Monochrome photos: Monochrome image is known as a binary image. Each pixel is stored as a single bit (0 or 1). A 640 x 480 monochrome image requires 37.5 KB of storage. It can also contain varying tones of only one colour.
Black & White photos:
It has no Gray. It can also be considered as a special case of monochrome images. A good example of black & white would be chess board. All black & white printings are monochrome but not all monochrome printings are black and white.
A grayscale image has values varying from black to white. In other words, it has varying shades of gray. In simple words, a grayscale image has more details than a black&white image. A 640 x 480 sized grayscale image would require over 300 KB of storage.
Color images or photos: Can be either 8 bit, 24 bit or 32 bit.
8 bit: One byte for each pixel. Supports only 256 out of the millions colour possible.(Calculated as 2^8)
24 bit: Each pixel is represented by 3 bytes as 1 byte=8 bits. These bytes are known as red, blue and green. 24-bit colour images offer the largest possible colour combinations. 256x256x256. (Calculated as 2^8 multiplied by 2^8 multiplied by 2^8).
32 bit: These images have 8 additional bits for alpha (transparency).
Tip for photoshop: Any RGB image that has 8 bits/channel is known as a 24-bit image. 8 Bit for each red, blue and green colour. Photoshop lists the bit depth as bits/channel. Sometimes organizations will ask for a 24 bit JPEG which means 24 bits in total or 8 bits/channel in the language of photoshop.
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