Canon T3i Manual page 33

Aperture ( lens o pening)
• Keep h olding d own t he A V b uFon a nd t urn
dial
• Lower n umbers c reate a l arger a perture: t his
increases e xposure ( more l ight)
• Higher n umbers d ecrease e xposure b y
narrowing t he l ens o pening
• Lower n umbers c reate a s hallower d epth o f
field