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.
A-Z Digital Cameras is the place
for unbiased information and the
best online merchants for
digital cameras,
accessories and photo processing. We invite you to browse dozens of articles by
clicking the links in the Site Menu at the right and on our
Related Articles
pages. Be sure to bookmark this site for
future reference.
|
