Photoshop: File types

PPI (Pixels Per Inch)
PPI determines the physical size of an image and the quality of the output.
– A computer screen typically displays images at 72ppi which is why this PPI is the industry standard for images going online.
300ppi is the industry standard for images for print.

Common file formats
JPEG (Joint Photographic Experts Group) uses lossy compression, which means the image loses quality but the file size will be smaller, making it better for the web but not for printing.
PNG (Portable Network Graphics) creates a better quality image but this can cause it to be read slower than, for example, JPEG on the web. It can be used for images going online but is more suited to image going to print.
TIFF (Tag Image File Format) is a more flexible format as it supports most colour spaces, such as RGB, CMYK. It’s the industry standard for high quality images and can compress an image without losing quality.
GIF (Graphic Interchange Format) doesn’t read an images PPI and also has a limited colour palette, making it useless for image printing but doesn’t affect online images.

RGB + CMYK
RGB stands for red, green and blue and represents the light emittance. Computer screens use this colour space making it more suitable for images going online.

RGB-color-additive-model

AdobeRGB represents a wider range of colours compared to RGB and therefore is good for editing images for the web.
CMYK stands for cyan, magenta, yellow and key (black). Printer inks create an image using these colours so this colour space is good for image printing. However it doesn’t represent the colour space used for computer screens (RGB) and therefore shouldn’t be used for online images.

CMYK-color-subtractive-model

Colour depth
Colour depth (or bit depth) identifies the variety of colours in a single pixel. For example, 8bit has a palette of 256 colours consisting of reds, greens and blues, whereas 24bit can display up to 16,777,215 colour combinations. The colour depth you use can significantly alter the quality of your image.

In summary, if you were saving an image for web it would be best to save it as 72ppi, RGB and as a JPEG (unless you want a transparent background and therefore you’d save it as a PNG).

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