Geometric distortion correction for angled shots — restores parallel verticals, removes trapezoid effect. Done in-camera or in post; tilt-shift lenses correct optically.
Keystone Correction
You know the problem: the camera is positioned low, the building high — and suddenly the facade seems to recede like a trapezoid. The vertical lines converge instead of remaining parallel. This is the classic keystone effect, and it annoys anyone who has ever had to shoot from bottom to top. The correction brings these lines back to the vertical, turning the trapezoidal shape back into a rectangle.
On set, you have several ways to deal with it. Optically, tilt-shift lenses (TS lenses) work against the effect during shooting — the sensor remains parallel to the wall, you only shift the image field. This is the clean solution if budget and time allow. For normal lenses, you have the sensor tilt itself — it works, but quickly destroys your depth of field. In-camera (on digital cameras, especially mirrorless) there are correction functions in live view that show you in real-time what the correction looks like. This is incredibly helpful for composing.
However: The in-camera method compresses your image field. You effectively crop the top and bottom to stretch the sides. This is sometimes okay, sometimes fatal for your frame. That's why many shots with keystone end up in post-production. There you have full control — and full pain if your image was too distorted. Modern grading software and even DaVinci have built-in keystone tools. It's also possible in editing (Premiere, Avid), but less elegant.
Practical tip: Consciously plan your perspective on location. If you know correction will be necessary, position yourself so that the effect remains moderate — correcting extreme angles always costs quality and image field. And remember: the more you tilt up or down, the more aggressive the distortion in the corners becomes. This is still visible even after correction. With wide-angle lenses, it becomes even more critical — the distortion multiplies. Normal to medium focal lengths forgive you more.