What problem could cause a prism in one's field of vision?

Usually you're born with it. Some folks who have a 'lazy eye' are sometimes prescibed a prism to try to straighten that out, but... I have also seen cases where labs have made glasses incorrectly and unintentionally put prism in the lenses because the proper measurements were not taken. Occassionally... the prescribed prism is a result of some trauma to the head... in those instances, it's a small amount, and *might* reverse itself once the muscles gain strength. Best case scenario, it was poorly made specs... best bet though is to see your Optometrist or Opthamalogist for a full & thorough examination of they eyes. Hope that helped! Cindy