What is a pixel? The Nuts and Bolts of Digital Images
If you’re not already a tech-head, trying to wrap your brain around the concept of image digitalization can be daunting. Most people don’t know what a pixel is, but I’ll try to explain it simply: Bit by Bit.
A pixel is the smallest unit of a digital image or display that can be controlled or processed by a computer. A pixel represents a single point in a digital image and is usually composed of three color components: red, green, and blue (RGB). The combination of these three colors in different intensities creates a range of different colors that can be used to represent an image. The number of pixels in an image determines the resolution and quality of the image.
If you still need help visualizing it, let’s break it down further…
In Photography, pictures you take with film are Analog, and give a continuous gradient scale from dark to light tones.
Digital photography “Digitizes” a scene through the lens by dividing the scene projected onto an image sensor into a pixel grid.
The word PIXEL is a combination of ‘Picture Element’.
What is a Byte?
Now that you know what is a pixel, let’s look at what is a byte?
Each Pixel is a square on the grid and has a number of bytes that correspond to its position, brightness, and color. Bytes also reflect the file size of an image: For instance, 1000 bytes is a Kilobyte, or 1 KB. 1,000,000 bytes is a Megabyte, or 1 MB.
A Byte can also be broken down into a grouping of 8 bits – a BIT is the basic binary value of 1 or 0, or in other words yes or no. A 1-bit pixel, therefore, would yield only one of two possible colors: for example Black or White. A 2-bit pixel, likewise, could express itself out of 4 possible tonal combinations as 11-(white), 00-(black), 01-(dark gray), or 10-(light gray).
The number of bits in a pixel is also referred to as bit-depth. In black and white photography, 8-Bits represent a basic gradient scale between black and white. Here, pure black would be represented as 00000000, while pure white would be represented as 11111111. Those 8 bits can combine themselves into 256 different gradient variations overall, including 254 unique shades of gray.
The deeper your bit-depth is, or the more bits per pixel you have, the richer your color tonal range will also be. In color environments, images can be broken down into Red, Green, and Blue (RGB). Here, each color represents one Byte (8 bits): for example, 8 different values of Red, combined with 8 different values of Green and 8 different values of Blue. These three colors combine then for 24-bits or 16,777,216 possible colors!
|1-bit||2 possible colors||Images can only have 2 Tones, often Black or White|
|8-bit||256 possible colors||Images have up to 256 Tones giving a limited color range|
|24-bit||16,777,216 possible colors||Colors with full-color range and tonality, comparable to color film|
So as you see, the more bits per Pixel – the more variety of pixels you can produce in your image, giving you the ability to have smoother transitions between colors and tones.
Pixels: The Bigger Picture
So, what is a pixel as it relates to photography? Cameras record pixels by capturing light and translating that light into digital data. The light enters the camera lens and is focused onto an image sensor, which is composed of a large array of photodetectors. Each photodetector is responsible for capturing light information for a single pixel.
When the image sensor is exposed to light, the photodetectors collect photons and convert the energy from the photons into electrical signals. These electrical signals are then processed by the camera’s image processor to create a digital representation of the image. The resulting digital data is stored as a grid of pixels, with each pixel representing a specific color and brightness level.
The number of pixels captured by the camera and the size of the image sensor determine the resolution of the image, or the number of pixels that make up the image. Higher resolution cameras have larger image sensors and capture more pixels, resulting in images with greater detail and clarity.
Your image quality can be measured by the sharpness of detail and smoothness of the tonal gradient in an image. We looked at how bit-depth can give you a better color range, but the clarity of your picture depends largely on the resolution of the image.
Resolution refers to both the Pixel Count and the Pixel Density of an image. Pixel Count can be described as simply the total number of pixels that comprise the image. This value is usually expressed as the number of pixels across the x-axis (width) of the pixel grid multiplied by the pixels across the y-axis (height). These values come into play when formatting images for video and digital display.
The product of the two axes yields the Mexapixel value (MP). The image sensor size on digital cameras is given MPs and reflects the resolution in terms of how many pixels the camera can record across a pixel grid.
To find out what your pixel count is on your camera, cell phone, or monitor, check out this helpful tool.
Pixels and Print Resolution
Pixel Density, on the other hand, is expressed through the number of pixels in a given area, most commonly ‘Pixels per Inch’ or ppi. This can also be written as dpi, or ‘Dots per Inch’. Logically, the more pixels packed in an inch, the sharper the image. Dpi can also have a direct effect on the print quality of your picture, allowing you to either enlarge a picture or print with a fineness that allows the pixels to become undetectable.
In the table above, the red areas show poor print quality, dark yellow shows poor below average quality, and yellow shows the edge of what could be acceptable for private use. As you can see, it is inadvisable to print anything under 72 dpi as the image resolution is too low and the photos will look “pixelated.”
The green areas are the recommended minimum dpi values for the given print sizes, while the dark green areas show high resolutions that are no longer distinguishable to the eye.
Ideally, you want to stay above 150 dpi to ensure crisp images. 300 dpi will give you a very fine resolution for printing. Anything greater than 300 dpi won’t be noticeable to the eye and will only result in larger file sizes.
The distance the viewer is to the photo will also determine how fine your print should be. Generally, the larger an image is, the further away the viewer will need to be to see it in its entirety. For instance, the common dpi for an advertisement on the side of a bus is 83dpi, while a billboard requires only about 11dpi!
Hope this clears up any questions you may have had about pixels, bytes, bits, and resolution. If you have any further questions about other nuts and bolts of digital images, leave a question in the comment section!
To read more about Light Scales and Differences in Image Contrast and Tone, check out this Article on Seeing in Black and White.