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Digital Imaging - The Secret to Modern Photography

Digital imaging began in early 1950's television when engineers discovered how to convert analog video images to electrical impulses for storage on magnetic tape. Twenty years later solid state image sensors were developed that could capture still images, paving the way to today's digital cameras.

Digital Images Explained

A digital image is an electronic map of an image translated to dots or pixels, stored on magnetic media. Reproduction of the image is accomplished by three measurable components: resolution, bit depth and dynamic range.

Resolution

Pixel dimensions represent an image's resolution on a digital camera and are calculated by multiplying image width times pixels per inch (ppi) and height times ppi (e.g., an 8 inch by 10 inch image at 300 ppi = 2400 x 3000 pixels). Dots per inch (dpi) is a similar term applied to printers (in most cases it takes much higher dpi to equal ppi resolution).

Bit Depth

Bit depth determines range of tones in the image and is measured by powers of two. For example a black & white digital image is 1 bit (2¹). Most grayscale images are 8 bit (2⁸) producing 256 shades from black to white. Typical color images are 24 bit (2²⁴), and produce 16.7 million colors (digital cameras achieve this by assigning 8 or more bits to each of three colors).

Dynamic Range

Dynamic range is the ability to reproduce differences in dark to light shading. Photos need a higher dynamic range for subtle transitions in the image such as shadows or the sky. A low dynamic range results in a contoured effect like we see in a posterized photo.

Image File Size Issues

Unless the image file size is reduced, even a 256 MB memory card would only be able to hold a few photos. File size is stated in bytes and calculated by multiplying pixel dimensions times bit depth and dividing by eight (1 byte = 8 bits).

For example, an 8" x 10" photo at 300 ppi is 2400 x 3000 or 7.2 million pixels. At 24 bits, file size is (2400 x 3000 x 24) / 8 = 21,600,000 bytes. Image file sizes are expressed in KB (1024 bytes) or MB (1024 KB) and rounded down, making this a 20.6 MB digital file.

Because this raw file is so large, digital cameras use file compression algorithms to discard visually insignificant data, resulting in smaller files. The most effective and widely used standard for compressing files in digital cameras is JPEG. In normal or fine mode JPEG will produce high resolution digital photographs only a highly trained expert can distinguish from a much larger raw file.

Another advantage to the complex JPEG algorithms is they are not linear. More data is discarded in the brightest gradations the human eye can't discern, and more is preserved in darker areas like shadows that the eye can distinguish. And the camera's processor does in mere milliseconds what could take hours with computer software.

Learn more about pixels, sensors and digital images at our Digital Camera Technology page.

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