Corners

Simulator

AI generated demo image

The lens is working hardest at the edge

A lens is designed around its middle. Out at the corner the light is arriving at an angle, through the edge of every element, and three different aberrations arrive at once.

• Why the edge is harder

  Light reaching the corner of the frame goes through the outer part of every element in the lens, at an angle, further from the axis everything was calculated around. Spherical aberration, coma and astigmatism all grow with that distance, and the first two grow fast.

• Coma smears outwards

  A point of light off-axis does not blur evenly, it grows a tail pointing away from the centre of the frame. Night skies and practical lamps show it first, which is why a lens that looks fine on faces can fall apart on a city at night.

• Field curvature

  Most lenses focus on a dish rather than a plane. Focus the middle and the corners sit in front of the focus; focus the corners and the middle goes. There is no aperture that fixes it, only one that hides it.

• Two stops buys most of it back

  Nearly everything off-axis improves sharply for the first two stops down and then stops improving. If a shot needs the corners, that is usually the cheapest thing you can do about it.

• Whether it matters at all

  Faces sit in the middle third of the frame almost always. Corner performance matters for architecture, for landscape, for anything with a horizon, and for titles. It matters much less than test charts suggest for everything else.

• Testing it honestly

  Shoot a flat wall, square on, at the distance you will actually use. A corner test at two metres on a lens you will use at twenty tells you about field curvature at two metres and nothing else.