using lens corrections (Ann)

DISTORTION: BARREL, PINCUSHION & PERSPECTIVE

This can give otherwise straight lines the appearance of bending inward or outward, and can influence the depiction of depth:

Pincushion Distortion

Barrel Distortion

Types & Causes. The most common categories of image distortion include:

Perspective "Distortion"

Blue dot represents direction of camera;
red lines mark converging parallel lines.

• Pincushion Distortion. Appears when otherwise straight lines curve inward. Typically caused by telephoto lenses, or at the telephoto end of a zoom lens.
• Barrel Distortion. Appears when otherwise straight lines curve outward. Typically caused by wide angle lenses, or at the wide end of a zoom lens.
• Perspective Distortion*. Appears when otherwise parallel lines converge. Caused by the camera not facing these parallel lines perpendicularly; with trees and architecture, this usually means that the camera isn't pointed at the horizon.

With landscapes, distortion of the horizon and trees are usually most detectable. Placing the horizon along the center of a photo can help minimize the appearance of all three types of distortion.

*Technical Note: Perspective distortion isn't technically a true distortion because it is a natural characteristic of 3D vision. We see it with our own eyes, but our mind knows the correct 3D positioning of objects and therefore doesn't perceive these lines as converging. For more, also see the tutorials on wide angle lenses and using tilt/shift lenses to control perspective.

Correction. Fortunately, each of the above types of distortion is correctable. However, this should only be performed when necessary, such as with subjects that include straight lines or are highly geometric. Architectural photography is often most sensitive, for example, whereas many landscapes aren't noticeably affected.

distortion correction sliders in photoshop

Editing software usually provides sliders for barrel/pincushion and perspective distortion in the vertical and horizontal directions. However, make sure to use the grid overlay feature (if available) so that you can more easily assess whether editing has made lines straight and parallel.
Disadvantages. Distortion correction usually requires cropping out curved edges of the corrected frame, which can influence composition. Correction also redistributes an image's resolution; with pincushion removal, the edges will appear slightly sharper (at the expense of the center), whereas with barrel removal the center will instead appear slightly sharper (at the expense of the edges). With wide angle lenses, barrel distortion is actually a helpful way of offsetting the corner softening that is common with these lenses, for example.