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.
Many sports use very fast shutter speeds to capture the peak moment and freeze it in time. If the subject of your photo is not moving, or you want to show the passage of time (with a running river or waterfall), then you can use a slower shutter speed.hutter speed is measured in fractions of seconds and seconds. Here is an example of shutter speed numbers, from fast to slow: 1/2000 1/1500 1/1000 1/750 1/500 1/350 1/250 1/180 1/125 1/90 1/60 1/45 1/30. Realize that 1/2000 of a second is incredibly fast and is not a shutter speed that you will typically use.