There are times I enjoy casting a fly to carp holding thirty-five or forty feet away. To me this presents more of a challenge in capturing carp. At these distances we lose total eye contact with the fly and depend on watching the behavior and movements of the carp to know when it's time to go for the hook-set.
Over the last three years I've caught a good number of carp making a roll cast of thirty-five or better feet and it's always rewarding to bring a fish to hand from these longer distance casts.
However, there have been many times I've watched carp spook well before the fly ever hits the water. It is if they can see the line coming their way while the line is still in mid-air.
This ability they own in seeing a neutral color line in flight is simply amazing to me, and for a long time it was a head-scratcher.
To understand how these fish can see our line in the air, all we have to do is look at the phenomenon known as Snell's Window. Snell's Window is a portal that fish, other creatures, and humans can see through the surface of water. Snell's Window is derived from Snell's Law named after Willebrord van Roijen Snell.
As anglers, the thing we need to remember is the size of the portal's diameter is 2.26 times the depth of the fish. So, if a carp is in one foot of water the diameter of the portal is 2.26 feet. The carp just upstream is in two foot of water and his portal is 5.52 feet (diameter).
The carp I cast to at longer distances are usually holding in about three feet of water and therefore have a diameter portal of surface vision of almost seven feet. Once my fly line entered the plane of their vision through Snell's Window they could easily see the line.
There are ways to combat the advantage Snell's Window give to fish. Keeping a low profile, wearing muted or natural blending colors, and making side-arm casts will serve as tools we can use to keep from spooking the fish we seek.