Canon EOS Rebel T5i 18-55mm IS STM Lens Kit Manual (en)

: Changing the Depth of Field
When using an aperture with a high f/number or shooting in low 
light scenes, note that camera shake can occur.
A higher aperture f/number will make the shutter speed slower. 
Under low light, the shutter speed can be as long as 30 sec. In such 
cases, increase the ISO speed and hold the camera steady or use a 
The depth of field depends not only on the aperture, but also on 
the lens and on the subject distance.
Since wide-angle lenses have a wide depth of field (range of 
acceptable focus in front of and behind the point of focus), you need 
not set a high aperture f/number to obtain a sharp picture from the 
foreground to the background. On the other hand, a telephoto lens 
has a narrow depth of field.
The closer the subject, the narrower the depth of field. A farther 
subject will have a wider depth of field.
Set the aperture so that the shutter speed display does not 
If you press the shutter button halfway and change 
the aperture while the shutter speed is displayed, 
the shutter speed display will also change to 
maintain the same exposure (amount of light 
reaching the image sensor). If you exceed the 
adjustable shutter speed range, the shutter speed 
display will blink to indicate that the standard 
exposure cannot be obtained.
If the picture will be too dark, the “30"” (30 sec.) shutter speed 
display will blink. If this happens, turn the <6> dial to the left to set 
a lower aperture f/number or increase the ISO speed.
If the picture will be too bright, the “4000” (1/4000 sec.) shutter 
speed display will blink. If this happens, turn the <6> dial to the 
right to set a higher aperture f/number or decrease the ISO speed.
Shooting Tips