Guide

Planning resolution: print and screen

How many pixels an image needs depends on its purpose. For a screen the pixel count is what matters; for print the area over which those pixels are spread matters as well.

Pixel count, megapixels and file size

The resolution of a sensor or display is stated as the product of width and height in pixels. 1920 × 1080 gives 2,073,600 pixels, about 2.07 megapixels. The megapixel uses the decimal prefix: 1 MP is exactly 1,000,000 pixels, not 1,048,576. File size does not follow directly, because compression depends on the image content.

Pixel density and print size

Printing needs the pixel density: how many pixels fall on a given length. It is stated in ppi and determines how large an image can be printed before individual pixels become visible. An image 3000 pixels wide gives exactly 10 inches or 25.4 centimetres at 300 ppi.

Resolution is not sharpness

More pixels do not automatically mean a better image. Sharpness depends on the lens, aperture, exposure time, focusing and post-processing. An image from a camera with many pixels and a weak lens can be less sharp than one with fewer pixels and good optics. Conversely a good lens on a low-resolution sensor delivers less detail than it could.

Aspect ratio and cropping

The aspect ratio describes width to height and is independent of the pixel count: 16:9, 4:3 and 3:2 are common formats. Changing an image to another ratio crops or adds pixels. The pixel count changes, the subject does not. A figure of “12 megapixels” without the aspect ratio is therefore incomplete.

Matching conversions

Matching tables

Unit family: Image resolution

Sources