Facial twitch: I believe these twitches are similar to the twitches people get in their eyelids. Usually it goes away within 6 weeks or so. There are other causes such as caffeine, stress, alcohol, electrolyte imbalances and excessive fatigue. Feel better.
A shutter speed of 1/30 for example will let in twice as much light as a speed of 1/60 since it is two times slower. When you decrease the shutter speed by a factor of two you are increasing the amount of light by one stop. Remember that fractions get larger as the number on the bottom gets smaller.
If you want to photograph something that is moving, you need a fast shutter speed. 1  Use 1/500 for general photography of everyday events, sports, and subjects. 2  Use 1/1000-1/4000 when shooting subjects that are extremely fast and up close. 1/1000-1/2000 works well when photographing birds. Typically only the denominator is marked on the camera. 2  For example, 125 means 1/125 second. 3  Some cameras allow you to take photos at a shutter speed in full seconds, such as 1, 2, or 10 seconds. 4  This is used for low light photography and a lot of movement.