OPEN LOOP FLY CASTING
Sometimes called "no loop." The lines (legs) are widely separated. This is produced by waving the rod so that the tip moves in an exaggerated convex path.
This type of loop takes a lot of energy to cast. It is not recommended to practice open loop fly casting in windy situations; it may end up in a disaster. This technique is best used for casting heavy flies or weighted rigs for nymphing to give the leader more time to unfurl with the added weight. Controlling the shape is vital to take advantage of this technique’s benefits when the situation calls for it.
TIGHT LOOP FLY CASTING
Sometimes called narrow loop. The lines (legs) are close together and parallel, produced by an almost straight line path of the rod tip.
Tight loops require less energy to control. A tight loop is extremely efficient in wind and enables you to present your fly with pin-point accuracy. The energy used in this loop is mostly one-directional and produces high line speed. Tight loops can be quite fun to cast.
Image courtesy of Guide Recommended.
ONE LAST WORD ABOUT FLY CASTING LINES
Whether tight loop or open loop fly casting, successfully creating fly line loops relies on a smooth transfer of energy. Your fly line knots are part of this transfer, including the fly line loop to leader connection. Be mindful that your connection is secure enough to stay put without interfering with the casting motion.