There are two concepts that I teach that every nymph fisher should know. The first concept is "Effective Drift Length (EDL),“ and the second is “Effective Natural Drift (END).”
We all unconsciously apply these two concepts when we dry fly fish. Because we can see where the rise is when dry fly fishing, we instinctively know that the fly must be in the right current seam, on (adult) or in (emerger) the surface film, AND drifting drag free. Only when all of those three things occur do we have a chance of catching the fish.
The identical principal applies when we are nymphing. The fly must not only be in the correct current seam laterally (1) but vertically (2) in depth as well, and the must be as drag free (3) as possible. Only when these three criteria of lateral and vertical location and drift speed are correct is the drift most effective in catching fish. The fishing method that gives keeps the fly at the correct location for the longest length has the best "Effective Drift Length". When the fly is moving naturally at the correct drift speed, it has “Effective Natural Drift.”
A fly is only effective if it is at the level of the feeding fish. If the fish are at the bottom, a fly that is drifting above them is ineffective. It is only after the fly sinks to the level of the fish that the fly has an “Effective Drift Length". So the time the fly takes to sink to the bottom is wasted. If your flies never get to the fish, the entire drift is ineffective. That is why fly tungsten beads are more effective than brass beads. They sink faster and have a longer “Effective Drift Length" for the same cast.
The second concept is “Effective Natural Drift.” Does the fly drift like a natural nymph or pupa? My view is that unweighted nymphs drift more naturally. My belief is that you only weight the nymph if it adds to the natural action or it does not detract from a natural behavior. Otherwise I weight the leader.
I believe split shot(s) do not distract the fish. If fish ignore a leader that is tied directly to the fly, why would they be distracted by a split shot that is on that same leader but 12 - 18 inches away. If they are distracted or put off by anything, it will be any unnatural behavior of the nymph and I believe weighted nymphs act unnaturally. The may have a long “Effective Drift Length" but during that drift, they do not act naturally. So “Effective Natural Drift” is absent.
A live nymph is mostly water and it has a mass density that is neutrally buoyant. An unweighted fly is more neutrally buoyant than a weighted nymph, so it will act more naturally.
If a bead head does not drift naturally, why does it catch fish? It does so because they get to the bottom faster for a long “Effective Drift Length" AND a bright bead can be an attractor. I believe bead head work in fast water when the fish have less time to examine the fly for natural drift and when the fish feed by reaction strikes. So in fast water with turbulent flow, “Effective Natural Drift” is less important.
You have two choices, one is to weight the nymph to get it down, but once down it will be less natural. The other is to weight the leader to get the unweighted fly down, but one down, the fly will act more naturally.
One needs to choose between the two different methods. If you believe that a weight on the leader distracts the fish, use a weighted fly. if you think the fish are feeding on reaction, you can weight the fly. If you believe a weighted fly acting unnaturally will put the fish off, weight the leader.
I believe the latter rather than the former. Adding weight to the leader also allows me to micro adjust the weight I want to use. If you use only a weighted fly, you need to tie flies of multiple weights. Even then, you may not be able to pick the right fly for all speeds of the water flow and water depth without adding weight anyway. So if you have to add weight to the leader when you use bead head flies, why not use un-weighted flies and use adjust the split shot to get the flies down.
I also think my method of using unweighted flies is in the minority. I believe more nymph fishers do use weighted flies or bead head flies.
I think that in heavily fished waters, I have the advantage. When most fly fishers are using weighted nymphs, they will catch the fish that are susceptible to those flies. The fish that are left are the ones more susceptible to unweighted nymphs. They are the fish that are sensitive to both “Effective Drift Length" and “Effective Natural Drift.”
Since the Madison River is often crowded, I can fish behind other fly fishers and catch those fish that will reject bead head flies. Here are two fish I caught from the same run following another fly fisher. Notice that both fish have no hook scars which means they are infrequently caught. The second has a talon scar on the head from either an eagle or an osprey.