Comparing Colour Depth and Resolution

Colour depth and resolution are independent properties of a bitmap image, so changing one does not automatically change the other. Colour depth is the number of bits used to store each pixel's colour, measured in bits per pixel. Resolution is the number of pixels the image is made from. You can give each pixel more bits without adding any pixels, and you can add pixels without giving each one more bits.

The four combinations

For example, a stencil of a bicycle cut from two colours can be stored at very high resolution, giving clean, accurate shapes from a small palette. A tiny thumbnail of the same bicycle can use thousands of colours and still look blurry and blocky, because there are too few pixels to show the spokes.

Quality and file size

Raising the colour depth improves colour accuracy, so gradients blend smoothly and there is less colour banding. Raising the resolution improves detail, so edges and patterns look sharper. Both generally improve image quality, but neither one can do the other's job: extra bits do not add detail, and extra pixels do not add colours. A higher colour depth does not make an image look sharper, because sharpness comes from how many pixels there are.

Both also increase file size. More bits per pixel means more data for every pixel, and more pixels means more pixel codes to store. Cutting either one reduces the file size and costs some quality.