Regarding eyeglasses:
I believe your camera has a diopter adjustment for the viewfinder, that lets you set the viewfinder to match your glasses or to match your eyes without glasses.
Here is a writeup on the subject:
http://www.digital-photography-school.com/using-diopter-adjustment
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Regarding covering the viewfinder:
Your camera probably came with a small plastic cover that is used to covering your viewfinder.
Most people lose it because they don't know what it is for.
For normal shooting there is no need to cover the viewfinder.
If you are doing long exposures (over 20 seconds) you may need to cover the viewfinder.
I ran into this while shooting fireworks.
I was using a 35 film SLR and was keeping the lens open for about 60 seconds to capture more fireworks bursts in one shot.
I was using a tripod.
There was a street light behind me.
Enough light got in through the viewfinder to spoil the shots.
Previously I had taken shots of fireworks where there was no light behind me, and they were OK.
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I have a new T3i and cannot use the regular viewfinder (eyeglass issues) and find myself always using the LCD screen. Do I have to cover the viewfinder when I use the screen? Or does using the LCD screen close the viewfinder so no extraneous light gets in?

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