Spot the Soft Edges

Most lenses, when making the picture, create softness on edges of the frame. The lens focuses light sharply in the middle of the camera's sensor but as the light reaches the rim of the lenses, the sharpness deteriorates, hence the soft edges. More expensive lenses allow for more complex lens configurations to treat the soft-edge condition. For any lens, using high f-stop helps prevent soft-edge effect but not always 100%.

1/1.3th second @ f8 ISO 100

tripod

Sometimes the soft-edge effect doesn't affect our photos much. Your audience won't be spotting lens deficiencies on the first look; they'll be looking at your subject matter more importantly. There are some cases where soft edges are undesirable: A solid or consistent pattern/texture across the whole image will reveal itself more easily, you can see soft, sharp, soft looking left to right on the frame. A subject who partly reaches beyond the frame, for example, an arm that crops off to the right causes sudden sharp to soft.
Edge softness also reduces contrast in those effected areas.