What makes a Digital Photograph?

Posted By BrokenClaw on March 3, 2008

Size, compression, and resolution are three characteristics which determine the quality of a digital photograph. The size of a digital photo is usually expressed in terms of pixel dimensions, such as 1024 x 768. The quality of digital cameras is usually expressed in megapixels, which represents one million photo sensors in the camera.

While megapixels and photo dimensions are not the exact same thing, it’s reasonable to think of them relative to each other: the more megapixels a camera has, the larger the dimensions and the higher the quality of the photo it produces. There are other differences among cameras, notably the quality of the lens and the size of the photo sensors, which impact the overall quality of the original photo. In the case of a compact point-and-shoot camera, there doesn’t seem to be much advantage to having more than 8 megapixels for producing photos to be displayed on a computer or printed in a reasonable size.

A pixel is a picture element, the smallest dot of color that makes up the digital photo. The more pixels that a photo contains increases the detail that the photo can hold.

It is a common misconception propagated in movies and television that you can “blow up” a digital photo and get more detail. But if the pixels weren’t there in the first place, creating more pixels won’t create more detail. In other words, if a person’s face is recorded on a digital photo as 4 tiny blocks of color, if you blow it up, it just becomes 4 bigger blocks of color.

Resolution is the general term used to describe the amount of detail that a photo contains. It is often used interchangeably with the pixel dimension size, but the comparison is only useful when the image is the original file from the photo sensors, and not simply re-sized to larger dimensions. In other words, an original photo taken with a digital camera that produces a 2048 x 1536 pixel image is a higher resolution than a photo taken with a cell phone at 640 x 480 which has been resized to 2048 x 1536.

Resolution is sometimes expressed in dots per inch (dpi), which refers to the number of pixels that are displayed on a monitor or printed on paper. However, changing the dpi has no effect on the actual detail contained in the photo; it merely changes the amount of detail which is displayed or printed.

Digital photo compression and digital photo size are two different concepts. Two photos can be the same size but have different levels of compression. The result would be a lower quality photo of the same pixel dimensions, but the file size (in bytes) would be smaller. This type of compression is commonly used for website construction, where the quality of the photo is not crucial.

Here are two versions of the exact same photograph of Hoover Dam, resized from an original 2160 x 1440 digital photograph to a small 375 x 250 web image. The top photo was simply resized without any further compression. The file size is 100,226 bytes.

Hoover Dam

The lower photo was compressed to a file size of merely 13,832 bytes — less than 15% of the size of its counterpart!

Hoover Dam

Looking closely, you can see the difference between these two photos, but in this situation, the advantages of compression for lower bandwidth and faster page loading far outweighs the 100 kb vs 14 kb difference. Nevertheless, it’s important to remember that compression of this type is not reversible from the finished photo. The compressed file is fine for viewing on this webpage, but it is unsuitable for printing or for any other type of manipulation.

Read an introduction to Data Compression.

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