I have just had a weekend of down-stream nymphing and whilst fishing a thought struck me. At what point in the drift does the nymph stop looking natural?

Taking the example of a cast straight out (90º angle from the bank) and allowing the line to be taken by the flow whilst mending at the same time. Normal drift will allow a fly to travel more or less to an angle of 45º to where the angler is... then the fly will start to move back towards the bank. In a relatively fast flow I am I correct in assuming that it is unatural for a Nymph to "swim" across the current?
Also I had understood that the most natural presentation point when down stream nymphing is when the nymph lifts at the end of the run.. would this be at the 45º point or when the line is parrallel with the bank?

I'd guess that it stops looking natural the second that "drag" is created by a tight line.

Even though naturals may move across the flow, they do not have a length of line pulling them and so slowing them down and lifting them up as they come across.

However, it may not really matter. Matching the origional exactly is not really possible in any fly fishing (put simply - your fly, no matter how lifelike, is not alive), it is more about putting something that a particuar fish wants to eat in front of that particular fish at a time that particular fish is ready to eat.
Having it behave just like the naturals may well get it eaten. But also, having it behave differently may well make the trout grab out of instinct, greed or curiousity.
Often refered to as the "induced take".

So for me, the "most natural" doesnt really come into it. I'm more a believer in trying different things out and then going with the "most productive" on the day.
I've had enough fish off a bad cast, or grab hold on the retrieve to make me try diffrent things out.

Very interesting spider316... thanks for the reply! I will follow your guide next time and just keep trying different things!