This is known as chromatic aberration, and it’s more of a problem with cheaper lenses. You may notice red, purple or blue fringes clinging to high-contrast edges in your shots. At shorter focal lengths the image will bulge outwards (barrel distortion), while at longer focal lengths the opposite effect can become apparent (pincushion distortion). Some lenses can cause straight lines to appear curved.
It’s especially noticeable in images that contain large areas of even tones, such as the sky in this photograph. Peripheral illumination distortion, or vignetting, appears as darkening at the edges of the frame. (Image credit: Future) (opens in new tab) At wide apertures in particular this can result in peripheral illumination, or vignetting, with the edges of the frame appearing darker than the rest of the image – this can spoil at shot with a blue sky like ours. Most lenses are designed so that the maximum amount of light is captured in the central area of the image, with the light falling off towards the edges of the frame. These geometric distortions affect most images in many cases they’re barely noticeable, but they can be a problem in architectural shots or other images containing prominent straight lines. The lens can also cause light to be bent, resulting in a subject’s horizontal and vertical lines bulging outwards (barrel distortion) or being ‘pinched’ inwards (pincushion distortion). In some cases different wavelengths of light aren’t quite focused on the same spot, causing fringes of colour to cling to contrasting edges – the technical term for this is chromatic aberration.
When light enters a lens it’s focused onto the imaging sensor.