There are some situations where you focus manually, but don’t take the shot straight away. Such situations tend to come about because your subject moves quickly and you won’t have time to focus and still get the shot.Wildlife and nature shots are classic examples where you’d do this.
For example, you may focus on a branch, then wait for the bird to perch there, or focus on the rapids and wait for the canoe to whiz past that exact spot. Another common situation is focusing at night, where you’ve set the focus earlier in the day, and waited for the light to fall before taking the shots as the badger emerges from its den, or the deer walks down the path.
For these shots the correct focus is critical so, once you’ve set it, you need to make sure that it doesn’t shift.
Unfortunately it’s all too easy for the lens to be knocked, for a loose focus ring to slip a little, or thermal expansion and contraction to shift the ring ever so slightly. As a result your carefully focused shots turn out to be not so crisp after all.
Luckily, this can be easily prevented with a small strip of sticky tape, or a sticking-plaster from the First Aid kit.
Focus the lens in manual mode, then stick some tape across the focus ring so it is attached to the lens body and can’t be turned. Check the focus is still spot on, then relax knowing it can’t be changed until you peel off the sticking tape.