The angler in the center of the picture had a pod of over 2 dozen risers constantly working the bank in front of him. We fished within site of him for several hours and the risers never stopped. In that amount of time, Connor and I were fortunate enough to catch dozens of fish each with less dense pods of fish. I watched the angler only land 4 or 5 and Connor and I both wanted to motor over and lend some help but couldn't bring ourselves to do it for fear of how we would be received. Though he might not have been the sharpest caster, it wasn't his skill that was keeping him from catching trout. He was fishing a fly 4 or so feet below an indicator. The problem was that the fish were less than a foot below the surface.
His lack of success led me to think about a common problem with many fly anglers' strategies. Trout (and other salmonids) are visual predators. True, they have lateral lines that sense vibration and nares that serve olfactory purposes, but by and large they find most of their prey through visual detection. (Different salmonids have different visual capabilities by the way. One of the characteristics that allow sympatric brown trout and Arctic char as well as sympatric Dolly Varden and cutthroat trout to coexist is the superior vision of the char species in low light.)
The positioning of eyes on a trout create monocular vision on each side with binocular vision in front and in a cone shape above them. A quick search on the web will give you a run down on the technical aspects but the main thing to remember is trout generally see at or above their level. They are more adept at column feeding than they are at benthic grubbing, though they occasionally use this strategy especially in weedy waters where scuds or sowbugs are their main table fare. Trout's column feeding strategy would generally suggest that your flies should also be at or above a trout's level if your goal is for them to actually see and eat your flies. This principle applies to stillwaters and rivers. This is generally why I like to start above the level where I think the trout are and work my way down. If my first depth guess is incorrect, a deeper increment in fly line choice or fly weight usually puts me in the zone where trout see my flies and are close enough to them to be willing to intercept as long as other aspects of my presentation are suitable. If I guess too deep to begin with then I'm left with the fate of the angler I met yesterday. Let's all avoid that fate and put a few more fish in our nets by working our way down instead of staying under the trout were after.