OK, so you have a variety of output values, and you are setting a threshold and connecting that into an Arduino digital input, is that right?
The easiest way may be to pick a midrange resistor and connect the signal into an Arduino analog input where you can read the value and then decide what to do??
Since the lighting is a transient signal, you'd have to sample it pretty often, though!
How often do you adjust the threshold, and why do you change it??
BTW some people use an antenna and RF detector to detect the lightning, like you hear on an AM radio. That can 'hear' lightning at a long distance so you can put the camera out and get ready..
Tell us more...
Also, here's a pointer to a camera controller I am working on, and Lightning detector is one of the things I want to add http://yourduino.com/camera-control.htm