November 23, 2015

What Is the Difference Between Image Size and File Size?

Recently I was helping a friend prepare pictures for his new website. He was confused about the difference between the size of the image in Megabytes (MB) or Kilobytes (KB) compared to the width or height of the photo itself. If you have been puzzled by this subject, let me explain.

Soda Butte Creek
Yellowstone National Park, Wyoming
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Picture Dimensions or Image Size 
Let's start with the dimensions of your picture, in other words, the width and height of the image. If it is a printed photo, you describe this using inches or centimeters. These are standard units of measure, so if you ask for an 8x10 inch image, you know what size the print will be.

For digital images displayed on screen, inches or centimeters are not useful because there are so many different sizes of monitors. Instead, we describe the picture dimensions or image size by the number of pixels wide and high.

Let's look at an example. The photo at the beginning of this post was taken with a Canon 6D camera. The Image Size is 5472 pixels wide x 3648 pixels high. If I use photo editing software to change the dimensions of the image, I can reduce the image size to 3000 x 2000 pixels. I'm changing the size of the photo by telling the software to throw away enough pixels to give me a smaller image size. So the photo becomes smaller because its dimensions (width and height) are less than the original.

These two pictures have different Image Sizes.

File Size
The size of the file that contains your photo is a different way to describe the photo size. File Size refers to the amount of space required to store your photo on your computer's hard drive or your camera's memory card. The amount of storage space is measured in Kilobytes (KB), Megabytes (MB) or Gigabytes (GB). Kilobytes are the smallest amount (think ounces), Megabytes a medium amount (think pounds) and Gigabytes the largest (think tons). [Of course, pixels have no actual weight.]

The type of file format you use to save your picture affects the amount of storage space it uses. Let's compare an image saved in three different file formats: Raw, TIFF and JPEG.

Using the same picture from the Canon 6D camera, its original picture dimensions are 5472 x 3648 pixels. When the photo is saved as a raw file, its File Size is 24 MB. If I open that picture and save a new copy as a TIFF file, the File Size changes to 57 MB, even though it still has the same image size in pixels as the raw file. Last, if I save this same picture as a JPEG file with its original picture dimensions, it is only 11.8 MB.

Here's a table to make this comparison easier:

Click to view larger

Each file type uses a different method to store the information that makes up your picture; hence, each file type needs a different amount of storage space, even when the dimensions of the picture remain the same.

Pixel Dimensions and File Size
Of course, the number of pixels in the image also has an effect on the file size. Fewer pixels means less picture information. This produces a smaller file size, regardless of file type. So if I shrink the number of pixels from the original 5472 x 3648 pixels to 3000 x 2000 pixels and save the photo as a TIFF file, it takes up only 17 MB of storage. If I save the 3000 x 2000 pixel image as a JPEG file, it needs just 4.4 MB of storage space. (I cannot save a raw file outside of the camera.)

Sharing Pictures
Web hosting companies, publishing companies (whether online or in print), photo labs and other vendors may tell you the recommended size for your image to get the best results from their products. Sometimes they describe the size of the pictures you can use in File Size, the amount of space it takes to store the photo on a hard drive (or memory card). Other times, they describe the size of the picture by the Image Size, the number of pixels per side. As I hope you have seen, these two methods of describing picture size are actually very different. Be sure to use the appropriate one when sharing your photos with others.