Originally posted by hcc Some PF member said: your feet are your zoom in/out with a prime. That is right. With a single prime, you have to learn to be at the right place, and this is an excellent skill to gain.
In addition, there are situations when you cannot change lenses: no time, or too dirty, or too risky. Changing lenses can be messy and you take the risks to contaminate your camera and your lens.
My walk around prime lens is the FA31mm. I love its IQ and I am prepared to use my feet to compensate for the lack of zoom (or the lack of other primes).
My 5 cents....
^I like the feet analogy, I'm pretty darn fresh to photography (imo at least) and this is how I view it when I shoot with my 50mm.
To the OP, I switch lenses very very often in the field. When I look for shots I am trying to identify not only how I'm going to compose, but where I need to be, what lens works with that, and whether the strengths/weaknesses of that lens is going be an issue. The lens switch happens if I think its worth it; if I don't think I have the equipment for the shot, I'll usually just try to figure out another line of attack.
I carry three lenses with me usually, and I try to streamline my bag so that lens swaps can happen ASAP. If I'm particularly active and in a relatively safe zone, I'll have lens caps and bayonet caps off all lenses the entire time, with the bayonet facing the bottom of the bag. Really darn risky, but I find that when I can just unlock the lens and chuck it into the bag (without putting on caps), I start kind of indexing where that little red alignment dot is, and lenses begin to pop in and out at the snap of a finger.