of pixels in the image from left to right and 2,304
pixels in each column of pixels from top to bottom. For the record, 1 million pixels are equal to 1 Megapixel.
To find the total # of pixels in an image, just multiply the # of pixels in the width of the image by the # of pixels in the height of the image.
(3,456 X 2,304=7,962,624 ) Usually the total amount of pixels is rounded up or down to the nearest megapixel. So in this case the image
might be called an 8 Megapixel picture although it does not contain a full 8 million pixels.
If 7,962,624 is also the maximum file size the camera can produce, the manufacturer will market the camera as an 8 Megapixel camera.
Although digital cameras are often marketed with emphasis on the total # of megapixels available for an image, they all have settings which
allow the photographer to take pictures with smaller file sizes. That feature can save space on your cameras memory card or in your
computer hard drive. Also, images with smaller file sizes are easier to send and download when using email. Taking pictures at different file
sizes will make more sense as you continue to read about image resolution and print sizes.
Image Resolution: In general terms, image resolution in digital photography refers to the amount of information contained in an image file.
That information is the amount of pixels contained in any digital picture you take. A picture file size 4,000 x 3,000 will have a higher picture
resolution than a file size 2,000 x 1,500. Higher resolution pictures will be sharper, clearer, and more accurately reproduced when printed or
viewed on a computer screen.
More specifically, image resolution refers to density in terms of Pixels per Inch (PPI) or Dots per Inch (DPI) rather than the size of the whole
file. Sometimes the terms PPI and DPI are often incorrectly used interchangeably. In this tutorial, we refer to the output picture resolution of
a computer screen as PPI. When we refer to the output picture resolution of a machine such as an ink jet printer, the term DPI will be used.
We will cover the technical differences of these terms in more detail in another tutorial.
Image resolution is usually written as numbers such as 72 PPI, or 300 DPI. What that means is that in a particular image file there will be 72
pixels or 300 dots within a one square inch area of the file. That will be 72 dots or 300 pixels from left to right and from top to bottom for
each inch of the file. That information is important when it comes to how your pictures will be displayed.
Computer screens are able to show clear and sharp images at low screen resolutions such as 72 PPI or 96 PPI. If you take pictures that will
be for on screen viewing only, you can set your camera to a low image quality setting such as one megapixel or less. Taking pictures at high
quality settings like 12 megapixels will not make the image look better on a 72 PPI screen. However, you will definitely need to set your
camera to higher quality settings if your are going to make enlarged prints of the pictures you take.
Calculating Maximum print sizes: As previously mentioned, to view pictures clearly on a computer monitor, the screen only needs an
output resolution of 72 PPI or 96 PPI. However, in order for a print to be sharp and clear, the output resolution from the printer needs to be
much higher. A printer output resolution from 140 DPI to 300 DPI is a better range for making prints from your images. (that is with 300 DPI
being professional grade printing)
The chart shown above will give you a general idea as to the approximate maximum print sizes (in inches) to which you may enlarge your
pictures and retain good quality. Take note that the maximum sizes listed are guidelines and prints can be enlarged to sizes slightly larger
than those listed. However, a 3MP image that is enlarged to 16 x 20 size will be a very poor quality print. Making prints smaller than the
recommended maximum for the file size listed is not a problem. The prints will still be very high quality.
Some of the more common print sizes are 4X6, 5X7, 8X10, 10X13, 11X14, and 16X20. (these sizes are for pictures taken when the camera
is held vertically but the sizes in the chart are for when the camera is held horizontally.)
It is actually pretty easy to determine the maximum print size you can make from your digital images. First you need to determine how many
DPI (dots per inch) will be used when the file is being printed. To keep it simple, lets say the printer output resolution will be 200 DPI. If your
image file size is 2,000 X 1,600 you will be able to get a quality print size up to about 10 X 8. The math involved in coming up with that size
print is to simply divide the # of pixels in the width of the file by the 200 DPI. (2,000/200=10). Next, divide the # of pixels in the height of the
file by 200. (1600/200=8). So there you have it. A file size of 2,000 pixels X 1600 pixels can be printed to make a good quality 10 X 8 photo
when printed at 200 DPI.
If you decide to make a print at 300DPI from the same image file, you will have a print with better resolution. However the maximum size for a
quality print will be smaller. Lets do the math.... 2,000/300=6.6. Next, 1,600/300=5.3. So if you round the numbers out, the maximum
standard size for that print will be about 5 X 7. Of course you won't have to do this type calculation every time you take a picture. Just keep
in mind that when you are planning to take pictures that will be printed and enlarged, use one of your cameras' larger file size settings.
Picture 1A
Picture 1B Example of pixels
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Photo Enlargement Chart