Generally, barrel distortion occurs when a wide angle lens is used to photograph a subject with strong vertical or horizontal lines. Basically a convex mirror effect is applied because the lense is actually bending the light, forcing a larger image area to be recorded on a smaller capture service. Or in super confusing mathematical formula format: R=(a* r ^ 3 + b * r ^2 + c ^r + d)*r. Simply, objects closer to the camera appear larger and straight lines are curved and objects in background appear smaller and further away.
Easy Tips to Avoid Barrel Distortion in your Photography
1) Shoot with a 50mm lens (full frame sensor)
On a 35mm format camera a focal length of 50mm is almost identical to that of the human eye. So, coincidentally what you see is what you get. Furthermore, I strongly believe every person interested in photography should learn on a 50mm lens first before moving on to different lens types. Any lens with a shorter focal length will cause barrel distortion.
2) Shoot Panoramic Images.
Shooting a panoramic image allows you to use a a lens with a longer focal length yet still capture a wide field of view. I will often shoot two or three image pans so I can create the exact image I want without adding distortion.
3) Avoid the Corners of the Lens.
When shooting with a wide angle lens always avoid putting subject matter in the fringes of the image. Barrel distortion increases exponentially as you move away from the centre of the lens. Photographing a group shot with a wide angle well often turn one or two members into a conehead.
4) Don't Use a Fisheye Lens.
Just don't. A Fisheye looks good in maybe 1% of the instances you will have the opportunity to use one, and even that is a stretch. They are expensive, make subjects look terrible and provide poor optic quality, so pretty much useless.
Preventing and properly managing distortion is a key step in the advancing the quality of your imagery. It is one of the many attributes that defines the quality of a professional photograph.
You can also view: Top 8 Hilarious Photography Mistakes