Digital Photography Math

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Digital Photography Storage Terms:
bit = A fundamental unit of information having just two possible values, as either of the binary digits 0 or 1.
byte = A sequence of adjacent bits, usually eight, operated on as a unit.
kilobyte = 1,024 bytes (2 ^ 10); a thousand bytes.
megabyte = 1,048,576 bytes (2 ^ 20); a million bytes.

With a digital darkroom, there are lots of numbers that you're going to need to remember. These numbers will help you plan how you will store your photos, how you will scan your photos, how you will share your photos and how you will archive your photos. If you are not new to how computers interact with images, you know that photographs can not only be big, but they can be HUGE. If you are using a TIFF or RAW file format on your digital camera, these image files can be anywhere from 3 or 4 megabytes up to 11 megabytes (thats what the megapixel rating on your camera means).

If you are scanning your image and using some native uncompressed image format, the image file can be absolutely huge. This past weekend I scanned in an old 8 1/2 x 11 photograph and stored it in a native format because I didnt want to lose any of the image information since reproduction was important to me. The size of the image was 96 megabytes. If you know that you can store 700 Megabytes on a recordable CD, that means you can store about 7 of these ultra-high resolution TIFF image files.

There are a few elements that go into the calculation of the potential file size:

  • Image dimensions. These are either portrayed in pixels or in inches, depending on the originating hardware. Cameras talk about pixels, scanners typically start with inches as their dimensions.
  • Scanner Resolution. Obviously this matters with using a scanner as the originating hardware. A typical consumer scanner can scan an image at various resolutions, ranging from 75 to 150 to 300, all the way up to 2400 dots per inch.
  • Bits per Pixel. This piece of information is used to determine just how many colors the image is capable of storing. The 2 most common image formats are 16-bit images and 24-bit images. 16-bit images can store 32,000 colors and 24-bit images are capable of 16 million colors.

In the example used above, the 8 1/2 x 11 inch image can be calculated as:

(8.5 * 11) * 600 bpi * 24 bits =
(5100 * 6600) * 24 bits =
(33,660,000) * 24 bits =
807,840,000 total bits = 96.3 megabytes
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